Bibliography - Walter Moxim

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. 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.

3. 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.

4. 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%.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.
15. 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.

16. 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.

17. 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 ]
18. 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.

19. Moxim, Walter, 1990: Simulated transport of NOy to Hawaii during August: A synoptic study. Journal of Geophysical Research, 95(D5), 5717-5729.
    [ Abstract ]

    Using data sets generated by the Geophysical Fluid Dynamics Laboratory general circulation/transport model's U.S.-Canada combustion nitrogen source experiment, a detailed analysis of the simulated transport mechanims producing the observed August NOy maximum at Hawaii is presented. Combustion nitrogen is not simply advected from the United States to Hawaii by the winds circulating around the climatological subtropical anticyclone. Rather, its transport results from a complicated three-way interaction of surface advection from source regions, enhanced vertical diffusion due to dry convection, and winds in the "free troposphere." Backward trajectories from Hawaii using model pressure and isentropic surfaces were insufficient in explaining the transport. Model-consistent three-dimensional trajectories revealed that the transport originated in the "free troposphere" along a path from northern Baja to the Texas Gulf coast. Combustion nitrogen from the source regions of southern California and as distant as the Texas Gulf area is advected along the surface toward the arid areas of Baja, the desert southwest, northern Mexico, and west Texas. Dry convection then vertically mixes the air to pressures of 800-650 mbar, where the subsiding wind flow from the east-northeast transports the NOy to Hawaii. Observed wind fields and heights of dry convection are compared to the model where data are available.

20. 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.
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21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

29. Moxim, Walter, and Jerry D Mahlman, 1980: Evaluation of various total ozone sampling networks using the GFDL 3-D tracer model. Journal of Geophysical Research, 85(C8), 4527-4539.
    [ Abstract ]

    Data sets generated by the Geophysical Fluid Dynamics Laboratory (GFDL) 3-D general circulation--tracer model for an ozone experiment are used to compare the accuracy of various total ozone networks in calculating global and hemispheric means of total ozone and annual trends of monthly mean total ozone. The advantage of this approach is that exact model integrals and trends are known, thus providing an ability to examine the errors expected in present and hypothesized sampling networks. The effects of both spatial and temporal sampling errors are presented. Because the 3-D tracer model uses the same time-dependent wind fields from year to year, the influence of interannual meteorological variability and the sampling error resulting from long-term ozone trends cannot be evaluated. By using varying numbers of observations per month, total ozone networks of 9, 53, and 181 stations are compared. In addition, a case of 53, plus 15 new, judiciously placed stations is examined. Model network evaluations of global mean ozone show underestimnates of 1-3% occurring because of a compensation of Northern and Southern Hemispheric errors as large as -6 and +3%, respectively. The error of global mean ozone from random sampling networks for various months is examined, showing rapid improvement from 9 to 1% for an increase in the number of random stations from 5 to 100. Markedly slower improvement is seen with further increases in the number of stations. One-year trend analyses of total ozone are compared for various networks and individual stations. Sampling errors of nearly 1%/yr. are seen for the 53 station case, when using four perfect, equally spaced observations per month. The errors grow substantially larger with fewer observations. The effect on global and hemispheric means from stations that did not take measurements during cloudy periods is also investigated. Results indicate that the weak annual mean cloud bias error (0.285%) is overwhelmed by the larger error produced by the decrease in effective network density.

30. 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.

31. Mahlman, Jerry D., and Walter Moxim, 1978: Tracer simulation using a global general circulation model: results from a midlatitude instantaneous source experiment. Journal of the Atmospheric Sciences, 35(8), 1340-1374.
    [ Abstract PDF ]

    An 11-level general circulation model with seasonal variation is used to perform an experiment on the dispersion of passive tracers. Specially constructed time-dependent winds from this model are used as input to a separate tracer model. The methodologies employed to construct the tracer model are described. The experiment presented is the evolution of a hypothetical instantaneous source of tracer on 1 January with maximum initial concentration at 65 mb, 36 degrees N, 180 degrees E. The tracer is assumed to have no sources or sinks in the stratosphere, but is subject to removal processes in the lower troposphere. The experimental results reveal a number of similarities to observed tracer behavior, including the average poleward-downward slope of mixing ratio isopleths, strong tracer gradients across the tropopause, intrusion of tracer into the Southern Hemisphere lower stratosphere, and the long-term interhemispheric exchange rate. The model residence times show behavior intermediate to those exhibited for particulate radioactive debris and gaseous C¹⁴O₂. This suggests that caution should be employed when either radioactive debris or C¹⁴O₂ data are used to develop empirical models for prediction of gaseous tracers which are efficiently removed in the troposphere. In this experiment, the tracer mixing ratio and potential vorticity evolve to very high correlations. Mechanisms for this correlation are discussed. The zonal mean tracer balances exhibit complex behavior among the various transport terms. At early stages, the tracer evolution is dominated by eddy effects. Later, a very large degree of self-cancellation between mean cell and eddy effects is observed. During seasonal transitions, however, this self-cancellation diminishes markedly, leading to significant changes in the zonal mean tracer distribution. A possible theoretical explanation is presented. For this tracer dispersion problem, probably the most significant model shortcoming is the inability of the general circulation model to produce the midwinter stratospheric sudden warming phenomenon.

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