Bibliography - Arlene M Fiore

1. Crevoisier, C, D Nobileau, Arlene M Fiore, R Armante, R A Chédin, and N A Scott, in press: A new insight on tropospheric methane in the Tropics - first year from IASI hyperspectral infrared observations. Atmospheric Chemistry and Physics Discussions. 3/09.
   [ Abstract ]

   Simultaneous observations from the Infrared Atmospheric Sounding Interferometer (IASI) and from the Advanced Microwave Sounding Unit (AMSU), launched together onboard the European MetOp platform in October 2006, are used to retrieve a mid-to-upper tropospheric content of methane (CH₄) in clear-sky conditions, in the Tropics, over sea, for the first 16 months of operation of MetOp (July 2007–October 2008). With its very high spectral resolution, IASI provides nine channels in the 7.7 μm band highly sensitive to CH₄ with reduced sensitivities to other atmospheric variables. These channels, sensitive to both CH₄ and temperature, are used in conjunction with AMSU channels, only sensitive to temperature, to decorrelate both signals through a non-linear inference scheme based on neural networks. A key point of this approach is that no use is made of prior information in terms of methane seasonality, trend, or geographical patterns. The accuracy of the retrieval is estimated to be about 16 ppbv (~0.9%). Features of the retrieved methane space-time distribution include: (1) a strong seasonal cycle of 30 ppbv in the Northern Tropics with a maximum in January–March and a minimum in July–September, and a flat seasonal cycle in the Southern Tropics, in agreement with in-situ measurements; (2) a latitudinal decrease of 30 ppbv from 20° N to 20° S, in boreal spring and summer, lower than what is observed at the surface but in excellent agreement with tropospheric aircraft measurements; (3) geographical patterns in good agreement with simulations from the atmospheric transport and chemistry model MOZART-2, but with a higher variability and a higher concentration in boreal winter; (4) signatures of CH₄ emissions transported to the middle troposphere such as a large plume of elevated tropospheric methane south of the Asian continent, which might be due to Asian emissions from rice paddies uplifted by deep convection during the monsoon period and then transported towards Indonesia. In addition to bringing a greatly improved view of methane distribution, these results from IASI should provide a means to observe and understand atmospheric transport pathways of methane from the surface to the upper troposphere.

2. Fiore, Arlene M., and Larry Horowitz, et al., February 2009: Multimodel estimates of intercontinental source-receptor relationships for ozone pollution. Journal of Geophysical Research, 114, D04301, doi:10.1029/2008JD010816.
   [ Abstract ]

   Understanding the surface O₃ response over a “receptor” region to emission changes over a foreign “source” region is key to evaluating the potential gains from an international approach to abate ozone (O₃) pollution. We apply an ensemble of 21 global and hemispheric chemical transport models to estimate the spatial average surface O₃ response over east Asia (EA), Europe (EU), North America (NA), and south Asia (SA) to 20% decreases in anthropogenic emissions of the O₃ precursors, NO_x, NMVOC, and CO (individually and combined), from each of these regions. We find that the ensemble mean surface O₃ concentrations in the base case (year 2001) simulation matches available observations throughout the year over EU but overestimates them by >10 ppb during summer and early fall over the eastern United States and Japan. The sum of the O₃ responses to NO_x, CO, and NMVOC decreases separately is approximately equal to that from a simultaneous reduction of all precursors. We define a continental-scale “import sensitivity” as the ratio of the O₃ response to the 20% reductions in foreign versus “domestic” (i.e., over the source region itself) emissions. For example, the combined reduction of emissions from the three foreign regions produces an ensemble spatial mean decrease of 0.6 ppb over EU (0.4 ppb from NA), less than the 0.8 ppb from the reduction of EU emissions, leading to an import sensitivity ratio of 0.7. The ensemble mean surface O₃ response to foreign emissions is largest in spring and late fall (0.7–0.9 ppb decrease in all regions from the combined precursor reductions in the three foreign regions), with import sensitivities ranging from 0.5 to 1.1 (responses to domestic emission reductions are 0.8–1.6 ppb). High O₃ values are much more sensitive to domestic emissions than to foreign emissions, as indicated by lower import sensitivities of 0.2 to 0.3 during July in EA, EU, and NA when O₃ levels are typically highest and by the weaker relative response of annual incidences of daily maximum 8-h average O₃ above 60 ppb to emission reductions in a foreign region (<10–20% of that to domestic) as compared to the annual mean response (up to 50% of that to domestic). Applying the ensemble annual mean results to changes in anthropogenic emissions from 1996 to 2002, we estimate a Northern Hemispheric increase in background surface O₃ of about 0.1 ppb a⁻¹, at the low end of the 0.1–0.5 ppb a⁻¹ derived from observations. From an additional simulation in which global atmospheric methane was reduced, we infer that 20% reductions in anthropogenic methane emissions from a foreign source region would yield an O₃ response in a receptor region that roughly equals that produced by combined 20% reductions of anthropogenic NO_x, NMVOC, and CO emissions from the foreign source region.

3. West, J J., V Naik, Larry Horowitz, and Arlene M Fiore, in press: Effect of regional precursor emission control on long-range ozone transport - Part 1: Short-term changes in ozone air quality. Atmospheric Chemistry and Physics Discussions. 3/09.
   [ Abstract ]

   Observations and models demonstrate that ozone and its precursors can be transported between continents and across oceans. We model the influences of 10% reductions in anthropogenic nitrogen oxide (NO_x) emissions from each of nine world regions on surface ozone air quality in that region and all other regions. In doing so, we quantify the relative importance of long-range transport between all source-receptor pairs, for direct short-term ozone changes. We find that for population-weighted concentrations during the three-month "ozone-season", the strongest inter-regional influences are from Europe to the Former Soviet Union, East Asia to Southeast Asia, and Europe to Africa. The largest influences per unit of NO_x reduced, however, are seen for source regions in the tropics and Southern Hemisphere, which we attribute mainly to greater sensitivity to changes in NO_x in the lower troposphere, and secondarily to increased vertical convection to the free troposphere in tropical regions, allowing pollutants to be transported further. Results show, for example, that NO_x reductions in North America are ~20% as effective per unit NO_x in reducing ozone in Europe during summer, as NO_x reductions from Europe itself. Reducing anthropogenic emissions of non-methane volatile organic compounds (NMVOCs) and carbon monoxide (CO) by 10% in selected regions, can have as large an impact on long-range ozone transport as NO_x reductions, depending on the source region. We find that for many source-receptor pairs, the season of greatest long-range influence does not coincide with the season when ozone is highest in the receptor region. Reducing NO_x emissions in most source regions causes a larger decrease in export of ozone from the source region than in ozone production outside of the source region.

4. West, J J., V Naik, Larry Horowitz, and Arlene M Fiore, in press: Effect of regional precursor emission controls on long-range ozone transport – Part 2: steady-state changes in ozone air quality and impacts on human mortality. Atmospheric Chemistry and Physics Discussions. 3/09.
   [ Abstract ]

   Large-scale changes in ozone precursor emissions affect ozone directly in the short term, and also affect methane, which in turn causes long-term changes in ozone that affect surface ozone air quality. Here we assess the effects of changes in ozone pre- cursor emissions on the long-term change in surface ozone via methane, as a function of the emission region, by modeling 10% reductions in anthropogenic nitrogen oxide (NO x ) emissions from each of nine world regions. Reductions in NO x emissions from all world regions increase methane and long-term surface ozone. While this long term increase is small compared to the intra-regional short-term ozone decrease, it is comparable to or larger than the short-term inter-continental ozone decrease for some source-receptor pairs. The increase in methane and long-term surface ozone per ton of NO x reduced is greatest in tropical and Southern Hemisphere regions, exceeding that from temperate Northern Hemisphere regions by roughly a factor of ten. We also assess changes in premature ozone-related human mortality associated with regional precursor reductions and long-range transport, showing that for 10% regional NOx reductions, the strongest inter-regional influence is for emissions from Europe affecting mortalities in Africa. Reductions of NOx in North America, Europe, the Former Soviet Union, and Australia are shown to reduce more mortalities outside of the source regions than within. Among world regions, NOx reductions in India cause the greatest number of avoided mortalities per ton, mainly in India itself. Finally, by increasing global methane, NOx reductions in one hemisphere tend to cause long-term increases in ozone concentration and mortalities in the opposite hemisphere. Reducing emissions of methane, and to a lesser extent carbon monoxide and non-methane volatile organic compounds, alongside NOx reductions would avoid this disbenefit.

5. Wu, S, B N Duncan, D J Jacob, Arlene M Fiore, and O Wild, March 2009: Chemical nonlinearities in relating intercontinental ozone pollution to anthropogenic emissions. Geophysical Research Letters, 36, L05806, doi:10.1029/2008GL036607.
   [ Abstract ]

   Model studies typically estimate intercontinental influence on surface ozone by perturbing emissions from a source continent and diagnosing the ozone response in the receptor continent. Since the response to perturbations is non-linear due to chemistry, conclusions drawn from different studies may depend on the magnitude of the applied perturbation. We investigate this issue for intercontinental transport between North America, Europe, and Asia with sensitivity simulations in three global chemical transport models. In each region, we decrease anthropogenic emissions of NO_x and nonmethane volatile organic compounds (NMVOCs) by 20% and 100%. We find strong nonlinearity in the response to NO_x perturbations outside summer, reflecting transitions in the chemical regime for ozone production. In contrast, we find no significant nonlinearity to NO_x perturbations in summer or to NMVOC perturbations year-round. The relative benefit of decreasing NO_x vs. NMVOC from current levels to abate intercontinental pollution increases with the magnitude of emission reductions.

6. Duncan, B N., J J West, Y Yoshida, Arlene M Fiore, and J R Ziemke, 2008: The influence of European pollution on ozone in the Near East and northern Africa. Atmospheric Chemistry and Physics, 8(8), 2267-2283.
   [ Abstract PDF ]

   We present a modeling study of the long-range transport of pollution from Europe, showing that European emissions regularly elevate surface ozone by as much as 20 ppbv in summer in northern Africa and the Near East. European emissions cause 50–150 additional violations per year (i.e. above those that would occur without European pollution) of the European health standard for ozone (8-h average >120 μg/m3 or ~60 ppbv) in northern Africa and the Near East. We estimate that European ozone pollution is responsible for 50 000 premature mortalities globally each year, of which the majority occurs outside of Europe itself, including 37% (19 000) in northern Africa and the Near East. Much of the pollution from Europe is exported southward at low altitudes in summer to the Mediterranean Sea, northern Africa and the Near East, regions with favorable photochemical environments for ozone production. Our results suggest that assessments of the human health benefits of reducing ozone precursor emissions in Europe should include effects outside of Europe, and that comprehensive planning to improve air quality in northern Africa and the Near East likely needs to address European emissions.

7. Ellingsen, K, M Gauss, R Van Dingenen, F Dentener, L Emberson, and Arlene M Fiore, et al., in press: Global ozone and air quality: a multi-model assessment of risks to human health and crops. Atmospheric Chemistry and Physics Discussions. 2/08.
   [ Abstract ]

   Within ACCENT, a European Network of Excellence, eighteen atmospheric models from the U.S., Europe, and Japan calculated present (2000) and future (2030) concentrations of ozone at the Earth's surface with hourly temporal resolution. Comparison of model results with surface ozone measurements in 14 world regions indicates that levels and seasonality of surface ozone in North America and Europe are characterized well by global models, with annual average biases typically within 5–10 nmol/mol. However, comparison with rather sparse observations over some regions suggest that most models overestimate annual ozone by 15–20 nmol/mol in some locations. Two scenarios from the International Institute for Applied Systems Analysis (IIASA) and one from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES) have been implemented in the models. This study focuses on changes in near-surface ozone and their effects on human health and vegetation. Different indices and air quality standards are used to characterise air quality. We show that often the calculated changes in the different indices are closely inter-related. Indices using lower thresholds are more consistent between the models, and are recommended for global model analysis. Our analysis indicates that currently about two-thirds of the regions considered do not meet health air quality standards, whereas only 2–4 regions remain below the threshold. Calculated air quality exceedances show moderate deterioration by 2030 if current emissions legislation is followed and slight improvements if current emissions reduction technology is used optimally. For the "business as usual" scenario severe air quality problems are predicted. We show that model simulations of air quality indices are particularly sensitive to how well ozone is represented, and improved accuracy is needed for future projections. Additional measurements are needed to allow a more quantitative assessment of the risks to human health and vegetation from changing levels of surface ozone.

8. 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.

9. Fiore, Arlene M., J J West, Larry Horowitz, V Naik, and M Daniel Schwarzkopf, April 2008: Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality. Journal of Geophysical Research, 113, D08307, doi:10.1029/2007JD009162.
   [ Abstract ]

   Reducing methane (CH₄) emissions is an attractive option for jointly addressing climate and ozone (O₃) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O₃ responds approximately linearly to changes in CH₄ emissions over a range of anthropogenic emissions from 0–430 Tg CH₄a⁻¹ (0.11–0.16 Tg tropospheric O₃ or ∼11–15 ppt global mean surface O₃ decrease per Tg a⁻¹ CH₄ reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH₄ emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost-effective CH₄ controls would offset the positive climate forcing from CH₄ and O₃ that would otherwise occur (from increases in NO_x and CH₄ emissions in the baseline scenario) and improve O₃ air quality. We estimate that anthropogenic CH₄ contributes 0.7 Wm⁻² to climate forcing and ∼4 ppb to surface O₃ in 2030 under the baseline scenario. Although the response of surface O₃ to CH₄ is relatively uniform spatially compared to that from other O₃ precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O₃ formation regime is NO_x-saturated. In the model, CH₄ oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O₃ than CH₄ oxidation in the free troposphere. In NO_x-saturated regions, the surface O₃ sensitivity to CH₄ can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH₄. Accurately representing the NO_x distribution is thus crucial for quantifying the O₃ sensitivity to CH₄.

10. Quinn, P K., T S Bates, E Baum, N Doubleday, and Arlene M Fiore, et al., 2008: Short-lived pollutants in the Arctic: their climate impact and possible mitigation strategies. Atmospheric Chemistry and Physics, 8(6), 1723-1735.
    [ Abstract PDF ]

    Several short-lived pollutants known to impact Arctic climate may be contributing to the accelerated rates of warming observed in this region relative to the global annually averaged temperature increase. Here, we present a summary of the short-lived pollutants that impact Arctic climate including methane, tropospheric ozone, and tropospheric aerosols. For each pollutant, we provide a description of the major sources and the mechanism of forcing. We also provide the first seasonally averaged forcing and corresponding temperature response estimates focused specifically on the Arctic. The calculations indicate that the forcings due to black carbon, methane, and tropospheric ozone lead to a positive surface temperature response indicating the need to reduce emissions of these species within and outside the Arctic. Additional aerosol species may also lead to surface warming if the aerosol is coincident with thin, low lying clouds. We suggest strategies for reducing the warming based on current knowledge and discuss directions for future research to address the large remaining uncertainties.

11. Sanderson, M, F Dentener, Arlene M Fiore, C Cuvelier, T J Keating, A Zuber, C Atherton, D J Bergmann, T Diehl, R Doherty, B N Duncan, P G Hess, and Larry Horowitz, et al., 2008: A multi-model study of the hemispheric transport and deposition of oxidised nitrogen. Geophysical Research Letters, 35, L17815, doi:10.1029/2008GL035389.
    [ Abstract ]

    Fifteen chemistry-transport models are used to quantify, for the first time, the export of oxidised nitrogen (NO_y) to and from four regions (Europe, North America, South Asia, and East Asia), and to estimate the uncertainty in the results. Between 12 and 24% of the NO_x emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NO_y, and East Asia the least. East Asia receives the most NO_y from the other regions. Between 8 and 15% of NO_x emitted in each region is transported over distances larger than 1000 km, with 3–10% ultimately deposited over the foreign regions.

12. Shindell, D, H Teich, M Chin, F Dentener, R Doherty, G Faluvegi, and Arlene M Fiore, et al., September 2008: A multi-model assessment of pollution transport to the Arctic. Atmospheric Chemistry and Physics, 8, 5353-5372.
    [ Abstract PDF ]

    We examine the response of Arctic gas and aerosol concentrations to perturbations in pollutant emissions from Europe, East and South Asia, and North America using results from a coordinated model intercomparison. These sensitivities to regional emissions (mixing ratio change per unit emission) vary widely across models and species. Intermodel differences are systematic, however, so that the relative importance of different regions is robust. North America contributes the most to Arctic ozone pollution. For aerosols and CO, European emissions dominate at the Arctic surface but Asian emissions become progressively more important with altitude, and are dominant in the upper troposphere. Sensitivities show strong seasonality: surface sensitivities typically maximize during boreal winter for European and during spring for East Asian and North American emissions. Mid-tropospheric sensitivities, however, nearly always maximize during spring or summer for all regions. Deposition of black carbon (BC) onto Greenland is most sensitive to North American emissions. North America and Europe each contribute ~40% of total BC deposition to Greenland, with ~20% from East Asia. Elsewhere in the Arctic, both sensitivity and total BC deposition are dominated by European emissions. Model diversity for aerosols is especially large, resulting primarily from differences in aerosol physics and removal. Comparison of aerosols with observations indicates problems in either the models or interpretation of the measurements. For gas phase pollutants such as CO and O₃, which are relatively well-simulated, the processes contributing most to uncertainties depend on the source region. Uncertainties in the Arctic surface CO response to emissions perturbations are dominated by emissions for East Asian sources, while uncertainties in transport, emissions, and oxidation are comparable for European and North American sources. At higher levels, model-to-model variations in transport and oxidation are most important. Differences in photochemistry appear to play the largest role in the intermodel variations in Arctic ozone sensitivity.

13. Donner, Leo J., Larry Horowitz, Arlene M Fiore, Charles J Seman, D Blake, and N J Blake, 2007: Transport of radon-222 and methyl iodide by deep convection in the GFDL Global Atmospheric Model AM2. Journal of Geophysical Research, 112, D17303, doi:10.1029/2006JD007548.
    [ Abstract ]

    Transport of radon-222 and methyl iodide by deep convection is analyzed in the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model 2 (AM2) using two parameterizations for deep convection. One of these parameterizations represents deep convection as an ensemble of entraining plumes; the other represents deep convection as an ensemble of entraining plumes with associated mesoscale updrafts and downdrafts. Although precipitation patterns are generally similar in AM2 with both parameterizations, the deep convective mass fluxes are more than three times larger in the middle- to upper troposphere for the parameterization consisting only of entraining plumes, but do not extend across the tropopause, unlike the parameterization including mesoscale circulations. The differences in mass fluxes result mainly from a different partitioning between convective and stratiform precipitation; the parameterization including mesoscale circulations detrains considerably more water vapor in the middle troposphere and is associated with more stratiform rain. The distributions of both radon-222 and methyl iodide reflect the different mass fluxes. Relative to observations (limited by infrequent spatial and temporal sampling), AM2 tends to simulate lower concentrations of radon-222 and methyl iodide in the planetary boundary layer, producing a negative model bias through much of the troposphere, with both cumulus parameterizations. The shapes of the observed profiles suggest that the larger deep convective mass fluxes and associated transport in the parameterization lacking a mesoscale component are less realistic.

14. Horowitz, Larry, Arlene M Fiore, G P Milly, R C Cohen, A Perring, P J Wooldridge, P G Hess, L K Emmons, and J F Lamarque, 2007: Observational constraints on the chemistry of isoprene nitrates over the eastern United States. Journal of Geophysical Research, 112, D12S08, doi:10.1029/2006JD007747.
    [ Abstract ]

    The formation of organic nitrates during the oxidation of the biogenic hydrocarbon isoprene can strongly affect boundary layer concentrations of ozone and nitrogen oxides (NO_x = NO + NO₂). We constrain uncertainties in the chemistry of these isoprene nitrates using chemical transport model simulations in conjunction with observations over the eastern United States from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign during summer 2004. The model best captures the observed boundary layer concentrations of organic nitrates and their correlation with ozone using a 4% yield of isoprene nitrate production from the reaction of isoprene hydroxyperoxy radicals with NO, a recycling of 40% NO_x when isoprene nitrates react with OH and ozone, and a fast dry deposition rate of isoprene nitrates. Simulated boundary layer concentrations are only weakly sensitive to the rate of photochemical loss of the isoprene nitrates. An 8% yield of isoprene nitrates degrades agreement with the observations somewhat, but concentrations are still within 50% of observations and thus cannot be ruled out by this study. Our results indicate that complete recycling of NO_x from the reactions of isoprene nitrates and slow rates of isoprene nitrate deposition are incompatible with the observations. We find that ~50% of the isoprene nitrate production in the model occurs via reactions of isoprene (or its oxidation products) with the NO₃ radical, but note that the isoprene nitrate yield from this pathway is highly uncertain. Using recent estimates of rapid reaction rates with ozone, 20–24% of isoprene nitrates are lost via this pathway, implying that ozonolysis is an important loss process for isoprene nitrates. Isoprene nitrates are shown to have a major impact on the nitrogen oxide (NO_x = NO + NO₂) budget in the summertime U.S. continental boundary layer, consuming 15–19% of the emitted NO_x , of which 4–6% is recycled back to NO_x and the remainder is exported as isoprene nitrates (2–3%) or deposited (8–10%). Our constraints on reaction rates, branching ratios, and deposition rates need to be confirmed through further laboratory and field measurements. The model systematically underestimates free tropospheric concentrations of organic nitrates, indicating a need for future investigation of the processes controlling the observed distribution.

15. West, J J., Arlene M Fiore, V Naik, Larry Horowitz, M Daniel Schwarzkopf, and D L Mauzerall, 2007: Ozone air quality and radiative forcing consequences of changes in ozone precursor emissions. Geophysical Research Letters, 37, L06806, doi:10.1029/2006GL029173.
    [ Abstract ]

    Changes in emissions of ozone (O₃) precursors affect both air quality and climate. We first examine the sensitivity of surface O₃ concentrations (O₃ ^srf) and net radiative forcing of climate (RF_net) to reductions in emissions of four precursors - nitrogen oxides (NO _x ), non-methane volatile organic compounds, carbon monoxide, and methane (CH₄). We show that long-term CH₄-induced changes in O₃, known to be important for climate, are also relevant for air quality; for example, NO _x reductions increase CH₄, causing a long-term O₃ increase that partially counteracts the direct O₃ decrease. Second, we assess the radiative forcing resulting from actions to improve O₃ air quality by calculating the ratio of ΔRF_net to changes in metrics of O₃ ^srf. Decreases in CH₄ emissions cause the greatest RF_net  decrease per unit reduction in O₃ ^srf, while NO _x reductions increase RF_net. Of the available means to improve O₃ air quality, therefore, CH₄ abatement best reduces climate forcing.

16. Dentener, F, J Drevet, J F Lamarque, I Bey, B Eickhout, Arlene M Fiore, D Hauglustaine, Larry Horowitz, M Krol, U C Kulshrestha, M G Lawrence, C Galy-Lacaux, S Rast, D Shindell, D Stevenson, T Van Noije, C Atherton, N Bell, D Bergman, T Butler, J Cofala, B Collins, R Doherty, K Ellingsen, J Galloway, M Gauss, V Montanaro, J F Müller, G Pitari, J Rodriguez, M Sanderson, F Solmon, S Strahan, M G Schultz, K Sudo, S Szopa, and O Wild, 2006: Nitrogen and sulfur deposition on regional and global scales: A multimodel evaluation. Global Biogeochemical Cycles, 20, GB4003, doi:10.1029/2005GB002672.
    [ Abstract ]

    We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60–70% of the model-calculated wet deposition rates agree to within ±50% with quality-controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 σ variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36–51% of all NOy, NHx, and SOx is deposited over the ocean, and 50–80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the “critical load” threshold of 1000 mg(N) m−2 yr−1. The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia (40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m−2 yr−1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.

17. Dentener, F, D Stevenson, K Ellingsen, T Van Noije, M G Schultz, Larry Horowitz, and Arlene M Fiore, et al., 2006: The Global Atmospheric Environment for the Next Generation. Environmental Science & Technology, 40(11), doi:10.1021/es0523845.
    [ Abstract ]

    Air quality, ecosystem exposure to nitrogen deposition, and climate change are intimately coupled problems: we assess changes in the global atmospheric environment between 2000 and 2030 using 26 state-of-the-art global atmospheric chemistry models and three different emissions scenarios. The first (CLE) scenario reflects implementation of current air quality legislation around the world, while the second (MFR) represents a more optimistic case in which all currently feasible technologies are applied to achieve maximum emission reductions. We contrast these scenarios with the more pessimistic IPCC SRES A2 scenario. Ensemble simulations for the year 2000 are consistent among models and show a reasonable agreement with surface ozone, wet deposition, and NO2 satellite observations. Large parts of the world are currently exposed to high ozone concentrations and high deposition of nitrogen to ecosystems. By 2030, global surface ozone is calculated to increase globally by 1.5 ± 1.2 ppb (CLE) and 4.3 ± 2.2 ppb (A2), using the ensemble mean model results and associated ±1 standard deviations. Only the progressive MFR scenario will reduce ozone, by -2.3 ± 1.1 ppb. Climate change is expected to modify surface ozone by -0.8 ± 0.6 ppb, with larger decreases over sea than over land. Radiative forcing by ozone increases by 63 ± 15 and 155 ± 37 mW m-2 for CLE and A2, respectively, and decreases by -45 ± 15 mW m-2 for MFR. We compute that at present 10.1% of the global natural terrestrial ecosystems are exposed to nitrogen deposition above a critical load of 1 g N m-2 yr-1. These percentages increase by 2030 to 15.8% (CLE), 10.5% (MFR), and 25% (A2). This study shows the importance of enforcing current worldwide air quality legislation and the major benefits of going further. Nonattainment of these air quality policy objectives, such as expressed by the SRES-A2 scenario, would further degrade the global atmospheric environment. --------------------------------------------------------------------------------

18. Fiore, Arlene M., Larry Horowitz, E J Dlugokencky, and J J West, 2006: Impact of meteorology and emissions on methane trends, 1990-2004. Geophysical Research Letters, 33, L12809, doi:10.1029/2006GL026199.
    [ Abstract ]

    Over the past century, atmospheric methane (CH4) rose dramatically before leveling off in the late 1990s. The processes controlling this trend are poorly understood, limiting confidence in projections of future CH4. The MOZART-2 global tropospheric chemistry model qualitatively captures the observed CH4 trend (increasing in the early 1990s and then leveling off) with constant emissions. From 1991–1995 to 2000–2004, the CH4 lifetime versus tropospheric OH decreases by 1.6%, reflecting increases in OH and temperature. The rise in OH stems from an increase in lightning NOx as parameterized in the model. A simulation including annually varying anthropogenic and wetland CH4 emissions, as well as the changes in meteorology, best reproduces the observed CH4 distribution, trend, and seasonal cycles. Projections of future CH4 abundances should consider climate-driven changes in CH4 sources and sinks.

19. Shindell, D, G Faluvegi, D Stevenson, M Krol, L K Emmons, J F Lamarque, G Pétron, F Dentener, M G Schultz, K Ellingsen, O Wild, Arlene M Fiore, and Larry Horowitz, et al., 2006: Multimodel simulations of carbon monoxide: Comparison with observations and projected near-future changes. Journal of Geophysical Research, 111, D19306, doi:10.1029/2006JD007100.
    [ Abstract ]

    We analyze present-day and future carbon monoxide (CO) simulations in 26 state-of-the-art atmospheric chemistry models run to study future air quality and climate change. In comparison with near-global satellite observations from the MOPITT instrument and local surface measurements, the models show large underestimates of Northern Hemisphere (NH) extratropical CO, while typically performing reasonably well elsewhere. The results suggest that year-round emissions, probably from fossil fuel burning in east Asia and seasonal biomass burning emissions in south-central Africa, are greatly underestimated in current inventories such as IIASA and EDGAR3.2. Variability among models is large, likely resulting primarily from intermodel differences in representations and emissions of nonmethane volatile organic compounds (NMVOCs) and in hydrologic cycles, which affect OH and soluble hydrocarbon intermediates. Global mean projections of the 2030 CO response to emissions changes are quite robust. Global mean midtropospheric (500 hPa) CO increases by 12.6 ± 3.5 ppbv (16%) for the high-emissions (A2) scenario, by 1.7 ± 1.8 ppbv (2%) for the midrange (CLE) scenario, and decreases by 8.1 ± 2.3 ppbv (11%) for the low-emissions (MFR) scenario. Projected 2030 climate changes decrease global 500 hPa CO by 1.4 ± 1.4 ppbv. Local changes can be much larger. In response to climate change, substantial effects are seen in the tropics, but intermodel variability is quite large. The regional CO responses to emissions changes are robust across models, however. These range from decreases of 10–20 ppbv over much of the industrialized NH for the CLE scenario to CO increases worldwide and year-round under A2, with the largest changes over central Africa (20–30 ppbv), southern Brazil (20–35 ppbv) and south and east Asia (30–70 ppbv). The trajectory of future emissions thus has the potential to profoundly affect air quality over most of the world's populated areas.

20. Stevenson, D, F Dentener, M Schulz, K Ellingsen, T Van Noije, O Wild, Arlene M Fiore, and Larry Horowitz, et al., 2006: Multimodel ensemble simulations of present-day and near-future tropospheric ozone. Journal of Geophysical Research, 111, D08301, doi:10.1029/2005JD006338.
    [ Abstract ]

    Global tropospheric ozone distributions, budgets, and radiative forcings from an ensemble of 26 state-of-the-art atmospheric chemistry models have been intercompared and synthesized as part of a wider study into both the air quality and climate roles of ozone. Results from three 2030 emissions scenarios, broadly representing “optimistic,” “likely,” and “pessimistic” options, are compared to a base year 2000 simulation. This base case realistically represents the current global distribution of tropospheric ozone. A further set of simulations considers the influence of climate change over the same time period by forcing the central emissions scenario with a surface warming of around 0.7K. The use of a large multimodel ensemble allows us to identify key areas of uncertainty and improves the robustness of the results. Ensemble mean changes in tropospheric ozone burden between 2000 and 2030 for the 3 scenarios range from a 5% decrease, through a 6% increase, to a 15% increase. The intermodel uncertainty (±1 standard deviation) associated with these values is about ±25%. Model outliers have no significant influence on the ensemble mean results. Combining ozone and methane changes, the three scenarios produce radiative forcings of −50, 180, and 300 mW m−2, compared to a CO2 forcing over the same time period of 800–1100 mW m−2. These values indicate the importance of air pollution emissions in short- to medium-term climate forcing and the potential for stringent/lax control measures to improve/worsen future climate forcing. The model sensitivity of ozone to imposed climate change varies between models but modulates zonal mean mixing ratios by ±5 ppbv via a variety of feedback mechanisms, in particular those involving water vapor and stratosphere-troposphere exchange. This level of climate change also reduces the methane lifetime by around 4%. The ensemble mean year 2000 tropospheric ozone budget indicates chemical production, chemical destruction, dry deposition and stratospheric input fluxes of 5100, 4650, 1000, and 550 Tg(O3) yr−1, respectively. These values are significantly different to the mean budget documented by the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). The mean ozone burden (340 Tg(O3)) is 10% larger than the IPCC TAR estimate, while the mean ozone lifetime (22 days) is 10% shorter. Results from individual models show a correlation between ozone burden and lifetime, and each model's ozone burden and lifetime respond in similar ways across the emissions scenarios. The response to climate change is much less consistent. Models show more variability in the tropics compared to midlatitudes. Some of the most uncertain areas of the models include treatments of deep tropical convection, including lightning NO x production; isoprene emissions from vegetation and isoprene's degradation chemistry; stratosphere-troposphere exchange; biomass burning; and water vapor concentrations.

21. Van Noije, T, H J Eskes, F Dentener, D Stevenson, K Ellingsen, M G Schultz, O Wild, M Amann, C Atherton, D J Bergmann, I Bey, K F Boersma, T Butler, J Cofala, J Drevet, Arlene M Fiore, M Gauss, D Hauglustaine, Larry Horowitz, I Isaksen, M Krol, J F Lamarque, M G Lawrence, R V Martin, V Montanaro, J F Müller, G Pitari, M J Prather, J A Pyle, A Richter, J Rodriguez, N H Savage, S Strahan, K Sudo, S Szopa, and M van Roozendael, 2006: Multi-model ensemble simulations of tropospheric NO2 compared with GOME retrievals for the year 2000. Atmospheric Chemistry and Physics, 6, 2943-2979.
    [ Abstract PDF ]

    We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval. Full Article in PDF (1875 KB) Discussion Paper Library Search ACP Library Search ACPD Special Services Printer-friendly Version Bookmark Download Acrobat Reader News ISI Impact Factor: 4.362 (2006) more ISI Special Report on ACP more Most Commented Papers more Personalised Publication Alert Service more New Licence and Copyright Agreement for Publications more We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.

22. West, J J., Arlene M Fiore, Larry Horowitz, and D L Mauzerall, 2006: Global health benefits of mitigating ozone pollution with methane emission controls. Proceedings of the National Academy of Sciences, 103(11), doi:10.1073/pnas.0600201103.
    [ Abstract ]

    Methane (CH4) contributes to the growing global background concentration of tropospheric ozone (O3), an air pollutant associated with premature mortality. Methane and ozone are also important greenhouse gases. Reducing methane emissions therefore decreases surface ozone everywhere while slowing climate warming, but although methane mitigation has been considered to address climate change, it has not for air quality. Here we show that global decreases in surface ozone concentrations, due to methane mitigation, result in substantial and widespread decreases in premature human mortality. Reducing global anthropogenic methane emissions by 20% beginning in 2010 would decrease the average daily maximum 8-h surface ozone by 1 part per billion by volume globally. By using epidemiologic ozone-mortality relationships, this ozone reduction is estimated to prevent 30,000 premature all-cause mortalities globally in 2030, and 370,000 between 2010 and 2030. If only cardiovascular and respiratory mortalities are considered, 17,000 global mortalities can be avoided in 2030. The marginal cost-effectiveness of this 20% methane reduction is estimated to be $420,000 per avoided mortality. If avoided mortalities are valued at $1 million each, the benefit is $240 per tonne of CH4 ($12 per tonne of CO2 equivalent), which exceeds the marginal cost of the methane reduction. These estimated air pollution ancillary benefits of climate-motivated methane emission reductions are comparable with those estimated previously for CO2. Methane mitigation offers a unique opportunity to improve air quality globally and can be a cost-effective component of international ozone management, bringing multiple benefits for air quality, public health, agriculture, climate, and energy. human health | mortality | tropospheric ozone | air quality

23. 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.

24. West, J J., and Arlene M Fiore, 2005: Management of Tropospheric Ozone by Reducing Methane Emissions. Environmental Science & Technology, 39(13), doi:10.1021/es048629f.
    [ Abstract ]

    Background concentrations of tropospheric ozone are increasing and are sensitive to methane emissions, yet methane mitigation is currently considered only for climate change. Methane control is shown here to be viable for ozone management. Identified global abatement measures can reduce ~10% of anthropogenic methane emissions at a cost-savings, decreasing surface ozone by 0.4-0.7 ppb. Methane controls produce ozone reductions that are widespread globally and are realized gradually (~12 yr). In contrast, controls on nitrogen oxides (NOX) and nonmethane volatile organic compounds (NMVOCs) target high-ozone episodes in polluted regions and affect ozone rapidly but have a smaller climate benefit. A coarse estimate of the monetized global benefits of ozone reductions for agriculture, forestry, and human health (neglecting ozone mortality) justifies reducing ~17% of global anthropogenic methane emissions. If implemented, these controls would decrease ozone by ~1 ppb and radiative forcing by ~0.12 W m-2. We also find that climate-motivated methane reductions have air quality-related ancillary benefits comparable to those for CO2. Air quality planning should consider reducing methane emissions alongside NOX and NMVOCs, and because the benefits of methane controls are shared internationally, industrialized nations should consider emphasizing methane in the further development of climate change or ozone policies.

25. Liu, H, D J Jacob, J E Dibb, Arlene M Fiore, and R M Yantosca, 2004: Constraints on the sources of tropospheric ozone from 210 Pb-7 Be-O3 correlations. Journal of Geophysical Research, 109(D7), D07306, doi:10.1029/2003JD003988.
    [ Abstract PDF ]

    The 210Pb-7Be-O 3 relationships observed in three aircraft missions over the western Pacific (PEM-West A and B, TRACE-P) are simulated with a global three-dimensional chemical tracer model (GEOS-CHEM) driven by assimilated meteorological observations. Results are interpreted in terms of the constraints that they offer on sources of tropospheric ozone (O3). Aircraft observations of fresh Asian outflow show strong 210Pb-O3 correlations in September–October, but such correlations are only seen at low latitudes in February–March. Observations further downwind over the Pacific show stronger 210Pb-O3 correlations in February–March than in September–October. The model reproduces these results and attributes the seasonal contrast to strong O3 production and vertical mixing over east Asia in September–October, seasonal shift of convection from China in September–October to Southeast Asia in February–March, and slow but sustained net O3 production in Asian outflow over the western Pacific in February–March. Seasonal biomass burning over Southeast Asia in February–March is responsible for the positive 210Pb-O3 correlations observed at low latitudes. The model reproduces the observed absence of 7Be-O3 correlations over the western Pacific during September–October, implying strong convective and weak stratospheric influence on O3. Comparison of observed and simulated 7Be-O3 correlations indicates that the stratosphere contributes less than 20–30% of O3 in the middle troposphere at northern midlatitudes even during spring.

26. Martin, R V., Arlene M Fiore, and A Van Donkelaar, 2004: Space-based diagnosis of surface ozone sensitivity to anthropogenic emissions. Geophysical Research Letters, 31, L06120, doi:10.1029/2004GL019416.
    [ Abstract ]

    We present a novel capability in satellite remote sensing with implications for air pollution control strategy. We show that the ratio of formaldehyde columns to tropospheric nitrogen dioxide columns is an indicator of the relative sensitivity of surface ozone to emissions of nitrogen oxides (NOx ≡ NO + NO2) and volatile organic compounds (VOCs). The diagnosis from these space-based observations is highly consistent with current understanding of surface ozone chemistry based on in situ observations. The satellite-derived ratios indicate that surface ozone is more sensitive to emissions of NOx than of VOCs throughout most continental regions of the Northern Hemisphere during summer. Exceptions include Los Angeles and industrial areas of Germany. A seasonal transition occurs in the fall when surface ozone becomes less sensitive to NOx and more sensitive to VOCs.

27. Fiore, Arlene M., T Holloway, and M G Hastings, 2003: A global perspective on Air Quality: Intercontinental transport and linkages with Climate. EM, 13-22.
    [ Abstract ]

    Linkages between climate and the intercontinental transport (ICT) of ozone and aerosols offer opportunities for coordinated mitigation of air pollution and global warming. This article considers the ICT of ozone and aerosols among Asia, Europe, and North America, and highlights linkages between air quality and climate that might benefit future emissions control strategies.

28. Fiore, Arlene M., D J Jacob, H Liu, R M Yantosca, T D A Fairlie, and Q Li, 2003: Variability in surface ozone background over the United States: Implications for air quality policy. Journal of Geophysical Research, 108, D24,2487, doi:10.1029/2003JD003855.
    [ Abstract PDF ]

    The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O3 level as a baseline in its O3 risk assessments. This background is defined as those concentrations that would exist in the absence of North American emissions. Lefohn et al. [2001] have argued that frequent occurrences of O3 concentrations above 50–60 ppbv at remote northern U.S. sites in spring are of stratospheric origin, challenging the EPA background estimate and implying that the current O3 standard (84 ppbv, 8-hour average) may be unattainable. We show that a 3-D global model of tropospheric chemistry reproduces much of the observed variability in U.S. surface O3 concentrations, including the springtime high-O3 events, with only a minor stratospheric contribution (always <20 ppbv). We conclude that the previous interpretations of a stratospheric source for these events underestimated the role of regional and hemispheric pollution. While stratospheric intrusions might occasionally elevate surface O3 at high-altitude sites, our results indicate that these events are rare and would not compromise the O3 air quality standard. We find that the O3 background is generally 15–35 ppbv, with some incidences of 40–50 ppbv in the west in spring at high-elevation sites (>2 km). It declines from spring to summer and further decreases during O3 pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O3 during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O3 concentration. Natural O3 levels are typically 10–25 ppbv and never exceed 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40-type standard (cumulative exposure to O3 above 40 ppbv) in the United States.

29. Fiore, Arlene M., D J Jacob, R. Mathur, and R V Martin, 2003: Application of empirical orthogonal functions to evaluate ozone simulations with regional and global models. Journal of Geophysical Research, 108(14), 4431, doi:10.1029/2002JD003151.
    [ Abstract ]

    Empirical orthogonal functions are used together with standard statistical metrics to evaluate the ability of models with different spatial resolutions to reproduce observed patterns of surface ozone (O3) in the eastern United States in the summer of 1995. We examine simulations with the regional Multiscale Air Quality Simulation Platform model (horizontal resolution of 36 km2) and the global GEOS-CHEM model (2° × 2.5° and 4° × 5°). As the model resolution coarsens, the ability to resolve local O3 maxima (O3 ≥ 90 ppbv) is compromised, but the spatial correlation improves. This result shows that synoptic-scale processes modulating O3 concentrations are easier to capture in models than processes occurring on smaller scales. Empirical orthogonal functions (EOFs) derived from the observed O3 fields reveal similar modes of variability when averaged onto the three model horizontal resolutions. The EOFs appear to represent (1) an east-west pattern associated with frontal passages, (2) a midwest-northeast pattern associated with migratory high-pressure systems, and (3) a southeast stagnation pattern linked to westward extension of the Bermuda High. All models capture the east-west and southeast EOFs, but the midwest-northeast EOF is misplaced in GEOS-CHEM. GEOS-CHEM captures the principal components of the observational EOFs when the model fields are projected onto these EOFs, implying that it can resolve the contribution of the EOFs to the observed variance. We conclude that coarse-resolution global models can successfully simulate the synoptic conditions leading to high-O3 episodes in the eastern United States.

30. Holloway, T, Arlene M Fiore, and M G Hastings, 2003: Intercontinental Transport of Air Pollution: Will Emerging Science Lead to a New Hemispheric Treaty? Environmental Science & Technology, 37(20), 4535-4542.
    [ Abstract PDF ]

    We examine the emergence of InterContinental Transport (ICT) of air pollution on the agendas of the air quality and climate communities and consider the potential for a new treaty on hemispheric air pollution. ICT is the flow of air pollutants from a source continent (e.g., North America) to a receptor continent (e.g., Europe). ICT of air pollutants occurs through two mechanisms: (i) episodic advection and (ii) increasing the global background, which enhances surface concentrations. We outline the current scientific evidence for ICT of aerosols and ozone, both of which contribute to air pollution and radiative forcing. The growing body of scientific evidence for ICT suggests that a hemispheric-scale treaty to reduce air pollutant concentra tions may be appropriate to address climate and air quality concerns simultaneously. Such a treaty could pave the way for future climate agreements. --------------------------------------------------------------------------------

31. Palmer, P I., D J Jacob, Arlene M Fiore, R V Martin, K Chance, and A Abe-Ouchi, 2003: Mapping isoprene emissions over North America using formaldehyde column observations from space. Journal of Geophysical Research, 108(D6), 4180, doi:10.1029/2002JD002153.
    [ Abstract ]

    We present a methodology for deriving emissions of volatile organic compounds (VOC) using space-based column observations of formaldehyde (HCHO) and apply it to data from the Global Ozone Monitoring Experiment (GOME) satellite instrument over North America during July 1996. The HCHO column is related to local VOC emissions, with a spatial smearing that increases with the VOC lifetime. Isoprene is the dominant HCHO precursor over North America in summer, and its lifetime (≃1 hour) is sufficiently short that the smearing can be neglected. We use the Goddard Earth Observing System global 3-D model of tropospheric chemistry (GEOS-CHEM) to derive the relationship between isoprene emissions and HCHO columns over North America and use these relationships to convert the GOME HCHO columns to isoprene emissions. We also use the GEOS-CHEM model as an intermediary to validate the GOME HCHO column measurements by comparison with in situ observations. The GEOS-CHEM model including the Global Emissions Inventory Activity (GEIA) isoprene emission inventory provides a good simulation of both the GOME data (r2 = 0.69, n = 756, bias = +11%) and the in situ summertime HCHO measurements over North America (r2 = 0.47, n = 10, bias = −3%). The GOME observations show high values over regions of known high isoprene emissions and a day-to-day variability that is consistent with the temperature dependence of isoprene emission. Isoprene emissions inferred from the GOME data are 20% less than GEIA on average over North America and twice those from the U.S. EPA Biogenic Emissions Inventory System (BEIS2) inventory. The GOME isoprene inventory when implemented in the GEOS-CHEM model provides a better simulation of the HCHO in situ measurements than either GEIA or BEIS2 (r2 = 0.71, n = 10, bias = −10%).

32. Fiore, Arlene M., D J Jacob, B D Field, D G Streets, S D Fernandes, and C. Jang, 2002: Linking ozone pollution and climate change: The case for controlling methane. Geophysical Research Letters, 29(19), 1919, doi:101.1029/2002GL015601.
    [ Abstract PDF ]

    Methane (CH4) emission controls are found to be a powerful lever for reducing both global warming and air pollution via decreases in background tropospheric ozone (O3). Reducing anthropogenic CH4 emissions by 50% nearly halves the incidence of U.S. high-O3 events and lowers global radiative forcing by 0.37 W m−2 (0.30 W m−2 from CH4, 0.07 W m−2 from O3) in a 3-D model of tropospheric chemistry. A 2030 simulation based upon IPCC A1 emissions projections shows a longer and more intense U.S. O3 pollution season despite domestic emission reductions, indicating that intercontinental transport and a rising O3 background should be considered when setting air quality goals.

33. Fiore, Arlene M., D J Jacob, I Bey, R M Yantosca, B D Field, A C Fusco, and J G Wilkinson, 2002: Background ozone over the United States in summer: Origin, trend, and contribution to pollution episodes. Journal of Geophysical Research, 107(D15), 4275, doi:10.1029/2001JD000982.
    [ Abstract PDF ]

    Methane (CH4) emission controls are found to be a powerful lever for reducing both global warming and air pollution via decreases in background tropospheric ozone (O3). Reducing anthropogenic CH4 emissions by 50% nearly halves the incidence of U.S. high-O3 events and lowers global radiative forcing by 0.37 W m−2 (0.30 W m−2 from CH4, 0.07 W m−2 from O3) in a 3-D model of tropospheric chemistry. A 2030 simulation based upon IPCC A1 emissions projections shows a longer and more intense U.S. O3 pollution season despite domestic emission reductions, indicating that intercontinental transport and a rising O3 background should be considered when setting air quality goals.

34. Li, Q, D J Jacob, I Bey, P I Palmer, B N Duncan, B D Field, R V Martin, and Arlene M Fiore, et al., 2002: Transatlantic transport of pollution and its effects on surface ozone in Europe and North America. Journal of Geophysical Research, 107(D13), 4166, doi:10.1029/2001JD001422.
    [ Abstract ]

    We examine the transatlantic transport of anthropogenic ozone and its impact on surface ozone in Europe and North America by using a 5-year (1993–1997) simulation with the GEOS-CHEM global three-dimensional model of tropospheric chemistry. Long-term time series of ozone and CO at Mace Head (Ireland) and Sable Island (Canada) are used to evaluate transatlantic transport in the model. North American anthropogenic emissions contribute on average 5 ppbv to surface ozone at Mace Head, and up to 10–20 ppbv during transatlantic transport events, which are forerunners of broader events in Europe. These events are associated with low-level westerly flow driven by an intense Icelandic low between Iceland and the British Isles. North American influence on ozone at Mace Head is strongly correlated with the North Atlantic Oscillation (NAO), implying that the NAO index can be used to forecast transatlantic transport of North American pollution to Europe. European anthropogenic emissions contribute on average less than 2 ppbv to surface ozone at Sable Island but up to 5–10 ppbv during transatlantic transport events. These events are associated with low-level easterly flow established by anomalous low pressure at 45°N over the North Atlantic. North American anthropogenic emissions enhance surface ozone in continental Europe by 2–4 ppbv on average in summer and by 5–10 ppbv during transatlantic transport events; transport in the boundary layer and subsidence from the free troposphere are both important mechanisms. We find in the model that 20% of the violations of the European Council ozone standard (55 ppbv, 8-hour average) in the summer of 1997 over Europe would not have occurred in the absence of anthropogenic emissions from North America. North American influence on surface ozone in Europe is particularly strong at the thresholds used for the European standards (55–65 ppbv).

35. Martin, R V., K Chance, D J Jacob, T P Kurosu, R J D Spurr, E Bucsela, J F Gleason, P I Palmer, I Bey, and Arlene M Fiore, et al., 2002: An improved retrieval of tropospheric nitrogen dioxide from GOME. Journal of Geophysical Research, 107(D20), 4437, doi:10.1029/2001JD001027.
    [ Abstract ]

    We present a retrieval of tropospheric nitrogen dioxide (NO2) columns from the Global Ozone Monitoring Experiment (GOME) satellite instrument that improves in several ways over previous retrievals, especially in the accounting of Rayleigh and cloud scattering. Slant columns, which are directly fitted without low-pass filtering or spectral smoothing, are corrected for an artificial offset likely induced by spectral structure on the diffuser plate of the GOME instrument. The stratospheric column is determined from NO2 columns over the remote Pacific Ocean to minimize contamination from tropospheric NO2. The air mass factor (AMF) used to convert slant columns to vertical columns is calculated from the integral of the relative vertical NO2 distribution from a global 3-D model of tropospheric chemistry driven by assimilated meteorological data (Global Earth Observing System (GEOS)-CHEM), weighted by altitude-dependent scattering weights computed with a radiative transfer model (Linearized Discrete Ordinate Radiative Transfer), using local surface albedos determined from GOME observations at NO2 wavelengths. The AMF calculation accounts for cloud scattering using cloud fraction, cloud top pressure, and cloud optical thickness from a cloud retrieval algorithm (GOME Cloud Retrieval Algorithm). Over continental regions with high surface emissions, clouds decrease the AMF by 20–30% relative to clear sky. GOME is almost twice as sensitive to tropospheric NO2 columns over ocean than over land. Comparison of the retrieved tropospheric NO2 columns for July 1996 with GEOS-CHEM values tests both the retrieval and the nitrogen oxide radical (NOx) emissions inventories used in GEOS-CHEM. Retrieved tropospheric NO2 columns over the United States, where NOx emissions are particularly well known, are within 18% of GEOS-CHEM columns and are strongly spatially correlated (r = 0.78, n = 288, p < 0.005). Retrieved columns show more NO2 than GEOS-CHEM columns over the Transvaal region of South Africa and industrial regions of the northeast United States and Europe. They are lower over Houston, India, eastern Asia, and the biomass burning region of central Africa, possibly because of biases from absorbing aerosols.

36. Martin, R V., D J Jacob, J Logan, I Bey, R M Yantosca, A C Staudt, Q B Li, and Arlene M Fiore, et al., 2002: Interpretation of TOMS observations of tropical tropospheric ozone with a global model and in situ observations. Journal of Geophysical Research, 107(D18), doi:10.1029/2001JD001480.
    [ Abstract ]

    We interpret the distribution of tropical tropospheric ozone columns (TTOCs) from the Total Ozone Mapping Spectrometer (TOMS) by using a global three-dimensional model of tropospheric chemistry (GEOS-CHEM) and additional information from in situ observations. The GEOS-CHEM TTOCs capture 44% of the variance of monthly mean TOMS TTOCs from the convective cloud differential method (CCD) with no global bias. Major discrepancies are found over northern Africa and south Asia where the TOMS TTOCs do not capture the seasonal enhancements from biomass burning found in the model and in aircraft observations. A characteristic feature of these northern tropical enhancements, in contrast to southern tropical enhancements, is that they are driven by the lower troposphere where the sensitivity of TOMS is poor due to Rayleigh scattering. We develop an efficiency correction to the TOMS retrieval algorithm that accounts for the variability of ozone in the lower troposphere. This efficiency correction increases TTOCs over biomass burning regions by 3–5 Dobson units (DU) and decreases them by 2–5 DU over oceanic regions, improving the agreement between CCD TTOCs and in situ observations. Applying the correction to CCD TTOCs reduces by ∼5 DU the magnitude of the “tropical Atlantic paradox” [ Thompson et al., 2000 ], i.e. the presence of a TTOC enhancement over the southern tropical Atlantic during the northern African biomass burning season in December–February. We reproduce the remainder of the paradox in the model and explain it by the combination of upper tropospheric ozone production from lightning NOx, persistent subsidence over the southern tropical Atlantic as part of the Walker circulation, and cross-equatorial transport of upper tropospheric ozone from northern midlatitudes in the African “westerly duct.” These processes in the model can also account for the observed 13–17 DU persistent wave-1 pattern in TTOCs with a maximum over the tropical Atlantic and a minimum over the tropical Pacific during all seasons. The photochemical effects of mineral dust have only a minor role on the modeled distribution of TTOCs, including over northern Africa, due to multiple competing effects. The photochemical effects of mineral dust globally decrease annual mean OH concentrations by 9%. A global lightning NOx source of 6 Tg N yr−1 in the model produces a simulation that is most consistent with TOMS and in situ observations.

37. Bey, I, D J Jacob, R M Yantosca, J Logan, B D Field, and Arlene M Fiore, et al., 2001: Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation. Journal of Geophysical Research, 106(D19), 23,073-23,095.
    [ Abstract PDF ]

    We present a first description and evaluation of GEOS-CHEM, a global threedimensional (3-D) model of tropospheric chemistry driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Data Assimilation Office (DAO). The model is applied to a 1-year simulation of tropospheric ozone-NO x -hydrocarbon chemistry for 1994, and is evaluated with observations both for 1994 and for other years. It reproduces usually to within 10 ppb the concentrations of ozone observed from the worldwide ozonesonde data network. It simulates correctly the seasonal phases and amplitudes of ozone concentrations for different regions and altitudes, but tends to underestimate the seasonal amplitude at northern midlatitudes. Observed concentrations of NO and peroxyacetylnitrate (PAN) observed in aircraft campaigns are generally reproduced to within a factor of 2 and often much better. Concentrations of HNO3 in the remote troposphere are overestimated typically by a factor of 2–3, a common problem in global models that may reflect a combination of insufficient precipitation scavenging and gas-aerosol partitioning not resolved by the model. The model yields an atmospheric lifetime of methylchloroform (proxy for global OH) of 5.1 years, as compared to a best estimate from observations of 5.5 +/− 0.8 years, and simulates H2O2 concentrations observed from aircraft with significant regional disagreements but no global bias. The OH concentrations are ∼20% higher than in our previous global 3-D model which included an UV-absorbing aerosol. Concentrations of CO tend to be underestimated by the model, often by 10–30 ppb, which could reflect a combination of excessive OH (a 20% decrease in model OH could be accommodated by the methylchloroform constraint) and an underestimate of CO sources (particularly biogenic). The model underestimates observed acetone concentrations over the South Pacific in fall by a factor of 3; a missing source from the ocean may be implicated.

38. Li, Q B., D J Jacob, J Logan, I Bey, R M Yantosca, H Liu, R V Martin, and Arlene M Fiore, et al., 2001: A Tropospheric Ozone Maximum Over the Middle East. Geophysical Research Letters, 28(17), 3235-3238.
    [ Abstract PDF ]

    The GEOS-CHEM global 3-D model of tropospheric chemistry predicts a summertime O3 maximum over the Middle East, with mean mixing ratios in the middle and upper troposphere in excess of 80 ppbv. This model feature is consistent with the few observations from commercial aircraft in the region. Its origin in the model reflects a complex interplay of dynamical and chemical factors, and of anthropogenic and natural influences. The anticyclonic circulation in the middle and upper troposphere over the Middle East funnels northern midlatitude pollution transported in the westerly subtropical jet as well as lightning outflow from the Indian monsoon and pollution from eastern Asia transported in an easterly tropical jet. Large-scale subsidence over the region takes place with continued net production of O3 and little mid-level outflow. Transport from the stratosphere does not contribute significantly to the O3 maximum. Sensitivity simulations with anthropogenic or lightning emissions shut off indicate decreases of 20-30% and 10-15% respectively in the tropospheric O3 column over the Middle East. More observations in this region are needed to confirm the presence of the O3 maximum.

39. Lin, C-Y C., D J Jacob, and Arlene M Fiore, 2001: Trends in exceedances of the ozone air quality standard in the continental United States, 1980-1998. Atmospheric Environment, 35(19), 3217-3228.
    [ Abstract PDF ]

    In 1997, the United States National Ambient Air Quality Standard (NAAQS) for ozone was revised from a 1-h average of 0.12 parts per million (ppm) to an 8-h average of 0.08 ppm. Analysis of ozone data for the ensemble of the contiguous United States and for the period 1980–1998 shows that the average number of summer days per year in exceedance of the new standard is in the range 8–24 in the Northeast and in Texas, and 12–73 in southern California. The probability of exceedance increases with temperature and exceeds 20% in the Northeast for daily maximum temperatures above 305 K. We present the results of several different approaches to analyzing the long-term trends in the old and new standards over the continental United States from 1980 to 1998. Daily temperature data are used to resolve meteorological variability and isolate the effects of changes in anthropogenic emissions. Significant negative trends are found in the Northeast urban corridor, in the Los Angeles Basin and on the western bank of Lake Michigan.Temperature segregation enhances the detection of negative trends. Positive trends occur at isolated sites, mostly in the Southeast; a strong positive trend is found in Nashville (Tennessee). There is some evidence that, except in the Southwest, air quality improvements from the 1980s to the 1990s have leveled off in the past decade.

40. Palmer, P I., D J Jacob, K Chance, R V Martin, R J D Spurr, T P Kurosu, I Bey, R M Yantosca, Arlene M Fiore, and Q B Li, 2001: Air mass factor formulation for spectroscopic measurements from satellites: Application to formaldehyde retrievals from the Global Ozone Monitoring Experiment. Journal of Geophysical Research, 106(D13), 14,539-14,550.
    [ Abstract PDF ]

    We present a new formulation for the air mass factor (AMF) to convert slant column measurements of optically thin atmospheric species from space into total vertical columns. Because of atmospheric scattering, the AMF depends on the vertical distribution of the species. We formulate the AMF as the integral of the relative vertical distribution (shape factor) of the species over the depth of the atmosphere, weighted by altitude-dependent coefficients (scattering weights) computed independently from a radiative transfer model. The scattering weights are readily tabulated, and one can then obtain the AMF for any observation scene by using shape factors from a three dimensional (3-D) atmospheric chemistry model for the period of observation. This approach subsequently allows objective evaluation of the 3-D model with the observed vertical columns, since the shape factor and the vertical column in the model represent two independent pieces of information. We demonstrate the AMF method by using slant column measurements of formaldehyde at 346 nm from the Global Ozone Monitoring Experiment satellite instrument over North America during July 1996. Shape factors are computed with the Global Earth Observing System CHEMistry (GEOS-CHEM) global 3-D model and are checked for consistency with the few available aircraft measurements. Scattering weights increase by an order of magnitude from the surface to the upper troposphere. The AMFs are typically 20–40% less over continents than over the oceans and are approximately half the values calculated in the absence of scattering. Model-induced errors in the AMF are estimated to be ∼10%. The GEOS-CHEM model captures 50% and 60% of the variances in the observed slant and vertical columns, respectively. Comparison of the simulated and observed vertical columns allows assessment of model bias.

41. Lin, C-Y C., D J Jacob, J W Munger, and Arlene M Fiore, 2000: Increasing Background Ozone in Surface Air Over the United States. Geophysical Research Letters, 27(21), 3465-3468.
    [ Abstract PDF ]

    The long-term trend of background O3 in surface air over the United States from 1980 to 1998 is examined using monthly probability distributions of daily maximum 8-hour average O3 concentrations at a large ensemble of rural sites. Ozone concentrations have decreased at the high end of the probability distribution (reflecting emission controls) but have increased at the low end. The cross-over takes place between the 30th and 50th percentiles in May-August and between the 60th and 90th percentiles during the rest of the year. The increase is statistically significant at a 5% level in spring and fall, when it is 3-5 ppbv. The maximum increase is in the Northeast. A possible explanation is an increase in the O3 background transported from outside the United States. Better understanding of the causes of the increase is needed because of its implications for meeting O3 air quality standards.

42. Fiore, Arlene M., D J Jacob, J Logan, and J Yin, 1998: Long-term trends in ground level ozone over the contiguous United States, 1980–1995. Journal of Geophysical Research, 103(D1), 1471-1480.
    [ Abstract PDF ]

    Long-term trends of median and 90th percentile summer afternoon O3 concentrations were examined at 549 sites across the United States for the 1980–1995 period. Daily temperature data were used to account for the variability in O3 concentrations associated with temperature. Both before and after segregating the O3 data by temperature, trends were insignificant over most of the continental United States. No region of the United States experienced a significant increase in O3 concentrations during the 1980–1995 period. Decreasing trends were predominantly clustered in the three largest metropolitan areas: New York City, Los Angeles, and Chicago. In these areas, additional sites with trends were identified in the temperature-segregated analysis. Correlation of trends with local anthropogenic emissions of nitrogen oxides (NO x = NO + NO2) and volatile organic compounds (VOC) indicates a greater frequency of decreasing trends for urban sites with high emission. National emission inventories for the United States indicate that anthropogenic VOC emissions decreased by 12% over the 1980–1995 period while NO x emissions remained constant. The observed O3 trends are consistent with the view that summertime O3 production over the United States is NO x -limited except in the largest metropolitan areas where it is partly VOC-limited.

43. Liang, J, Larry Horowitz, D J Jacob, Y Wang, and Arlene M Fiore, et al., 1998: Seasonal budgets of reactive nitrogen species and ozone over the United States, and export fluxes to the global atmosphere. Journal of Geophysical Research, 103(D11), 13,435-13,450.
    [ Abstract PDF ]

    A three-dimensional, continental-scale photochemical model is used to investigate seasonal budgets of O3 and NO y species (including NO x and its oxidation products) in the boundary layer over the United States and to estimate the export of these species from the U.S. boundary layer to the global atmosphere. Model results are evaluated with year-round observations for O3, CO, and NO y species at nonurban sites. A seasonal transition from NO x to hydrocarbon-limited conditions for O3 production over the eastern United States is found to take place in the fall, with the reverse transition taking place in the spring. The mean NO x /NO y molar ratio in the U.S. boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, in accord with observations, and reflecting largely the seasonal variation in the chemical lifetime of NO x . Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, is estimated to account for 30% of the chemical sink of NO x in the U.S. boundary layer in summer. Model results indicate that peroxyacylnitrates (PANs) are most abundant in the U.S. boundary layer in spring (25% of total NO y .), reflecting a combination of active photochemistry and low temperatures. About 20% of the NO x emitted from fossil fuel combustion in the United States in the model is exported out of the U.S. boundary layer as NO x or PANs (15% in summer, 25% in winter). This export responds less than proportionally to changes in NO x emissions in summer, but more than proportionally in winter. The annual mean export of NO x and PANs from the U.S. boundary layer is estimated to be 1.4 Tg N yr−1, representing an important source of NO x on the scale of the northern hemisphere troposphere. The eventual O3 production in the global troposphere due to the exported NO x and PANs is estimated to be twice as large, on an annual basis, as the direct export of O3 pollution from the U.S. boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O3 in the northern hemisphere troposphere on an annual basis

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