Bibliography - Hiram Levy II

1. Cassar, N, M Bender, B A Barnett, Song-Miao Fan, Walter Moxim, Hiram Levy II, and B Tilbrook, 2008: Response to comment on. Science, 319(5860), doi:10.1126/science.1150011.
   [ Abstract ]

   Net community production in the Southern Ocean is correlated with simulated local dust deposition, and more so with modeled deposition of soluble iron. Model simulations of the latter two properties are consistent with observations in both hemispheres. These results provide strong evidence that aerosol iron deposition is a first-order control on net community production and export production over large areas of the Southern Ocean.

2. Fang, Y, Arlene M Fiore, Larry Horowitz, Anand Gnanadesikan, Hiram Levy II, Y Hu, and A G. Russell, in press: Estimating the episodic contribution to total pollutant export from the United States in summer. Journal of Geophysical Research. 8/08.
   [ Abstract ]

   Emissions from the United States can affect downwind region air quality through strong episodic outflow or by increasing the hemispheric burden. We use the MOZART model to estimate the episodic contribution to the total pollutant export during summer. Focusing on the major export pathway from the United States, namely eastward export to the North Atlantic, we develop a criterion to identify episodic export. Our diagnosed episodic export is confirmed by strong outflow plumes sampled on subsequent days during the 2004 INTEX-NA field campaign. Both model and observations indicate that strong episodic outflow occurs in the boundary layer and in the upper troposphere. Enhanced surface export is associated with the passage of cyclones while upper tropospheric export is driven by lifting of surface pollutants by Warm Conveyor Belts. Simulated pollutant concentrations in anthropogenic plumes agree with observed concentrations to within 30% for CO and O3, while surface PAN is sometimes overestimated by a factor of 2. Episodic export accounts for 15% to 35% of total pollutant export in each summer, and it is poorly correlated with total export. Due to a weak Bermuda High, episodic export of CO in summer 2004 contributes over 30% to the total export while total export is relatively low. Episodic contributions to NOy and O3 export are similar to that of CO which implies that export of these species is largely driven by meteorology. We conclude that focusing on episodic export neglects a majority of the total export since most export occurs under non-episodic conditions.

3. Levy II, Hiram, M Daniel Schwarzkopf, Larry Horowitz, V Ramaswamy, and Kirsten L Findell, March 2008: Strong sensitivity of late 21st Century climate to projected changes in short-lived air pollutants. Journal of Geophysical Research, 113, D06102, doi:10.1029/2007JD00917.
   [ Abstract PDF ]

   This study examines the impact of projected changes (A1B “marker” scenario) in emissions of four short-lived air pollutants (ozone, black carbon, organic carbon, and sulfate) on future climate. Through year 2030, simulated climate is only weakly dependent on the projected levels of short-lived air pollutants, primarily the result of a near cancellation of their global net radiative forcing. However, by year 2100, the projected decrease in sulfate aerosol (driven by a 65% reduction in global sulfur dioxide emissions) and the projected increase in black carbon aerosol (driven by a 100% increase in its global emissions) contribute a significant portion of the simulated A1B surface air warming relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally, 0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40% of the total) in the summertime United States. These projected changes are also responsible for a significant decrease in central United States late summer root zone soil water and precipitation. By year 2100, changes in short-lived air pollutants produce a global average increase in radiative forcing of ∼1 W/m²; over east Asia it exceeds 5 W/m². However, the resulting regional patterns of surface temperature warming do not follow the regional patterns of changes in short-lived species emissions, tropospheric loadings, or radiative forcing (global pattern correlation coefficient of −0.172). Rather, the regional patterns of warming from short-lived species are similar to the patterns for well-mixed greenhouse gases (global pattern correlation coefficient of 0.8) with the strongest warming occurring over the summer continental United States, Mediterranean Sea, and southern Europe and over the winter Arctic.

4. Shindell, D, Hiram Levy II, M Daniel Schwarzkopf, Larry Horowitz, J F Lamarque, and G Faluvegi, June 2008: Multimodel projections of climate change from short-lived emissions due to human activities. Journal of Geophysical Research, 113, D11109, doi:10.1029/2007JD009152.
   [ Abstract ]

   We use the GISS (Goddard Institute for Space Studies), GFDL (Geophysical Fluid Dynamics Laboratory) and NCAR (National Center for Atmospheric Research) climate models to study the climate impact of the future evolution of short-lived radiatively active species (ozone and aerosols). The models used mid-range A1B emission scenarios, independently calculated the resulting composition change, and then performed transient simulations to 2050 examining the response to projected changes in short-lived species and to changes in both long-lived and short-lived species together. By 2050, two models show that the global mean annual average warming due to long-lived GHGs (greenhouse gases) is enhanced by 20–25% due to the radiatively active short-lived species. One model shows virtually no effect from short-lived species. Intermodel differences are largely related to differences in emissions projections for short-lived species, which are substantial even for a particular storyline. For aerosols, these uncertainties are usually dominant, though for sulfate uncertainties in aerosol physics are also substantial. For tropospheric ozone, uncertainties in physical processes are more important than uncertainties in precursor emissions. Differences in future atmospheric burdens and radiative forcing for aerosols are dominated by divergent assumptions about emissions from South and East Asia. In all three models, the spatial distribution of radiative forcing is less important than that of climate sensitivity in predicting climate impact. Both short-lived and long-lived species appear to cause enhanced climate responses in the same regions of high sensitivity rather than short-lived species having an enhanced effect primarily near polluted areas. Since short-lived species can significantly influence climate, regional air quality emission control strategies for short-lived pollutants may substantially impact climate over large (e.g., hemispheric) scales.

5. Balaji, Ventakramani, Thomas L Delworth, Robert W Hallberg, Hiram Levy II, Ronald J Stouffer, and Michael Winton, 2007: Toward a new generation of ice sheet models. EOS, 88(52), 578-579.
6. Cassar, N, M Bender, B A Barnett, Song-Miao Fan, Walter Moxim, Hiram Levy II, and B Tilbrook, 2007: The Southern Ocean biological response to aeolian iron deposition. Science, 317(5841), doi:10.1126/science.1144602.
   [ Abstract ]

   Biogeochemical rate processes in the Southern Ocean have an important impact on the global environment. Here, we summarize an extensive set of published and new data that establishes the pattern of gross primary production and net community production over large areas of the Southern Ocean. We compare these rates with model estimates of dissolved iron that is added to surface waters by aerosols. This comparison shows that net community production, which is comparable to export production, is proportional to modeled input of soluble iron in aerosols. Our results strengthen the evidence that the addition of aerosol iron fertilizes export production in the Southern Ocean. The data also show that aerosol iron input particularly enhances gross primary production over the large area of the Southern Ocean downwind of dry continental areas.

7. Fan, Song-Miao, Walter Moxim, and Hiram Levy II, 2006: Aeolian input of bioavailable iron to the ocean. Geophysical Research Letters, 33, L07602, doi:10.1029/2005GL024852.
   [ Abstract ]

   Atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean. Presumably only the readily soluble fraction is available for biological uptake. Previous ocean models assumed this fraction was constant. Here the variable solubility of Fe in aerosols and precipitation is parameterized with a two-step mechanism, the development of a sulfate coating followed by the dissolution of iron (hydr)oxide on the dust aerosols. The predicted soluble Fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration. The soluble fraction is ~1 percent near sources, but often 10–40 percent farther away producing a significant increase in soluble Fe deposition in remote ocean regions. Our results may require more rapid biological and physicochemical scavenging of Fe than used in current ocean models. We further suggest that increasing SO2 emission alone could have caused significant Fe fertilization in the modern northern hemisphere oceans.

8. Fan, Song-Miao, Walter Moxim, and Hiram Levy II, 2005: Implications of droplet nucleation to mineral dust aerosol deposition and transport. Geophysical Research Letters, 32, L10805, doi:10.1029/2005GL022833.
   [ Abstract ]

   Calculations from a microphysics model are shown which indicate the factors that control droplet nucleation scavenging of hydrophilic mineral dust particles over a large range of conditions including the size, chemical composition, and number density of particles in both cumulus and stratus clouds. We focus specifically on the activation threshold radius (ATR) for droplet nucleation which determines the particles that are activated and those available for further transport and subsequent iron deposition to the remote ocean. Results suggest: the ATR is typically found in the range of clay-sized particles (radius = .1 to 1. µm), a spectrum over which the amount of dust removed declines ~60% both in surface area and particle number; nucleation of silt-sized particles (1.–10. µm) occurs under most conditions; larger fractions of mineral aerosols are removed in cumulus clouds than in stratus; and while acid coating of dust particles in polluted environments acts to decrease the ATR, the effect is reduced by competition with soluble aerosols. Regional mineral dust environments exhibit potentially diverse aerosol wet removal impacts. The ATR representative of the tropical Atlantic ocean basin (<.2 µm) indicates ~80% removal of the total dust surface area, while in the pristine southern hemisphere mid latitudes an ATR ~.5 µm implies ~60%. In contrast, varying conditions in the polluted region of East Asia suggest a large ATR spectrum (.2 to 3. µm) with dust surface area removal ranging from >80% to <10%.

9. Fiore, Arlene M., Larry Horowitz, D W Purves, Hiram Levy II, M J Evans, Y Wang, Q Li, and R M Yantosca, 2005: Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States. Journal of Geophysical Research, 110, D12303, doi:10.1029/2004JD005485.
   [ Abstract ]

   Reducing surface ozone (O3) to concentrations in compliance with the national air quality standard has proven to be challenging, despite tighter controls on O3 precursor emissions over the past few decades. New evidence indicates that isoprene emissions changed considerably from the mid-1980s to the mid-1990s owing to land-use changes in the eastern United States (Purves et al., 2004). Over this period, U.S. anthropogenic VOC (AVOC) emissions decreased substantially. Here we apply two chemical transport models (GEOS-CHEM and MOZART-2) to test the hypothesis, put forth by Purves et al. (2004), that the absence of decreasing O3 trends over much of the eastern United States may reflect a balance between increases in isoprene emissions and decreases in AVOC emissions. We find little evidence for this hypothesis; over most of the domain, mean July afternoon (1300–1700 local time) surface O3 is more responsive (ranging from -9 to +7 ppbv) to the reported changes in anthropogenic NOx emissions than to the concurrent isoprene (-2 to +2 ppbv) or AVOC (-2 to 0 ppbv) emission changes. The estimated magnitude of the O3 response to anthropogenic NOx emission changes, however, depends on the base isoprene emission inventory used in the model. The combined effect of the reported changes in eastern U.S. anthropogenic plus biogenic emissions is insufficient to explain observed changes in mean July afternoon surface O3 concentrations, suggesting a possible role for decadal changes in meteorology, hemispheric background O3, or subgrid-scale chemistry. We demonstrate that two major uncertainties, the base isoprene emission inventory and the fate of isoprene nitrates (which influence surface O3 in the model by -15 to +4 and +4 to +12 ppbv, respectively), preclude a well-constrained quantification of the present-day contribution of biogenic or anthropogenic emissions to surface O3 concentrations, particularly in the high-isoprene-emitting southeastern United States. Better constraints on isoprene emissions and chemistry are needed to quantitatively address the role of isoprene in eastern U.S. air quality.

10. Fan, Song-Miao, Larry Horowitz, Hiram Levy II, and Walter Moxim, 2004: Impact of air pollution on wet deposition of mineral dust aerosols. Geophysical Research Letters, 31, L02104, doi:10.1029/2003GL018501.
    [ Abstract PDF ]

    Mineral dust aerosols originating from arid regions are simulated in an atmospheric global chemical transport model. Based on model results and observations of dust concentration, we hypothesize that air pollution increases the scavenging of dust by producing high levels of readily soluble materials on the dust surface, which makes dust aerosols effective cloud condensation nuclei (CCN). This implies that air pollution could have caused an increase of dust deposition to the coastal oceans of East Asia and a decrease by as much as 50% in the eastern North Pacific.

11. Yang, H, and Hiram Levy II, 2004: Sensitivity of photodissociation rate coefficients and O3 photochemical tendencies to aerosols and clouds. Journal of Geophysical Research, 109, D24301, doi:10.1029/2004JD005032.
    [ Abstract ]

    We examine the sensitivity of the daily integrated photodissociation rate (DIPR) coefficients of O3(O1 D) and NO2, the daily averaged O3 photochemical tendency, and OH concentration to variations of clouds and of four aerosol components: black carbon (BC), mineral dust, sea salt, and sulfate. Clouds and BC aerosols are found to be the most important. A comparison between the sensitivity study and some representative observations showed that the global average cloud reduction of DIPR at the surface level is ∼20% and it is ∼30% in storm track zones. At the surface level the average negative BC impact is ∼10% in urban areas and could reach ∼40% in some heavily polluted urban areas. Mineral dust aerosol, which is the next most important, can reduce the photochemistry at the surface level by over 17% in some seasons over the desert or along its long-range transport paths. The negative impact of sulfate aerosols is around 2% at the surface level, and the impact is as positive as 4% at the top of the sulfate aerosol layer in some urban areas. BC, sulfate, and mineral dust all have much smaller impacts away from their source regions. The impact of sea-salt aerosol is generally less than 1%. While the fractional impacts on O3 production and destruction and OH concentration do not depend much on NO x , the magnitude of the impact on O3 chemical tendency and OH concentration depends strongly on the concentration of NO x : When NO x is very low, the impacts are also very small even if DIPRs are strongly affected. Different mixing states of absorbing and scattering aerosol components, external, coating, and internal, are studied. It is found that with the same amount of each component, the external mixing state produces the weakest impact while the internal mixing state produces the strongest impact. Coating has almost the same impact as internal mixing. There is very little synergy between cloud and absorbing aerosols when clouds are located above the aerosol. The aerosol absorption is strengthened when clouds and aerosol are located in the same layer. The synergy is strong when clouds are located below the absorbing aerosol, where the impact of absorbing aerosol dominates.

12. Holloway, T, Hiram Levy II, and G R Carmichael, 2002: Transfer of reactive nitrogen in Asia: development and evaluation of a source-receptor model. Atmospheric Environment, 36(26), 4251-4564.
    [ Abstract PDF ]

    A simple model of chemistry and transport, ATMOS-N, has been developed to calculate source-receptor relationships for reactive nitrogen species within Asia. The model is intended to support discussion of energy and environmental issues in Asia, to compare sulfate and nitrate contributions to regional acidification, and to estimate how each nation's acid deposition and air quality relates to domestic versus foreign emissions. ATMOS-N is a Lagrangian "puff" model in which non-interacting puffs of emissions are advected horizontally and mixed between three vertical layers. Results are compared with wet nitrate deposition observations in Asia. On an annual average, the model estimates that long-range transport contributes a significant percentage of total nitrate deposition throughout east Asia. China, the largest emitter of the region, contributes 18% to nitrate deposition inTaiwan, 18% in Japan, 46% in North Korea, and 26% in South Korea. South Korea contributes 12% to nitrate deposition in Japan, due to its close upwind proximity. Compared with total acid deposition (nitrate + sulfate), nitrate contributes 30–50% over northern Japan, 30–60% in India, and 50–90% in southeast Asia where biomass burning emits high levels of NOx. The percentage contribution of nitrate is very low in China, where emissions and deposition of sulfur are extraordinarily high.

13. Phadnis, M J., Hiram Levy II, and Walter Moxim, 2002: On the evolution of pollution from South and Southeast Asia during the winter-spring monsoon. Journal of Geophysical Research, 107(D24), 4790, doi:10.1029/2002JD002190.
    [ Abstract PDF ]

    The NOAA Geophysical Fluid Dynamics Laboratory three-dimensional Global Chemical Transport Model (GFDL GCTM) is used to examine the winter-spring evolution of pollution (fossil fuel combustion and biomass burning) from South and Southeast Asia with special focus on the Indian Ocean region. We find that during the monsoonal winter-spring outflow, pollution over the Indian Ocean north of the ITCZ is concentrated in the maritime boundary layer and originates from both regions. South Asian emissions dominate over the Arabian Sea and the Western Indian Ocean, while the Southeast Asian emissions have the greatest impact over the Bay of Bengal and Eastern Indian Ocean. Over these oceanic regions, CO pollution in both source regions, most of which is from biomass burning, acounts for 30–50% of the boundary layer CO. It is transported equatorward from South and Southeast Asian source regions and episodically lofted into the upper troposphere by tropical convection events. This transport path has a noticable impact (10–20%) on total CO at 300 mb and produces a maximum in a tropical belt over and north of the ITCZ. Another free troposphere transport path, primarily open to Southeast Asian emissions, carries CO from the region out over the North Pacific and around the Northern Hemisphere. O3 production is driven by NOx, which, unlike CO, comes almost equally from biomass burning and fossil fuel combustion in this region and has a chemical lifetime of a few days or less. The resulting NOx distrributions, while qualitatively similar to CO, have much steeper gradients, are transported much less widely, have a much lower background, and over the Indian and Pacific Oceans, are strongly dominated by pollution. O3 resulting from these anthropogenic sources generally exhibits patterns similar to those found for CO and NOx. Pollution accounts for 20–50% of the near-surface O3 and 5–10% of the O3 in the upper troposphere. South and Southeast Asian emissions only produce 25% of the boundary layer O3 in the continental source regions. The maximum impact of the emissions occurs over the Indian Ocean (25–40%) with comparable contributions from O3 produced in the continental emission regions and O3 produced over the ocean by transported precursors. Convective lifting of the transported pollution O3 supplies ~10% of the O3 in the tropical upper troposphere. While both emission regions have modest impacts on O3 (5–10%) outside of the Indian Ocean region, Southeast Asian pollution impacts free troposphere O3 in a midlatitude belt across the North Pacific, similar to NOx.

14. Galanter, M, Hiram Levy II, and G R Carmichael, 2000: Impacts of biomass burning on tropospheric CO, NOX, and O3. Journal of Geophysical Research, 105(D5), 6633-6653.
    [ Abstract PDF ]

    This study utilizes the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory three-dimensional global chemical transport model to quantify the impacts of biomass burning on tropospheric concentrations of carbon monoxide (CO), nitrogen oxides (NOx), and ozone (O3). We construct updated global sources that emit 748 Tg CO/yr and 7.8 Tg N/yr in the surface layer. Both sources include six types of biomass: forest, savanna, fuelwood, agricultural residues, domestic crop residues (burned in the home for cooking and/or heating), and dried animal waste. Timing for the burning of forest, savanna, and agricultural residues is based upon regional cultural use of fire, vegetation type, local climate, and information gathered from satellite observations, while emissions from the burning of fuelwood, domestic crop residues, and dried animal waste are constant throughout the year. Based on agreement with observations, particularly of CO, we conclude that the collective uncertainty in our biomass burning sources is much less than the factor of two suggested by previous estimates of biomass burned in the tropics annually. Overall, biomass burning is a major source of CO and NOx in the northern high latitudes during the summer and fall and in the tropics throughout most of the year. While it contributes more than 50% of both the NOx and CO in the boundary layer over major source regions, it has a much larger global impact on the CO distribution in comparison to either NOx or O3, contributing 15 to 30% of the entire tropospheric CO background. The only significant biomass burning contribution to NOx at 500 mbar, due to the short lifetime of NOx in the lower troposphere, is a plume occurring July through October in the Southern Hemisphere subtropical free troposphere, stretching from South America to the western Pacific. The largest impacts on O3 are limited to those regions where NOx impacts are large as well. Near the surface, biomass burning indirectly contributes less than half of the total O3 concentrations over major tropical source regions, up to 15% throughout the year in the tropics, and 10 to 20% throughout the Southern Hemisphere during September through November. At 500 mbar, the largest contribution to O3 (20-30%) is correlated with the NOx plume during July through November. Biomass burning contributes less than 15% of either NOx or O3 in the upper troposphere.

15. Holloway, T, Hiram Levy II, and P S Kasibhatla, 2000: Global distribution of carbon monoxide. Journal of Geophysical Research, 105(D10), 12,124-12,147.
    [ Abstract PDF ]

    This study explores the evolution and distribution of carbon monoxide (CO) using the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory three-dimensional global chemical transport model (GFDL GCTM). The work aims to gain an improved understanding of the global carbon monoxide budget, specifically focusing on the contribution of each of the four source terms to the seasonal variability of CO. The sum of all CO sources in the model is 2.5 Pg CO/yr (1 Pg = 103 Tg), including fossil fuel use (300 Tg CO/yr), biomass burning (748 Tg CO/yr), oxidation of biogenic hydrocarbons (683 Tg CO/yr), and methane oxidation (760 Tg CO/yr). The main sink for CO is destruction by the hydroxyl radical, and we assume a hydroxyl distribution based on three-dimensional monthly varying fields given by Spivakovsky et al. [1990], but we increase this field by 15% uniformly to agree with a methyl chloroform lifetime of 4.8 years [Prinn et al., 1995]. Our simulation produces a carbon monoxide field that agrees well with available measurements from the NOAA/Climate Monitoring and Diagnostics Laboratory global cooperative flask sampling network and from the Jungfraujoch observing station of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) (93% of seasonal-average data points agree within ±25%) and flight data from measurement campaigns of the NASA Global Tropospheric Experiment (79% of regional- average data points agree within ±25%). For all 34 ground-based measurement sites we have calculated the percentage contribution of each CO source term to the total model-simulated distribution and examined how these contributions vary seasonally due to transport, changes in OH concentration, and seasonality of emission sources. CO from all four sources contributes to the total magnitude of CO in all regions. Seasonality, however, is usually governed by the transport and destruction by OH of CO emitted by fossil fuel and/or biomass burning. The sensitivity to the hydroxyl field varies spatially, with a 30% increase in OH yielding decreases in CO ranging from 4-23%, with lower sensitivities near emission regions where advection acts as a strong local sink. The lifetime of CO varies from 10 days over summer continental regions to well over a year at the winter poles, where we define lifetime as the turnover time in the troposphere due to reaction with OH.

16. Kasibhatla, P S., Hiram Levy II, Walter Moxim, S N Pandis, J J Corbett, M C Peterson, R E Honrath, K Knapp, D D Parrish, T B Ryerson, and G Frost, 2000: Do emissions from ships have a significant impact on concentrations of nitrogen oxides in the marine boundary layer? Geophysical Research Letters, 27(15), 2229-2232.
    [ Abstract PDF ]

    The potential impact of ship emissions on concentrations of nitrogen oxides and reactive nitrogen compounds in the marine boundary layer is assessed using a global chemical transport model. The model predicts significant enhancements of these compounds over large regions, especially over the northern midlatitude oceans. This result is consistent with a recently published study, though the impacts predicted here are more widespread and the peak enhancements are not as large. However, comparisons of model results with recent measurements over the central North Atlantic Ocean do not provide support for these model predictions. While one cannot completely overlook the possibility that emissions of nitrogen oxides from ships may be overestimated, our analysis suggests that there may be a gap in our understanding of the chemical evolution of ship plumes as they mix into the background atmosphere in the marine boundary layer. On a related note, it is also possible that the overestimate of the impacts of ships on nitrogen oxides in the marine boundary layer by global models is due to the lack of parameterized representations of plume dynamics and chemistry in these models.

17. Moxim, Walter, and Hiram Levy II, 2000: A model analysis of the tropical South Atlantic Ocean tropospheric ozone maximum: The interaction of transport and chemistry. Journal of Geophysical Research, 105(D13), 17,393-17,415.
    [ Abstract PDF ]

    The meteorological and photochemical nature of the South Atlantic Ocean tropospheric column ozone maximum is examined by analyzing the Geophysical Fluid Dynamics Laboratory (GFDL) Global Chemical Transport Model (GCTM) simulation during the Southern Hemisphere late winter. An ozone maximum of greater than 40 Dobson units is produced by the GCTM over the South Atlantic Ocean. The model is evaluted against available meteorological and ozone data and found to be in good qualitative agreement with observed wind fields, satellite measurements of tropospheric column ozone, tropospheric column ozone produced from ozonesonde data, and vertical profiles from ozonesondes. A quantitative analysis is performed over an area of the South Atlantic Ocean essentially devoid of local NOx sources and for a time, September, when the regional tropospheric ozone mass is at a maximum. The tropospheric mass of reactive nitrogen transported into the region is a result of source contributions from lightning (49%), biomass burning (36%), and 15% from the remaining NOx sources (fossil fuel plus biogenic plus stratosphere plus aircraft). Even with the removal of biomass burning NOx from the ozone photochemical system, the GCTM still produces an oceanic tropospheric column ozone maximum, suggesting the ozone phenomenon existed before agricultural burning by humans. The structure of clean air CO/CH4 net chemistry consists of ozone production in the upper troposphere (+2.2 Tg/month), weak destruction in the middle troposphere (-1.8 Tg/month), and strong destruction in the lower troposphere (-4.2 Tg/month). Through photochemistry, the two largest NOx sources help control the vertical profile of ozone with lightning dominating in the upper troposphere, while the relative importance of biomass burning is virtually constant throughout the troposophere. A mass budget analysis of ozone over the tropospheric South Atlantic Ocean reveals that net mass transport of ozone into the domain is nearly balanced by net chemical destruction and deposition and that the mass transport into and out of the region are comparable to the chemical production and destruction terms. The three-dimensional circulation governing the ozone vertical structure is one of horizontal mass convergence and net chemical production supplying ozone to the upper troposphere which is fluxed downward by subsidence and removed in the boundary layer by net chemical destruction, deposition, and horizontal mass divergence.

18. Rasch, P J., J Feichter, K Law, N Mahowald, J Penner, C Benkovitz, C Genthon, C Giannakopoulos, P S Kasibhatla, D Koch, and Hiram Levy II, et al., 2000: A comparison of scavenging and deposition processes in global models: results from the WCRP Cambridge Workshop of 1995. Tellus B, 52B(4), 1025-1056.
    [ Abstract PDF ]

    We report on results from a World Climate Research Program workshop on representations of scavenging and deposition processes in global transport models of the atmosphere. 15 models were evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against each other, and against observations of these constituents. This paper provides a survey on the simulation differences between models. It identifies circumstances where models are consistent with observations or with each other, and where they differ from observations or with each other. The comparison shows that most models are able to simulate seasonal species concentrations near the surface over continental sites to within a factor of 2 over many regions of the globe. Models tend to agree more closely over source (continental) regions than for remote (polar and oceanic) regions. Model simulations differ most strongly in the upper troposphere for species undergoing wet scavenging processes. There are not a sufficient number of observations to characterize the climatology (long-term average) of species undergoing wet scavenging in the upper troposphere. This highlights the need for either a different strategy for model evaluation (e.g., comparisons on an event by event basis) or many more observations of a few carefully chosen constituents.

19. Yienger, J J., M Galanter, T Holloway, M J Phadnis, S K Guttikunda, G R Carmichael, Walter Moxim, and Hiram Levy II, 2000: The episodic nature of air pollution transport from Asia to North America. Journal of Geophysical Research, 105(D22), 26,931-26,945.
    [ Abstract PDF ]

    We employ the Geophysical Fluid Dynamics Laboratory (GFDL) global chemistry transport model (GCTM) to address the episodic nature of trans-Pacific pollution. The stronger Asian CO episodes over North America (NA), occurring most frequently between February and May, are often associated with disturbances that entrain pollution over eastern Asia and amplify over the western Pacific Ocean. Using 55 ppb of Asian CO as a criterion for major events, we find that during a typical year three to five Asian pollution events analogous to those observed by Jaffe et al. [1999] are expected in the boundary layer all along the U.S. West Coast between February and May. In contrast to CO, Asia currently has a small impact on the magnitude and variability of background ozone arriving over NA from the west. Direct and indirect Asian contributions to episodic O3 events over the western United States are generally in the 3 - 10 ppbv range. The two largest total O3 events (>60 ppbv), while having trajectories which pass over Asia, show negligible impact from Asian emissions. However, this may change. A future emission scenario in which Asian NOx emissions increase by a factor of 4 from those in 1990 produces late spring ozone episodes at the surface of California with Asian contributions reaching 40 ppb. Such episodic contributions are certain to exacerbate local NA pollution events, especially in elevated areas more frequently exposed to free tropospheric and more heavily Asian-influenced air.

20. Levy II, Hiram, Walter Moxim, A Klonecki, and P S Kasibhatla, 1999: Simulated tropospheric NO_x : Its evaluation, global distribution and individual source contributions. Journal of Geophysical Research, 104(D21), 26,279-26,306.
    [ Abstract PDF ]

    Using the 11-level Geophysical Fluid Dynamics Laboratory global chemical transport model, we simulate global tropospheric fields of NO_x, peroxyacetyl nitrate (PAN), HNO₃,and NO_y, as well as the deposition of nitrate, extensively evaluate them against available observations from surface stations and aircraft missions, and quantify the contributions of individual natural and anthropogenic sources. The patterns and magnitudes of simulated and observed HNO₃ wet deposition are generally in good agreement around the globe. Scatterplots of model simulations versus aircraft observations for NO_x and NO_y find ~50% of the points within ±25%, find ~75% within ±50%, and show no systematic global biases. Both simulated and observed vertical profiles have similar shapes with high levels (~1 ppbv or greater) in the polluted boundary layer (BL), very low values in the remote BL, and values increasing from the middle to the upper troposphere. Simulated NO_y , HNO₃, and NO_x + PAN are also in good agreement with extensive lower free tropospheric (FT) observations made at Mauna Loa Observatory. In general, the level of agreement between simulation and observation is as good as the agreement between separate, but simultaneous, observations of NO, NO_x or NO_y. As previous studies have shown, fossil fuel combustion and biomass burning control NO_x levels in most of the lower half of the troposphere with a significant contribution from biogenic emissions. The exceptions are the remote low-NO_x regions where BL and FT sources make comparable contributions. Unlike most previous studies, we find that the much smaller in situ FT sources generally dominate in the upper half of the troposphere. Lightning dominates in the tropics and summertime midlatitudes, and stratospheric injection is the major source in the summer high latitudes. The exception is transported emissions from fossil fuel combustion, which dominate in winter high latitudes. Though seldom dominant, aircraft emissions do have a significant impact on the upper troposphere and lower stratosphere of the northern hemisphere extratropics.

21. Wang, S W., Hiram Levy II, G Li, and H Rabitz, 1999: Fully equivalent operational models for atmospheric chemical kinetics within global chemistry-transport models. Journal of Geophysical Research, 104(D23), 30,417-30,426.
    [ Abstract PDF ]

    A major portion of the computational effort in simulations by three-dimensional (3-D) chemistry-transport models is consumed in chemical kinetics calculations which repeatedly solve coupled ordinary differential equations. To address this burden, this paper introduces a high-speed fully equivalent operational model (FEOM) for chemical kinetics calculations. The FEOM consists of a hierarchical correlated-function expansion capturing the input-output relationships of chemical kinetics. As an initial test of the FEOM approach for chemical kinetics calculations in 3-D models, this paper develops the FEOMs for CO-CH₄-NO_y-H₂O chemistry to obtain the time-dependent chemical ozone production and destruction rates in global chemistry-transport model (GCTM) simulations. The FEOMs are constructed for all GCTM model levels, all 12 months of the year, every 10° of latitude, for two types of surface albedo, and for all tropospheric values of H₂O, CO, NO_x, and O₃. It is shown that the simulated global ozone fields using the FEOMs in the GCTM ozone simulation are at least as accurate and in some cases better than those obtained by using four-dimensional interpolative look-up tables. Future work will expand the FEOM approach to more detailed chemical schemes, including nonmethane hydrocarbon chemistry in 3-D chemistry-transport model simulations.

22. Yienger, J J., A Klonecki, Hiram Levy II, Walter Moxim, and G R Carmichael, 1999: An evaluation of chemistry's role in the winter-spring ozone maximum found in the northern midlatitude free troposphere. Journal of Geophysical Research, 104(D3), 3655-3667.
    [ Abstract PDF ]

    We employ the Geophysical Fluid Dynamics Laboratory global chemical transport model to investigate the contribution of photochemistry to the winter-spring ozone maximum in the northern hemisphere (NH) midlatitude free troposphere (770-240 mbar; 30°N-60°N). Free tropospheric ozone mass slowly builds up in the winter and early spring, with net chemistry and transport playing comparable roles. Winter and early spring conditions are favorable to net ozone production for two reasons: (1) Winter conditions (cold, low Sun angle, and dry) reduce HO_x and lower the level of NO_x needed for chemical production to exceed destruction (balance point); and (2) throughout the winter and early spring, NO_x, because of its longer chemical lifetime, increases above normally net-destructive levels in the remote atmosphere. Interestingly, net production in the midlatitude NH free troposphere maximizes in early spring because relatively high NO_xand low balance point conditions are present at a time when increasing insolation is speeding up photochemistry. Conceptually, the net ozone production is associated with an annual atmospheric "spring cleaning" in which high levels of NO_x are removed via OH oxidation. Further, we find that human activity has a major impact on both the levels of tropospheric ozone and the role of chemistry in the NH midlatitude, where anthropogenic NO_x emissions dominate. In that region, modern ozone levels have increased by ~20% in the winter and ~45% in the spring, winter-spring chemistry has switched from net destructive to net productive, the winter-spring balance between transport and chemistry has switched from transport dominance in preindustrial times to the present parity, and the preindustrial February maximum has progressed to March-April. Estimated 2020 levels of NO_x emissions were found to lead to even greater net production and to push the O₃ spring maximum later into April-May. Figure 5 was reproduced clearly in Journal of Geophysical Research, 104(D7), 8329.

23. Oltmans, S J., A S Lefohn, H E Scheel, J M Harris, and Hiram Levy II, et al., 1998: Trends of ozone in the troposphere. Geophysical Research Letters, 25(2), 139-142.
    [ Abstract PDF ]

    Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid-1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

24. Klonecki, A, and Hiram Levy II, 1997: Tropospheric chemical ozone tendencies in CO-CH₄ -NO_y-H₂O system: Their sensitivity to variations in environmental parameters and their application to a global chemistry transport model study. Journal of Geophysical Research, 102(D17), 21,221-21,237.
    [ Abstract PDF ]

    A photochemical box model with CO-CH₄-NO_y-H₂O chemistry is used to calculate the diurnally averaged net photochemical rate of change of ozone (hereinafter called the chemical ozone tendency) in the troposphere for different values of parameters: NO_x and ozone concentration, temperature, humidity, CO concentration, and surface albedo. To understand the dependency of the chemical ozone tendency on the input parameters, a detailed sensitivity study is performed. Subsequently, the expected variations of the ozone tendencies with altitude, latitude, and season are analyzed. The magnitude of the tendency decreases rapidly with height mostly as a result of lower absolute humidity and temperature. In the upper troposphere (at 190 mbar) the maximum tendencies are below 2 parts per billion by volume/day. Lower temperature and specific humidity cause a shift of the value of NO_x at which the ozone production balances the destruction of ozone (balance point) to lower NO_x values; these two parameters are also, to a large extent, responsible for lower magnitudes of the tendency at higher latitudes and in winter. In the upper troposphere we find that the net tendency is at least as sensitive to variations in H₂O concentration as to NO_x. This suggests a possible synergism between direct NO_x pollution by aircraft and the indirect modification of H₂O by climate change. In the second part of the paper the box model calculated rates are used as ozone's chemical tendency terms during a simulation conducted with the three-dimensional global chemistry transport model (GCTM). The box model is used to calculate the tendencies as a function of NO_x and ozone at all tropospheric levels of the GCTM, at nine latitudes and for four seasons using zonally and monthly averaged data: water vapor and temperature from observations and model CO. These tables together with the NO_x fields obtained in an earlier GCTM simulation are used in the GCTM simulation of O₃ if nonmethane hydrocarbon levels are low. The global monthly averaged chemical ozone tendency fields saved during the simulation are presented and analyzed for the present-day and preindustrial conditions. The chemical tendency fields show a strong correlation with the NO_x fields. In contrast with the lower and middle troposphere where the tendencies are negative in remote regions over the oceans, in the upper troposphere, where NO_x is generally greater than 50 parts per trillion by volume and the balance point is low, the tendencies are generally small but positive. The GCTM simulations of the preindustrial ozone show that in the upper troposphere the present-day ozone tendencies are greater than the simulated preindustrial tendencies. In the boundary layer and in the midtroposphere the present-day tendencies are greater near anthropogenic NO_x sources and smaller (generally more negative), due to higher ozone levels, in regions not affected by these sources.

25. Levy II, Hiram, P S Kasibhatla, Walter Moxim, A Klonecki, A I Hirsch, S J Oltmans, and W L Chameides, 1997: The global impact of human activity on tropospheric ozone. Geophysical Research Letters, 24(7), 791-794.
    [ Abstract PDF ]

    Within a conceptual framework of stratospheric injection, CO-CH₄ background tropospheric chemistry, parameterized pollution production in the continental boundary layer and surface deposition, we use an 11 level GCTM to simulate global distributions of present and pre-industrial tropospheric O₃. The chemistry is driven by previously simulated present and pre-industrial NO_x fields, while prescribed fields of CO, CH₄ and H₂O are held constant. An evaluation with measurements from 12 surface sites, 21 ozonesonde sites and 1 aircraft campaign finds agreement within +/- 25% for 73% of the observations while identifying systematic errors in the wintertime high-latitude Northern Hemisphere (NH), the Southern Hemisphere (SH) tropics during biomass burning, and the remote SH. We predict that human activity has increased the annual integral of tropospheric ozone by 39% with 3/4's of that increase in the free troposphere, though the boundary layer [BL] annual integral has increased by 66%. The 2 largest components of the global O₃ budget are stratospheric injection at 696 TgO₃/yr, and loss through dry deposition, which increases from 459 TgO₃/yr to a present level of 825 TgO₃/yr. While tropospheric chemistry's net contribution is relatively small, changing from a pre-industrial destruction of - 236 TgO₃/yr to a present production of + 128 TgO₃/yr, it is a balance between two much larger terms, - 558 TgO₃/yr of destruction in the background troposphere and + 686 TgO₃/yr of production in the polluted boundary layer. Human impact on O₃ predominates in the summertime extratropical NH and in the tropics during their biomass burning seasons [increases of 50% - 100% or more]. Conversely, there has been little increase in most of the upper troposphere [<20%], where ozone's influence on tropospheric climate is strongest.

26. Levy II, Hiram, 1996: A global three-dimensional time-dependent lightning source of tropospheric NOx. Journal of Geophysical Research, 101(D17), 22,911-22,922.
    [ Abstract PDF ]

    The spatial and temporal distribution for a global three-dimensional, time-dependent lightning source of NOx is constructed from a general circulation model's (GCM) deep moist convection statistics [Manabe et al., 1974; Manabe and Holloway, 1975], observations of cloud-to-cloud and intracloud lightning fractions and the vertical distribution of lightning discharge [Proctor, 1991], and empirical/theoretical estimates of relative lightning frequency resulting from deep moist convection over ocean and over land [Price and Rind, 1992]. We then bracket the annual global emission of NOx from lightning between 2 and 6 Tg N/yr., with a most probable range of 3 to 5 Tg N/yr, by comparing tropospheric NOx simulations from the Geophysical Fluid Dynamics Laboratory Global Chemical Transport Model with measurements of NOx and/or NOy in the mid and upper troposphere where lightning is a major, if not the dominant source. With this approach, the global magnitude of the lightning source is constrained by observed levels of NOx, while the temporal and spatial distributions of the source are under the control of the parent GCM. Although our lightning source is smaller than many previous estimates, it is still the major source of NOx and NOy in the mid and upper troposphere for a latitude belt running from 30°N to 30°S, an important contributor to summertime free tropospheric levels over the midlatitudes, and a major contributor, even in the lower troposphere, to the low NOx and NOy levels over the remote oceans.

27. Moody, J L., and Hiram Levy II, et al., 1996: Meteorological mechanisms for transporting O3 over the western North Atlantic Ocean: A case study for August 24-29, 1993. Journal of Geophysical Research, 101(D22), 29,213-29,227.
    [ Abstract PDF ]

    A large-scale view of O₃ transport over the western North Atlantic Ocean (WNAO) in summer illustrates distinct sources of O₃, and separate transport mechanisms are important at different vertical levels in the troposphere. The week-long period presented covers a sequence of O₃ sondes released from Bermuda and encompasses two surface O₃ events in the month-long NARE intensive. O₃ and CO peaked at Chebogue Point on the evening of August 25 and after midnight on the morning of August 28. At Sable Island, peaks occurred during early morning of August 26 and late morning of August 28. These events occurred under W-SW winds associated with advancing low-pressure systems that transported anthropogenic pollutants over the WNAO. The concentrations dropped with the passage of a trough or a cold front. Evidence suggests the surface was occasionally isolated from polluted air during favorable transport with pollutants lifted in warm sector flow riding over a wedge of cool, thermodynamically stable air. In addition to surface O₃, the O₃-sonde profile over Bermuda on the morning of August 27 showed a deep layer of O₃ from 6 to 12 km. Using back trajectories and two tracers (isentropic potential vorticity and water vapor), we illustrate that stratospheric ozone exchanged into the upper troposphere in conjunction with surface cyclogenesis was advected through the middle to upper troposphere over the midlatitudes with the potential to reach lower altitudes through subsidence in regions of anticyclonic motion.

28. Moxim, Walter, Hiram Levy II, and P S Kasibhatla, 1996: Simulated global tropospheric PAN: Its transport and impact on NOx. Journal of Geophysical Research, 101(D7), 12,621-12,638.
    [ Abstract PDF ]

    Using the 11-level Geophysical Fluid Dynamics Laboratory (GFDL) global chemical transport model (GCTM) with all known sources of tropospheric NOx, we simulate the global tropospheric distribution of peroxyacetyl nitrate (PAN) and quantify its impact on tropospheric NOx. The model's global distribution of PAN is in reasonable agreement with most available observations. In the atmospheric boundary layer, PAN is concentrated over the continental sites of NOx emissions, primarily the midlatitudes in the northern hemisphere and the subtropics in the southern hemisphere. PAN is distributed relatively zonally throughout the free troposphere of the northern hemisphere, with the maximum levels found in the coldest regions, while in the southern hemisphere the maximum PAN levels are found in an equator to 30°S belt stretching from South America to Australia. Overall, the simulated three-dimensional fields of seasonal PAN are a result of the interaction of the type of transport meteorology (convective or synoptic scale storms) occurring in the PAN formation regions and PAN's temperature-dependent lifetime. We find the impact of PAN chemistry on NOx to be rather subtle. The magnitude and the seasonal cycle of the global tropospheric integral of NOx, which has its maximum in January and the formation of HNO3 as its dominant loss path, are barely affected by the inclusion of PAN chemistry, however PAN, as a result of its temperature sensitivity and transport, regionally provides an efficient mechanism for redistributing NOx far from its source areas. With the inclusion of PAN chemistry, monthly mean NOx concentrations increase by up to a factor of 5 in the remote lower troposphere and show a spring maximum over areas of the North Atlantic and North Pacific Oceans. In contrast, PAN has only a minor impact in the upper half of the troposphere (+ 10%). Examining local time series of NOx and PAN, the monthly mean mixing ratios in remote regions are shown to be composed of numerous short-term (1-2 days) large magnitude events. These episodes are large enough to potentially result in ozone production even when the monthly mean NOx values are in the ozone destruction range. While both the direct transport of NOx and its indirect transport as PAN contribute to the elevated NOx episodes over the remote extratropical oceans, events over the remote subtropical oceans are dominated by midtropospheric PAN that sinks anticyclonically equatorward and decomposes to NOx in the warmer air.

29. Oltmans, S J., and Hiram Levy II, et al., 1996: Summer and spring ozone profiles over the North Atlantic from ozonesonde measurements. Journal of Geophysical Research, 101(D22), 29,179-29,200.
    [ Abstract PDF ]

    Ozone profiles obtained by near-daily ozonesonde observations during campaigns at several sites in the North Atlantic are used to construct time-height cross sections of ozone concentration through the troposphere. Strong day-to-day ozone variability on the scale of synoptic meteorological disturbances is found both in the spring and in the summer throughout much of the troposphere. Layers of high ozone concentration (-100 ppb) are frequently seen in the middle and upper troposphere and are invariably associated with transport characteristics that strongly support a stratospheric source for these layers. Regions of low ozone (<40 ppb) are seen in the middle and upper troposphere associated with higher relative humidity. The connection of these events with low surface mixing ratios suggests that convective processes mix air low in ozone up through the troposphere. Vertical layering of ozone mixing ratio, which is seen at all of the observing locations, is a result of differing sources of air in the different layers.

30. Galloway, J, W H Schlesinger, Hiram Levy II, A Michaels, and J L Schnoor, 1995: Nitrogen fixation: anthropogenic enhancement-environmental response. Global Biogeochemical Cycles, 9(2), 235-252.
    [ Abstract PDF ]

    In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90-130 Tg N yr^-1 (Tg = 10¹²g) on the continents. Human activities have resulted in the fixation of an additional ~140 Tg N yr^-1 by energy production (~20 Tg N yr^-1), fertilizer production (~80 Tg N yr^-1), and cultivation of crops (e.g., legumes, rice) (~40 Tg N yr^-1). We can only account for part of this anthropogenic N. N₂O is accumulating in the atmosphere at a rate of 3 Tg N yr^-1. Coastal oceans receive another 41 Tg N yr^-1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr^-1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO₃, N₂). The remaining 80 Tg N yr^-1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N₂. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N-fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil-fuel combustion. About two-thirds of the increase will occur in Asia, which by 2020 will account for over half of the global anthropogenic N fixation.

31. Lawrence, M G., W L Chameides, P S Kasibhatla, Hiram Levy II, and Walter Moxim, 1995: Lightning and atmospheric chemistry: the rate of atmospheric NO production In Handbook of Atmospheric Electrodynamics, Vol. 1, Bonn, Germany, CRC Press, 189-202.
32. Levy II, Hiram, J J Yienger, Walter Moxim, P S Kasibhatla, and W L Chameides, 1995: The increase of pollutants (nitrogen oxides and ozone) in the summertime Midwest In Preparing for Global Change: A Midwestern Perspective, Amsterdam; The Netherlands, SPB Academic Publishing, 11-19.
    [ Abstract ]

    We use the Geophysical Fluid Dynamics Laboratory (GFDL) Global Chemical Transport Model, with the six known sources for tropospheric NO_x and off-line calculations of daytime gas-phase nitrogen photochemistry and night-time heterogeneous chemistry, to simulate the summertime concentrations of reactive nitrogen compounds for pre-industrial, current and future emission scenarios. The simulated levels are less than 0.5 ppbv throughout the Midwest during the pre-industrial period. For present conditions, the simulated NO_x levels range from [1.5 - 2 ppbv] in the west to [5-10 ppbv] in the east and are in reasonable agreement with summertime measurements in Bondville, Illinois. We predict that NO_x levels will increase another 30% by 2020. Using a simple relationship that relates NO_y and NO_x concentrations to the net chemical production of ozone at rural sites (Trainer et al. 1993), we estimate, conservatively, that the ozone, which was at relatively harmless levels in the pre-industrial period, is now at the crop-damage threshold of 50-70 ppbv in parts of Illinois, Indiana and Ohio. Furthermore, we estimate that this threshold will be reached throughout most of the Midwest east of the Mississippi River and even exceeded in parts of Illinois, Indiana, and Ohio by the year 2020, unless the continued increase in midwestern nitrogen fertilizer application and fossil fuel combustion ceases.

33. Moody, J L., S J Oltmans, Hiram Levy II, and J Merrill, 1995: Transport climatology of tropospheric ozone: Bermuda, 1988-1991. Journal of Geophysical Research, 100(D4), 7179-7194.
    [ Abstract PDF ]

    We determined the major transport patterns for Bermuda and quantified the degree to which they influenced variability in ozone concentrations by applying cluster analysis to isentropic trajectories from September 1988, through September 1991. Concentration distributions of ozone associated with these transport patterns were slightly different. The highest concentrations of ozone in each season were associated with transport off the North American continent; the lowest concentrations were during low-level maritime transport around the Bermuda high. Using the vertical component of the isentropic trajectories, we also showed that the most extreme concentrations of ozone occurred with rapidly descending air from midtropospheric levels. This pattern was most pronounced in April and May when more than 50% of the O3 variability was related to transport differences. We conclude that this relatively remote marine site, which normally experienced low maritime ozone levels (~30 parts per billion by volume (ppbv)), periodically entrained dry, ozone-rich (~55 ppbv) midtropospheric air in association with strong subsidence in high pressure behind spring low-pressure systems. Although the ultimate source of these midtroposphere, midlatitude, elevated-ozone concentrations is still being investigated, the synoptic meteorology associated with these transport patterns supports a significant contribution from the upper troposphere and lower stratosphere.

34. Prospero, J M., and Hiram Levy II, et al., 1995: Temporal variability of summer-time ozone and aerosols in the free troposphere over the eastern North Atlantic. Geophysical Research Letters, 22(21), 2925-2928.
    [ Abstract PDF ]

    In the free troposphere over Tenerife in the summer, O₃ concentrations are anti-correlated with major pollutant aerosols and with 210Pb, a tracer for boundary layer sources. In contrast O₃ is highly correlated with ⁷Be, a product of cosmic ray interactions in the upper troposphere and stratosphere. This suggests that natural O₃ sources (i.e., the stratosphere) might be playing an important role. Nonetheless our results do not preclude the possibility that substantial amounts of pollution-related O₃ could be tranpsorted in the free troposphere. However, to be consistent with our results, the transport mechanisms would have to incorporate efficient processes for the removal of pollutant aerosol species and ²¹⁰Pb.

35. Yienger, J J., and Hiram Levy II, 1995: Empirical model of global soil-biogenic NO_x emissions. Journal of Geophysical Research, 100(D6), 11,447-11,464.
    [ Abstract PDF ]

    We construct a global, temperature and precipitation dependent, empirical model of soil biogenic NOx emissions using 6-hour general circulation model forcing. New features of this source relative to the latest published ones by Dignon et al. [1992] and Muller [1992] include synoptic-scale modeling of "pulsing" (the emissions burst following the wetting of a dry soil), a biome dependent scheme to estimate canopy recapture of NOx, and an explicit linear dependence of emission on N fertilizer rate for agricultural soils. Our best estimate for annual above-canopy emissions is 5.5 Tg N (NOx) with a range of 3.3-7.7 Tg N. Globally, the strongest emitters are agriculture, grasslands, and tropical rain forests, accounting for 41%, 35%, and 16% of the annual budget, respectively. "Pulsing" contributes 1.3 Tg N annually. In temperate regions, agriculture dominates emission, and in tropical regions, grassland dominates. Canopy recapture is significant, consuming, on average, possibly 50% of soil emissions. In temperate regions, periodic temperature changes associated with synoptic-scale distu;rbances can cause emission fluctuations of up to 20 ng N m^-2 s^-1, indicating a close correlation between emission and warm weather events favorable to O3/smog formation. By the year 2025, increasing use of nitrogen fertilizer may raise total annual emissions to 6.9 Tg N with agriculture accounting for more than 50% of the global source. Finally, biomass burning may add up to an additional 0.6 Tg N globally by stimulating emissions for a short period after the burn.

36. Chameides, W L., P S Kasibhatla, J J Yienger, and Hiram Levy II, 1994: Growth of continental-scale metro-agro-plexes, regional ozone pollution, and world food production. Science, 264, 74-77.
    [ Abstract PDF ]

    Three regions of the northern mid-latitudes, the continental-scale metro-agro-plexes presently dominate global industrial and agricultural productivity. Although these regions cover only 23 percent of the Earth's continents, they account for most of the world's commercial energy consumption, fertilizer use, food-crop production, and food exports. They also account for more than half of the world's atmospheric nitrogen oxide (NO_x) emissions and, as a result, are prone to ground-level ozone (O₃) pollution during the summer months. On the basis of a global simulation of atmospheric reactive nitrogen compounds, it is estimated that about 10 to 35 percent of the world's grain production may occur in parts of these regions where ozone pollution may reduce crop yields. Exposure to yield-reducing ozone pollution may triple by 2025 if rising anthropogenic (NO_x) emissions are not abated.

37. Galloway, J, Hiram Levy II, and P S Kasibhatla, 1994: Year 2020: Consequences of population growth and development on deposition of oxidized nitrogen. Ambio, 23(2), 120-123.
    [ Abstract PDF ]

    With a current world population of 5.3 billion, fossil fuel and biomass burning have already greatly increased the emission of fixed nitrogen to the global atmosphere. In 2020, with a projected population of 8.5 billion and an assumed 100% increase in per capita energy consumption relative to 1980 by the lesser developed countries, we predict an approximate 25% increase in total nitrogen deposition in the more-developed-country source regions such as North America. In addition, reactive nitrogen deposition will at least double in less-developed regions, such as SE Asia and Latin America, and will increasae by more than 50% over the oceans of the Northern Hemisphere. Although we also predict significant increases in the deposition of nitrogen from fossil-fuel sources over most of the Southern Hemisphere, particularly Africa, the tropical eastern Pacific, and the southern Atlantic and Indian Oceans, biomass burning and the natiral sources of nitrogen oxides (lightning and biogenic soil emissions) are also important in these regions. This increased deposition has the potential to fertilize both terrstrial and marine ecosystems, resulting in the sequestering of carbon. Increases in nitrogen deposition have also been shown not only to acidify ecosystems but also to increase emissions of nitric oxide (NO), nitrous oxide (N₂O), carbonyl sulfide (COS), and carbon + sulfur (CS₂) to the atmosphere and decrease methane (CH₄) consumption in forest soils. We also find that the atmospheric levels of nitrogen oxides increase significantly throughout much of the Northern Hemisphere and populated regions of the Southern Hemisphere. This increase may lead to larger ozone concentrations with resulting increases in the oxidative capacity of the remote atmosphere and in its ability to absorb IR radiation.

38. Oltmans, S J., and Hiram Levy II, 1994: Surface ozone measurements from a global network. Atmospheric Environment, 28(1), 9-24.
    [ Abstract PDF ]

    From a network of sites, primarily in the Atlantic and Pacific Ocean regions, measurements of the surface ozone concentration yield information on the seasonal, synoptic, and diurnal patterns. These sites, generally removed from the effects of local pollution sources, show characteristics that typify broad geographical regions. At Barrow, AK; Mauna Loa, HI; American Samoa; and South Pole, data records of 15-20 years show trends that in all cases are a function of season. This dependence on season is important in understanding the causes of the long-term changes. At Barrow, the summer (July, August, September) increase of 1.7% per year is probably indicative of photochemical production. At South Pole, on the other hand, the summer (December, January, February) decrease is related to photochemical losses and enhanced transport from the coast of Antarctica. At all the sites there is a pronounced seasonal variation. In the Southern Hemisphere (SH), all locations which run from 14 to 90°S show a winter (July-August) maximum and summer minimum. In the Northern Hemisphere (NH) most of the sites show a spring maximum and autumn minimum. At Barrow (70°North) and Barbados (14°), however, the maxima occur during the winter, but for very different reasons. At many of the sites, the transport changes associated with synoptic scale weather patterns dominate the day-to-day variability. This appears to result from photochemical destruction during the day in regions with very low concentrations of nitrogen oxides. At Niwot Ridge, CO, and Mace Head, Ireland, there is clear evidence of photochemical ozone production in the summer during transport from known regional pollution sources.

39. Kasibhatla, P S., Hiram Levy II, and Walter Moxim, 1993: Global NO_x, HNO₃, PAN, and NO_y distributions from fossil fuel combustion emissions: A model study. Journal of Geophysical Research, 98(D4), 7165-7180.
    [ Abstract ]

    The 11-level Geophysical Fluid Dynamics Laboratory global chemical transport model (GCTM) which explicitly treats NO_x, HNO₃, and PAN as transported species has been used to assess the impact of fossil fuel combustion emissions on the distribution of reactive nitrogen compounds (NO_y) in various regions of the troposphere. The GCTM is driven by 6-hour time-averaged wind and total precipitation fields derived from a parent general circulation model. PAN production rates are calculated using background, two-dimensional ethane and propane fields, which are then adjusted to parameterize the effect of short-lived hydrocarbons over continental regions. From an analysis of our model results, we conclude that (1) the model reproduces the observed spatial patterns of wet deposition near the major fossil fuel combustion source regions. Wet and dry deposition in source regions account for 30% and 40-45% of the emissions, respectively, with the remainder being exported over the adjacent ocean basins; (2) the fossil fuel source accounts for a large fraction of the observed surface concentrations and wet deposition fluxes of HNO₃ in the extra tropical North Atlantic; (3) while it appears that a significant fraction of NO_y observed in the marine free troposphere during the NASA Global Tropical Experiment/CITE 2 experiment in the eastern North Pacific cannot be explained in terms of fossil fuel source, this may simply indicate that in this region subgrid-scale transport from adjacent continental source regions is not being adequately resolved by the model; (4) at the more remote Mauna Loa, Hawaii site, less that 30% of the observed NO_y during May 1988, appears to be due to distant fossil fuel sources; (5) even with the explicit treatment of PAN as a transported species, the fossil fuel source has only a minor impact on NO_y levels in the remote tropics and in the southern hemisphere; (6) model calculations indicate that the relatively high levels of NO_y observed over western Alaska during the ABLE 3A experiment in July-August 1988, cannot be explained in terms of long-range transport of fossil fuel combustion emissions from the northern hemisphere mid-latituded surface source regions; and (7) away from source regions, PAN is a major component of fossil fuel NO_y, and is the dominant component poleward of 45 degress N. However, the relative impact of this sequestered PAN on regional spring time NOx levels has yet to be established.

40. Levy II, Hiram, Walter Moxim, and P S Kasibhatla, 1993: Impact of global NOx sources on the northern latitudes In The Tropospheric Chemistry of Ozone in the Polar Regions, NATO ASI Series I, Vol. 7, Springer-Verlag, 77-88.
    [ PDF ]
41. Oltmans, S J., and Hiram Levy II, 1992: Seasonal cycle of surface ozone over the western North Atlantic. Nature, 358, 392-394.
    [ Abstract PDF ]

    The possible impact of pollution from North America and Europe on tropospheric ozone throughout the Northern Hemisphere is a major environmental concern. We report here continuous measurements of ozone from Bermuda (32°N, 65°W) and Barbados (13°N, 60°W), which suggest that despite their proximity to the eastern US seaboard, natural processes rather than pollution control surface ozone in these regions. Although springtime daily average ozone concentrations ar Bermuda are greater than 70 parts per billion (109) by volume (p.p.b.v.) and hourly values in 1989 sometimes exceeded the Canadian Air Quality limit of 80 p.p.b.v., trajectory analyses indicate that these high levels of ozone are transported from the unpolluted upper troposphere >5 km above the northern United States and Canada. During the summer, when surface ozone concentrations over the eastern United States can exceed 70 p.p.b.v. owing to pollution, typical values at Bermuda are between 15 and 25 p.p.b.v. At Barbados, both the seasonal and diurnal variations in surface ozone are nearly identical to those at Samoa in the tropical South Pacific, where the isolation from anthropogenic sources and low levels of NOx ensure that natural processes control surface ozone.

42. Savoie, D L., J M Prospero, S J Oltmans, W C Graustein, K K Turekian, J Merrill, and Hiram Levy II, 1992: Sources of nitrate and ozone in the marine boundary layer of the tropical North Atlantic. Journal of Geophysical Research, 97(D11), 11,575-11,589.
    [ Abstract PDF ]

    During the period April 1989 through December 1990, O₃ concentrations in the marine boundary layer at Barbados, West Indies, show a pronounced seasonal cycle. Daily averaged values in the winter and spring often fall in the range of 25-35 ppbv for periods of several days, and they seldom fall below 20 ppbv. In contrast, during the summer, values typically fall in the range of 10-20 ppbv. During the winter-spring period, there is a very strong negative correlation between O₃ and a number of aerosol species, including NO₃-. These anticorrelations appear to be driven by changing transport patterns over the North Atlantic as opposed to chemical reactions involving O₃ and nitrogen species in the atmosphere. Analyses of isentropic trajectories clearly show that high O₃ and low NO₃- are associated with transport from higher latitudes and high altitudes. Conversely, high NO₃- and relatively low O₃ are associated with transport from Africa. Our study suggests that North America and the middle troposphere (and stratosphere) are not strong sources for NO₃- over the tropical North Atlantic. The strong correlation of NO₃- with 210Pb and the weaker correlation with Saharan dust indicates that NO₃- is derived principally from continental surface sources, probably in Europe and North Africa, but not from the Saharan soil material itself. During several extended periods, NO₃- and ²¹⁰Pb were strongly correlated and their concentrations were high relative to nss SO₄ =; these factors, coupled with trajectories originating in Africa, suggest that African biomass burning was a significant source at these times. In contrast, biomass burning appears to be a minor source for O₃ as measured at Barbados, perhaps accounting for an enhancement of about 5 ppbv at most during these periods.

43. Kasibhatla, P S., Hiram Levy II, Walter Moxim, and W L Chameides, 1991: The relative impact of stratospheric photochemical production of tropospheric NO_y levels: A model study. Journal of Geophysical Research, 96(D10), 18,631-18,646.
    [ Abstract PDF ]

    The 11-level Geophysical Fluid Dynamics Laboraotory (GFDL) global chemical transport model has been used to assess the impact of stratospheric NO_x production on tropospheric reactive nitrogen (NO_y) concentrations. A temporally varying source function was constructed using specified two-dimensional, monthly average O₃, N₂O, temperature, and surface pressure data generated by the GFDL "SKYHI" model. The calculated yearly NO_y production rate is 0.64 Tg N (0.64 x 10¹² g N). A wet removal scheme, which distinguishes between stable and convective rain based on the bulk Richardson number, is introduced. Simulations have been performed with a simplified chemical mechanism which fractionates NO_y into soluble and insoluble species. The role of peroxyacetyl nitrate (PAN) in determining the impact of stratospheric injection on the tropospheric NO_y budget is studied by comparing results of simulations with and without PAN chemistry. We conclude that (1) the stratospheric source is too small to account for background surface NO_y concentrations observed in the remote (i.e., regions a few thousand kilometers from continental source regions) troposphere. Surface NO_y mixing ratios seldom exceed 10 parts per trillion by volume (pptv) in the model northern hemisphere and are always below 20 pptv. Together, fossil fuel combustion emissions and stratospheric injection account for less than 10% of observed surface nitrate concentrations in the remote tropical Pacific. (2) The impact of the stratospheric source is comparable to that of the fossil fuel combustion source in terms of NO_y mixing ratios in the northern hemisphere at the 500 mbar model level and is more important in the middle and high latitudes of the southern hemisphere. At the 315 mbar model level the stratospheric source contribution to NO_y levels is more important than that of the fossil fuel source at all latitiudes, except in the tropics. However, substantial contributions from other NO_y sources are needed to explain observations in the remote middle and upper troposphere. (3) Inclusion of PAN chemistry has the effect of increasing model-calculated surface NO_y mixing ratios in the northern hemisphere middle and high latitudes by factors of 1.5-3 during winter/spring and by smaller increase due to slower rates of PAN formation. This is a direct result of lower hydrocarbon concentrations in the southern hemisphere.

44. Levy II, Hiram, 1990: Regional and global transport and distribution of trace species released at the earth's surface In Long Range Transport of Pesticides, Lewis Publishers, 83-95.
    [ PDF ]
45. Levy II, Hiram, 1989: Simulated global deposition of reactive nitrogen emitted by fossil fuel combustion In Atmospheric Deposition, IAHS Publication No. 179, 3-9.
    [ Abstract ]

    We use the medium resolution (265 km horizontal grid) Geophysical Fluid Dynamics Laboratory (GFDL) general circulation transport model to simulate the global deposition of reactive nitrogen emitted by fossil fuel combustion. The nirtogen species are transported as a single tracer, the global parameter for wet deposition is based on the observed wet deposition of nitrogen over North America, and constant bulk coefficients for dry depostion over land and sea are pre-calculated from measured concentrations and deposition velocities. The simulated yearly wet depositions in Europe, as well as nearby and distant export sites, are in reasonable agreement with observations. The agreement is generally quite good and almost always within a factor of 2. No more than 1.4 Tg of the 21.3 Tg of nitrogen emitted by fossil fuel combustion are deposited in the Southern Hemisphere, yet this source accounts for less than 10% of the apparent background deposition. The 4 Tg of nitrogen exported by the three major source regions (US/Canada, Europe, and Asia) accounts for most of the deposition over the remote Northern Hemisphere. The simulated deposition over the North Pacific, which is in good agreement with estimates based on recent observations, is dominated by emissions from Asia, while US/Canadian emissions dominate deposition over the North Atlantic.

46. Levy II, Hiram, and Walter Moxim, 1989: Examining the global impact of local/regional air pollution: The role of global chemical transport models In Air Pollution Modeling and its Application VII, Plenum Press, 139-157.
    [ PDF ]
47. Levy II, Hiram, and Walter Moxim, 1989: Influence of long-range transport of combustion emissions on the chemical variability of the background atmosphere. Nature, 338(6213), 326-328.
    [ Abstract PDF ]

    Maunal Loa Observatory, located 3,400 m above sea level on the island of Hawaii in the middle of the Pacific Ocean, is a critical site for determining the background chemical reactivity of the unpolluted atmosphere and for monitoring its rate of change on a global scale. However, recent measurements of soluble nitrogen (principally HNO3) at the observatory find mixing ratios rising from their expected background values of 0.02-0.03 parts per 10⁹ by volume (p.p.b.v.) in the winter to 0.07-0.12 p.p.b.v. in late summer with three-hour events as high as 0.25 p.p.b.v. This raises the specific question of contamination by the long-range transport of pollution and a broader question of the chemical variability of the background atmosphere. Here we show that a general circulation transport model which simulated the global spread and depostion of emissions from fossil-fuel combustion can reproduce the nitrogen measurements at the Mauna Loa Observatory. By isolating individual source regions, we show that US emissions are responsible for the late summer increase and that Asian emissions cause a smaller increase in the spring. These simulations, together with the earlier observations, indicate frequent contamination of the Mauna Loa Observatory by the long-range tranport of reactive trace gases, such as HNO₃, and suggest a highly variable background atmosphere. It is essential that we are aware of such variability in order to discern anthropogenic effects on the atmosphere.

48. Levy II, Hiram, and Walter Moxim, 1989: Simulated global distribution and deposition of reactive nitrogen emitted by fossil fuel combustion. Tellus B, 41B, 256-271.
    [ Abstract PDF ]

    We use the medium resolution (~ 265 km horizontal grid) GFDL general circulation transport model to simulate the global spread and deposition of reactive nitrogen emitted by fossil fuel combustion. The nitrogen species are transported as a single tracer with no explicit chemistry. Chemical reactions are only present implicitly in the bulk coefficients for dry and wet removal. The observed wet deposition of nitrogen over North America is used to determine the global parameter for wet deposition, and constant bulk coefficients for dry deposition over land and sea are pre-calculated from measured concentrations and deposition velocities. The simulated yearly depositions in Western Europe and at regional export sites, as well as simulated yearly concentrations and their seasonal variation over the North Pacific, are compared with available observations. The agreement is generally quite good and almost always within a factor of 2. This model is then used to identify a number of important source regions and long-range transport mechanisms: (1) Asian emissions supply two-thirds of the soluble nitrogen compounds over the North Pacific. In the summer, North American emissions are important over the subtropical North Pacific. (2) Nitrogen emissions from Europe dominate the nitrogen component of Arctic haze in the lower troposphere, while North American and Asian emissions are only important locally. The model predicts a large gradient in the Arctic with average winter mixing ratios ranging from less than 0.1 ppbv over Alaska to more than 1 ppbv over eastern Russia. (3) Throughout the Southern Hemisphere, the emissions from fossil fuel combustion account for 10% or less of the observed soluble nitrogen at remote sites, an amount less than a previously simulated contribution from stratospheric injection. The long-range transport of PAN, NO_x production by lightning and biomass burning, and some, as yet, unknown marine biogenic source may all supply part of this background soluble nitrogen. However, the similarity between the seasonal cycles observed at Samoa for soluble nitrogen and for O₃, a species known to be supplied from the stratosphere, suggests a major role for either stratospheric injection or an upper tropospheric source.

49. Levy II, Hiram, 1988: Global transport of ozone In Tropospheric Ozone, D. Reidel Publishing Company, 319-325.
    [ Abstract PDF ]

    The three principal mechanisms for large scale atmospheric transport of tropospheric ozone [injection from the stratosphere, transport from regions of net production in the boundary layer and distribution of O₃ precursors resulting from either stratospheric injection or surface emissions] are examined in the light of current observations. While the actual O₃ climatology may be much more complex than it currently appears, the limited data suggests that ozone in the Southern Hemisphere and the northern tropics and subtropics is strongly influenced by transport from the stratosphere. At this time, the major questions are in the southern tropics and the northern mid-latitudes. The high levels of ozone observed over South America appear to be either the result of local chemical production or transport from higher latitudes. Both the latitude gradient and the seasonal cycle in the northern mid-latitudes suggest a significant, if not dominant, role for the transport of both O₃ and its precursors from source regions in the boundary layer, though transport from the upper troposphere also plays a role.

50. Levy II, Hiram, 1987: Tracers of atmospheric transport. Nature, 325, 761-762.
    [ PDF ]
51. Levy II, Hiram, and Walter Moxim, 1987: Fate of US and Canadian combustion nitrogen emissions. Nature, 328(6129), 414-416.
    [ Abstract PDF ]

    While approximately 7.5 x 10¹² grams (7.5 Tg) of combustion nitrogen are emitted yearly in the United States and Canada, only 1.5-2.3 Tg are deposited as acid rain over that same region. Past nitrogen budget estimates as well as recent observations suggest that dry deposition and export play major roles. Here we report how the yearly accumulated deposition and transport of combustion nitrogen is simulated for the first time by a global transport model with realistic meteorology. The model predicts that dry deposition over the United States and Canada accounts for at least 2.1 and most probably 3.5 Tg of the emissions. The remainder is exported, principally over the North Atlantic. But at most 0.2 Tg, less than 3% of the estimated European emissions, is predicted to reach Europe from North America. Furthermore, the model predicts that, while less than 40% of the nitrogen deposited as acid rain in the northeastern United States and eastern Canada comes from emissions in that region, almost all of the dry-deposited nitrogen and over half of the total acid nitrogen deposition comes from there.

52. Mahlman, Jerry D., Hiram Levy II, and Walter Moxim, 1986: Three-dimensional simulations of stratospheric N2O: predictions for other trace constituents. Journal of Geophysical Research, 91(D2), 2687-2707.
    [ Abstract PDF ]

    The Geophysical Fluid Dynamics Laboratory (GFDL) three-dimensional general circulation/tracer model has been used to investigate the stratospheric behavior of N₂O under a range of photodestruction hypotheses. A comparison of observations with these simulations shows that the atmospheric N₂O lifetime lies between 100 and 130 years. For the three experiments conducted, it was found that the model-derived global one-dimensional eddy diffusion coefficients Kz for one experiment are appropriate for the other two experiments as well. In addition, the meridional slopes of N₂O mixing ratio isolines are virtually identical in the lower stratosphere for all three experiments. The generality of these two results was explored with a simple "two-slab" model. In this model the equilibrium meridional slopes of trace gas isolines and Kz values are solved directly. The model predicts that long-lived gases with weak photodestruction rates should have similar meridional slopes, but the effect of faster destruction is to flatten the meridional slopes. The simple model also predicts that Kz depends upon chemical processes through a direct dependence upon the meridional slope for a given gas as well as upon the intensity with which upward propagating tropospheric disturbances force the stratospheric zonal winds. The three N₂O experiments have been compared against detailed observational analyses. These analyses show that the model meridional N₂O slopes are too flat by about 30%. The simple two-slab model indicates that this results from a somewhat weak forcing of the model stratospheric zonal winds. A comparision of the temporal variability of model N₂O against the "Delta" statistics of Ehhalt et al. (1983) shows good agreement. Another simple theoretical model is proposed that shows why Delta statistics are so useful and predicts the circumstances under which different destruction chemistries should lead to different Delta statistics. These results have allowed a very general extrapolation of the three N₂O numerical experiments to predicted structure for a wide class of long-lived trace gases. Specifically, the supporting theoretical developments allow predictions for the effect of chemistry on the global (one-dimensional) behavior, meridional-height (two-dimensional) structure and local temporal variability. Finally, some examples of transient behavior are presented through model time series at points corresponding to available measurements. These time series, with the support of horizontal N₂O charts, show complex behavior, including pronounced seasonal cycles, transport-produced N₂O "inversions", and detailed meridional transport events associated with transient stratospheric disturbances.

53. Levy II, Hiram, Jerry D Mahlman, Walter Moxim, and S C Liu, 1985: Tropospheric ozone: The role of transport. Journal of Geophysical Research, 90(D2), 3753-3772.
    [ Abstract PDF ]

    The Geophysical Fluid Dynamics Laboratory general circulation/transport model, with photochemistry in the top level (middle stratosphere) only, is used to simulate global tropospheric ozone distributions for upper and lower limits of surface removal rates. We compared these simulations with available observations and find that large-scale atmospheric transport plays a major role in the behavior of tropospheric ozone. Furthermore, we identify potential roles for tropospheric chemistry, discover defects in the model's mean cross-tropopause flux is in the range of previous estimates, and the shapes of the simulated vertical profiles of mixing ratio and percent standard deviation are in good agreement with observations. South of 40 degrees N, the simulated and observed latitude gradients are the same, the upper and lower limit calculations bracket measured values, and the seasonal cycles are well reproduced. While the transport model simulates a wide tange of tropospheric ozone climatology, there are a significant number of disagreements. The need for additional ozone destruction in the maritime boundary layer suggests a role for chemical destruction, while in the continental boundary layer, it appears that chemical production, a seasonal cycle in surface deposition, and improved boundary layer transport are all required. The two major defects in the simulated "free troposphere" are (1) excess ozone at high latitudes of the northern hemisphere (NH) and (2) spring rather than summer maxima and fall rather than winter minima at NH mid- and high latitudes. While defect 1 has a number of possible causes, deficiencies in model transport play a major role. Although similar transport defects have not been ruled out for defect 2, tropospheric chemistry appears to be needed. Separate calculations of the net chemical production and loss demonstrate that this is a complex problem. The most likely solution involves the transport control of NO_x which controls the ozone chemistry.

54. Levy II, Hiram, Jerry D Mahlman, and Walter Moxim, 1982: Tropospheric N₂O variability. Journal of Geophysical Research, 87(C4), 3061-3080.
    [ Abstract PDF ]

    Tropospheric N₂O climatologies are simulated with the GFDL general circulation/tracer model for three idealized source specifications: (1) a constant surface flux of 1.44 x 10⁹ molecules cm^-2 s^-1 distributed uniformly over the earth's surface, with a global source strength of 17 tg N₂O yr^-1 and an atmospheric lifetime of 131 yr; (2) a 13 month integration of the stable N₂O field from experiment 1 with its N₂O source removed; (3) a constant surface flux of 2.24 x 10¹⁰ molecules cm^-1 s^-1 over only those land areas with precipitation in excess of an arbitrary limit of 127 cm yr-1, but with the same global strength (17 tg N²O yr-1) and atmospheric lifetime (131 yr) as in experiment 1. In the boundary layer, the model produces an interhemispheric gradient with a minimum in the northern hemisphere (NH). This is due to the greater downward transport in the NH which results in more dilution of NH boundary layer N₂O mixing ratios by the N₂O-poor air from the lower stratosphere. The boundary layer distribution of N₂O is also influenced by the distribution of the surface source. The lack of an N₂O maximum in the model's NH boundary layer suggests that, unlike the model's idealized source, the true source has a large excess in the Northern Hemisphere. Above the boundary layer, the N-S gradient is controlled by the large-scale vertical transport that produces a NH minimum in N₂O mixing ratio. The impact of the surface source distribution is small. Current measurements at 500 mb have too low a precision to confirm or disprove the model prediction of an interhemispheric gradient with a NH minimum in the middle troposphere. The sources of temporal variability in the model's N₂O fields are transient motions of all scales acting on mixing ratio gradients, both vertcal and horizontal. The model finds that a small surface source of 17 tg N₂O yr^-1, sufficient to balance stratospheric destruction, is more than able to maintain the observed variability in the boundary layer.

55. Liu, S C., D Kley, M McFarland, Jerry D Mahlman, and Hiram Levy II, 1981: Reply. Journal of Geophysical Research, 86(C12), 12,165-12,166.
56. Levy II, Hiram, Jerry D Mahlman, and Walter Moxim, 1980: A stratospheric source of reactive nitrogen in the unpolluted troposphere. Geophysical Research Letters, 7(6), 441-444.
    [ Abstract ]

    A GFDL 3-D global generalized tracer field is adapted to provide a preliminary simulation of the reactive nitrogen (NOY) climatology in an unpolluted troposphere which has, as its sole source, downward transport of stratospheric NOY. The tracer field is scaled so that its downward cross-tropopause flux is balanced by the stratospheric production of NOY. While the model results show stratospheric NOY to be a significant source for the remote troposphere, they do not rule out additional contributions from either the long-range transport of combustion NOY or the insitu production by lightning. The model NOY climatology in the unpolluted troposphere shows a strong interhemispheric asymmetry due to greater downward NOY flux in the northern hemisphere and a steep drop off to a minimum in the tropics resulting from a combination of model features (tropical rainbelt, ITCZ, and Indidan monsoon) which have been well documented in the real atmosphere.

57. Liu, S C., D Kley, M McFarland, Jerry D Mahlman, and Hiram Levy II, 1980: On the origin of tropospheric ozone. Journal of Geophysical Research, 85(C12), 7546-7552.
    [ Abstract PDF ]

    The effects of NO_x (NO + NO₂) intrusion from the stratosphere on photochemical ozone production in the upper troposphere are investigated. Using the currently accepted reaction rate coefficients, we find that this upper tropospheric ozone source may be significantly larger than the direct injection of ozone from the stratosphere. Many features of the observed tropospheric temporal and spatial ozone distributions appear to be better explained by this upper tropospheric ozone source hypothesis than by either the classical 'dynamical control' or 'photochemical control' hypotheses. In addition, we find that NO_x emissions from high flying subsonic aircraft in the northern hemisphere may cause an ozone increase in the troposphere. The calculated tropospheric ozone increase due to these NO_x emissions is not inconsistent with the increases observed by the northern hemispheric ozonesonde stations.

58. Mahlman, Jerry D., Hiram Levy II, and Walter Moxim, 1980: Three-dimensional tracer structure and behavior as simulated in two ozone precursor experiments. Journal of the Atmospheric Sciences, 37(3), 655-685.
    [ Abstract PDF ]

    The GFDL, II-level, general circulation-tracer model is used for two experiments designed to prepare the way for a self-consistent model of atmospheric ozone. The first experiment invokes a simple condition at the top model level: an instantaneous relaxation to a specified 10-mb average observed ozone value. The tracer is inert below the top level until it is removed in the lower troposphere. The second experiment introduces a simplified, but reasonably realistic, ozone chemistry at the top level, including Chapman, nitrogen, and hydrogen loss processes. Below the top level, ozone is inert and is removed in the lower troposphere by the same mechanism as in the first experiment. These two experiments, in spite of their different middle stratospheres, show remarkably similar behavior in the lower stratosphere. A comparison of model values and fluxes with available observations shows general qualitative agreement as well as some notable discrepancies. In the second experiment, a detailed analysis of the processes affecting the 10-mb, zonal-mean mixing ratio is presented. The results show that the midstratospheric ozone production and losses are strongly sensitive to circulation features, changing overhead sun angle, and temperature. These various effects lead to some substantial interhemispheric and seasonal asymmetries in ozone production. An analysis of the transport processes is performed, leading to the pronounced poleward-downward slope of tracer isopleths. The results demonstrate that adiabatic and diabatic effects in the eddies, as well as diabatic effects in the zonal mean, all contribute importantly to the creation of these sloping surfaces. As an aid to tracer transport analysis, a Lagrangian nontransport theorem is derived for an integration following a fluid particle. Some Lagrangian drift-type calculations are performed in the model January mean flow. The results show a slow, but substantial, particle convergence just to the cyclonic shear side of the time-mean jet stream axis. This is a region in which the traditional zonal-mean budget analysis shows a large cancellation between eddy and meridional circulation flux convergence. Also, the analysis demonstrates indirectly the important contributions of transient disturbances to the movement of heat and tracers irreversibly into the stratospheric polar vortex.

59. Levy II, Hiram, Jerry D Mahlman, and Walter Moxim, 1979: Preliminary report on the numerical simulation of the three-dimensional structure and variability of atmospheric N₂O. Geophysical Research Letters, 6(3), 155-158.
    [ Abstract PDF ]

    A numerical simulation of atmospheric N₂O using the GFDL 3-D tracer model with a small uniform surface source (15 Mton yr^-1) and stratospheric destruction (150-yr lifetime) was run to a state near transport and chemical statistical equilibrium. The resulting N₂O tropospheric distribution is relatively uniform with a slight excess in the Southern Hemisphere. In the model stratosphere, there is a sharp poleward decrease in N₂O mixing ratio away from high values in the Tropics, with pronounced winter minima at 50 degrees S and 60 degrees N. Even with the small uniform surface source, relative standard deviations of N₂O in the surface layer range from 0.1 to 0.8%, are well within the range of recent measurements. Additional experiments suggest that motions acting upon N₂O accumulation in the source region boundary layer and upon the mixing ratio gradient between the troposphere and lower stratosphere are the major sources of tropospheric N₂O variability.

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