Sensitivity of Tropospheric Ozone Over the Southeast USA to Dry Deposition

2020 ◽  
Vol 47 (7) ◽  
Author(s):  
Colleen B. Baublitz ◽  
Arlene M. Fiore ◽  
Olivia E. Clifton ◽  
Jingqiu Mao ◽  
Jingyi Li ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Southeast Usa

scholarly journals Revising global ozone dry deposition estimates based on a new mechanistic parameterisation for air-sea exchange and the multi-year MACC composition reanalysis

2017 ◽  
Author(s):  
Ashok K. Luhar ◽  
Matthew T. Woodhouse ◽  
Ian E. Galbally
Keyword(s):  
Chemical Reaction ◽  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Surface Resistance ◽  
Chemical Reactivity ◽  
Deposition Velocity ◽  
Water Layer ◽  
Turbulent Transfer ◽  
Dominant Term ◽  
Deposition Velocities

Abstract. Dry deposition at the Earth’s surface is an important sink of atmospheric ozone. Currently, dry deposition of ozone to the ocean surface in atmospheric chemistry models has the largest uncertainty compared to deposition to other surface types, with implications for global tropospheric ozone budget and associated radiative forcing. Most models assume that the dominant term of surface resistance in the parameterisation of ozone dry deposition velocity at the oceanic surface is constant. We present a consistent, process-based parameterisation scheme for air-sea exchange in which the surface resistance accounts for the simultaneous waterside processes of ozone solubility, molecular diffusion, turbulent transfer, and a first-order chemical reaction of ozone with dissolved iodide. The new scheme makes the following realistic assumptions: (a) the thickness of the top water layer is of the order of a reaction-diffusion length scale (a few micrometres) within which ozone loss is dominated by chemical reaction and the influence of waterside turbulent transfer is negligible; (b) in the water layer below, both chemical reaction and waterside turbulent transfer act together and are accounted for; and (c) iodide (hence chemical reactivity) is present through the depth of the oceanic mixing layer. The asymptotic behaviour of the new scheme is consistent with the known limits when either chemical reaction or turbulent transfer dominates. It has been incorporated into the ACCESS-UKCA global chemistry-climate model and the results are evaluated against dry deposition velocities from currently best available open-ocean measurements. In order to better quantify the global dry deposition loss and its interannual variability, the modelled 3-h ozone deposition velocities are combined with the 3-h MACC (Monitoring Atmospheric Composition and Climate) reanalysis ozone for the years 2003–2012. The resulting ozone dry deposition is found to be 98.4 ± 4.5 Tg O3 yr−1 for the ocean and 722.8 ± 20.9 O3 yr−1 globally. The new estimate of the ocean component is approximately a third of the current model estimates. This reduction corresponds to an approximately 20 % decrease in the total global ozone dry deposition, which is equivalent to an increase of approximately 5 % in the modelled tropospheric ozone burden and a similar increase in tropospheric ozone lifetime.

Surface ozone and land-atmosphere coupling

2020 ◽  
Author(s):  
Tamara Emmerichs ◽  
Huug Ouwersloot ◽  
Astrid Kerkweg ◽  
Silvano Fares ◽  
Ivan Mammarella ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Amazon Basin ◽  
Surface Ozone ◽  
Stomatal Closure ◽  
Circulation Model ◽  
Vapour Pressure Deficit ◽  
Air Pollutant ◽  
Global Models

<p>Surface ozone is a harmful air pollutant, heavily influenced by chemical production and loss processes. Dry deposition to vegetation is a relevant loss process responsible for 20 % of the total tropospheric ozone loss. Its parametrization in atmospheric chemistry models represents a major source of uncertainty for the global tropospheric ozone budget and might account for the mismatch with observations. The model used in this study, the Modular Earth Submodel System (MESSy2) linked to ECHAM5 as atmospheric circulation model (EMAC) is no exception. Like many global models, EMAC employs a “resistances in series” scheme with the major surface deposition via plant stomata which is hardly sensitive to meteorology depending only on solar radiation. Unlike many global models, however, EMAC uses a simplified high resistance for non-stomatal deposition which makes this pathway negligible.                             </p><p>Hence, a revision of the dry deposition scheme of EMAC is desirable. The scheme has been extended with empirical adjustment factors to predict stomatal responses to temperature and vapour pressure deficit. Furthermore, an explicit formulation of humidity depending non-stomatal deposition at the leaf surface (cuticle) has been implemented based on established schemes. Next, the soil moisture availability function for plants has been critically reviewed and modified in order to avoid a stomatal closure where the model shows a strong soil dry bias, e.g. Amazon basin in dry season.</p><p>The last part of the presentation will show comparisons of dry deposition velocities and fluxes comparing simulations with data obtained from four experimental sites where ozone deposition is measured with micrometeorological techniques. The impacts of the changes on daily and seasonal patterns of ozone dry deposition will be discussed with a highlight on surface ozone, global distribution and budget.</p>

scholarly journals Late-Spring and Summertime Tropospheric Ozone and NO<sub>2</sub> in Western Siberia and the Russian Arctic: Regional Model Evaluation and Sensitivities

2020 ◽  
Author(s):  
Thomas Thorp ◽  
Stephen R. Arnold ◽  
Richard J. Pope ◽  
Dominic V. Spracklen ◽  
Luke Conibear ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Western Siberia ◽  
Surface Ozone ◽  
Late Spring ◽  
The Arctic ◽  
Transport Sector ◽  
Anthropogenic Emissions ◽  
Negative Bias ◽  
Russian Arctic

Abstract. We use a regional chemistry transport model (WRF-Chem) in conjunction with surface observations of tropospheric ozone and Ozone Monitoring Instrument (OMI) satellite retrievals of tropospheric column NO2 to evaluate processes controlling the regional distribution of tropospheric ozone over Western Siberia for late-spring and summer in 2011. This region hosts a range of anthropogenic and natural ozone precursor sources, and serves as a gateway for near-surface transport of Eurasian pollution to the Arctic. However, there is a severe lack of in-situ observations to constrain tropospheric ozone sources and sinks in the region. We show widespread negative bias in WRF-Chem tropospheric column NO2 when compared to OMI satellite observations from May – August, which is reduced when using ECLIPSE v5a emissions (FMB= -0.82 to -0.73) compared with the EDGAR-HTAP-2 emissions data (FMB= -0.80 to -0.70). Despite the large negative bias, the spatial correlations between model and observed NO2 columns suggest that the spatial pattern of NOx sources in the region is well represented. Based on ECLIPSE v5a emissions, we assess the influence of the two dominant anthropogenic emission sectors (transport and energy) and vegetation fires on surface NOx and ozone over Siberia and the Russian Arctic. Our results suggest regional ozone is more sensitive to anthropogenic emissions, particularly from the transport sector, and the contribution from fire emissions maximises in June and is largely confined to latitudes south of 60° N. Large contributions to surface ozone from energy emissions are simulated in April north of 60° N, due to emissions associated with oil and gas extraction. Ozone dry deposition fluxes from the model simulations show that the dominant ozone dry deposition sink in the region is to forest, averaging 6.0 Tg of ozone per month, peaking at 9.1 Tg of ozone deposition during June. The impact of fires on ozone dry deposition within the domain is small compared to anthropogenic emissions, and is negligible north of 60° N. Overall, our results suggest that surface ozone in the region is controlled by an interplay between seasonality in atmospheric transport patterns, vegetation dry deposition, and a dominance of transport and energy sector emissions.

scholarly journals A revised global ozone dry deposition estimate based on a new two-layer parameterisation for air–sea exchange and the multi-year MACC composition reanalysis

2018 ◽  
Vol 18 (6) ◽  
pp. 4329-4348 ◽  
Author(s):  
Ashok K. Luhar ◽  
Matthew T. Woodhouse ◽  
Ian E. Galbally
Keyword(s):  
Chemical Reaction ◽  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Surface Resistance ◽  
Chemical Reactivity ◽  
Water Layer ◽  
Turbulent Transfer ◽  
Dominant Term ◽  
Deposition Velocities ◽  
Oceanic Surface

Abstract. Dry deposition at the Earth's surface is an important sink of atmospheric ozone. Currently, dry deposition of ozone to the ocean surface in atmospheric chemistry models has the largest uncertainty compared to deposition to other surface types, with implications for global tropospheric ozone budget and associated radiative forcing. Most global models assume that the dominant term of surface resistance in the parameterisation of ozone dry deposition velocity at the oceanic surface is constant. There have been recent mechanistic parameterisations for air–sea exchange that account for the simultaneous waterside processes of ozone solubility, molecular diffusion, turbulent transfer, and first-order chemical reaction of ozone with dissolved iodide and other compounds, but there are questions about their performance and consistency. We present a new two-layer parameterisation scheme for the oceanic surface resistance by making the following realistic assumptions: (a) the thickness of the top water layer is of the order of a reaction–diffusion length scale (a few micrometres) within which ozone loss is dominated by chemical reaction and the influence of waterside turbulent transfer is negligible; (b) in the water layer below, both chemical reaction and waterside turbulent transfer act together and are accounted for; and (c) chemical reactivity is present through the depth of the oceanic mixing layer. The new parameterisation has been evaluated against dry deposition velocities from recent open-ocean measurements. It is found that the inclusion of only the aqueous iodide–ozone reaction satisfactorily describes the measurements. In order to better quantify the global dry deposition loss and its interannual variability, modelled 3-hourly ozone deposition velocities are combined with the 3-hourly MACC (Monitoring Atmospheric Composition and Climate) reanalysis ozone for the years 2003–2012. The resulting ozone dry deposition is found to be 98.4 ± 30.0 Tg O3 yr−1 for the ocean and 722.8 ± 87.3 Tg O3 yr−1 globally. The new estimate of the ocean component is approximately a third of the current model estimates. This reduction corresponds to an approximately 20 % decrease in the total global ozone dry deposition, which (with all other components being unchanged) is equivalent to an increase of approximately 5 % in the modelled tropospheric ozone burden and a similar increase in tropospheric ozone lifetime.

scholarly journals The influence of weather-driven processes on tropospheric ozone

2021 ◽  
Author(s):  
Tamara Emmerichs ◽  
Bruno Franco ◽  
Catherine Wespes ◽  
Vinod Kumar ◽  
Andrea Pozzer ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Surface Ozone ◽  
Air Pollutant ◽  
Chemical Production ◽  
Ozone Monitoring Instrument ◽  
Near Surface ◽  
The Impact

Abstract. Near-surface ozone is an harmful air pollutant, which is determined to a considerable extent by weather-controlled processes, and may be significantly impacted by water vapour forming complexes with peroxy radicals. The role of water in the reaction of HO2 radical with nitrogen oxides is known from the literature, and in current models the water complex is considered by assuming a linear dependence on water concentrations. In fact, recent experimental evidence has been published, showing the significant role of water on the kinetics of one of the most important reaction for ozone chemistry, namely NO2 + OH. Here, the available kinetic data for the HOx + NOx reactions have been included in the atmospheric chemistry model ECHAM5/MESSy (EMAC) to test its global significance. Among the modified kinetics, the newly added HNO3 channel from HO2 + NO, dominates, significantly reducing NO2. A major removal process of near-surface ozone is dry deposition accounting for 20 % of the total tropospheric ozone loss mostly occurring over vegetation. However, parameterizations for modelling dry deposition represent a major source of uncertainty for tropospheric ozone simulations. This potentially belongs to the reasons why global models, such as EMAC used here, overestimate ozone with respect to observations. In fact, the employed parameterization is hardly sensitive to local meteorological conditions (e.g., humidity) and lacks non-stomatal deposition. In this study, a dry deposition scheme including these features have been used in EMAC, affecting not only the deposition of ozone but of its precursors, resulting in lower chemical production of ozone. Additionally, we improved the emissions of isoprene and nitrous acid (HONO). Namely, for isoprene emissions we have accounted for the impact of drought stress which confers a higher model sensitivity to meteorology leading to reduced annual emissions down to 32 %. For HONO, we have implemented soil emissions, which depend on soil moisture and thus on precipitation. We estimate for the first time a global source strength of 7 Tg(N) a−1. Furthermore, the usage of a parameterization for the production of lightning NOx that depends on cloud top height contributes to a more realistic representation of NO2 columns over remote oceans with respect to the satellite measurements of the Ozone Monitoring Instrument (OMI). The combination of all the model modifications reduces the simulated global ozone burden by ≈ 20 % to 337 Tg, which is in better agreement with recent estimates. By comparing simulation results with measurements from the Infrared Atmospheric Sounding Interferometer (IASI) and the Tropospheric Ozone Assessment Report (TOAR) databases (of 2009) we demonstrate an overall reduction of the ozone bias by a factor of 2.

scholarly journals Late-spring and summertime tropospheric ozone and NO<sub>2</sub> in western Siberia and the Russian Arctic: regional model evaluation and sensitivities

2021 ◽  
Vol 21 (6) ◽  
pp. 4677-4697
Author(s):  
Thomas Thorp ◽  
Stephen R. Arnold ◽  
Richard J. Pope ◽  
Dominick V. Spracklen ◽  
Luke Conibear ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Western Siberia ◽  
Late Spring ◽  
The Arctic ◽  
Transport Sector ◽  
Anthropogenic Emissions ◽  
Negative Bias ◽  
Russian Arctic ◽  
The Impact

Abstract. We use a regional chemistry transport model (Weather Research and Forecasting model coupled with chemistry, WRF-Chem) in conjunction with surface observations of tropospheric ozone and Ozone Monitoring Instrument (OMI) satellite retrievals of tropospheric column NO2 to evaluate processes controlling the regional distribution of tropospheric ozone over western Siberia for late spring and summer in 2011. This region hosts a range of anthropogenic and natural ozone precursor sources, and it serves as a gateway for near-surface transport of Eurasian pollution to the Arctic. However, there is a severe lack of in situ observations to constrain tropospheric ozone sources and sinks in the region. We show widespread negative bias in WRF-Chem tropospheric column NO2 when compared to OMI satellite observations from May–August, which is reduced when using ECLIPSE (Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants) v5a emissions (fractional mean bias (FMB) = −0.82 to −0.73) compared with the EDGAR (Emissions Database for Global Atmospheric Research)-HTAP (Hemispheric Transport of Air Pollution) v2.2 emissions data (FMB = −0.80 to −0.70). Despite the large negative bias, the spatial correlations between model and observed NO2 columns suggest that the spatial pattern of NOx sources in the region is well represented. Scaling transport and energy emissions in the ECLIPSE v5a inventory by a factor of 2 reduces column NO2 bias (FMB = −0.66 to −0.35), but with overestimates in some urban regions and little change to a persistent underestimate in background regions. Based on the scaled ECLIPSE v5a emissions, we assess the influence of the two dominant anthropogenic emission sectors (transport and energy) and vegetation fires on surface NOx and ozone over Siberia and the Russian Arctic. Our results suggest regional ozone is more sensitive to anthropogenic emissions, particularly from the transport sector, and the contribution from fire emissions maximises in June and is largely confined to latitudes south of 60∘ N. Ozone dry deposition fluxes from the model simulations show that the dominant ozone dry deposition sink in the region is to forest vegetation, averaging 8.0 Tg of ozone per month, peaking at 10.3 Tg of ozone deposition during June. The impact of fires on ozone dry deposition within the domain is small compared to anthropogenic emissions and is negligible north of 60∘ N. Overall, our results suggest that surface ozone in the region is controlled by an interplay between seasonality in atmospheric transport patterns, vegetation dry deposition, and a dominance of transport and energy sector emissions.

scholarly journals Modelling the global tropospheric ozone budget: exploring the variability in current models

2007 ◽  
Vol 7 (10) ◽  
pp. 2643-2660 ◽  
Author(s):  
O. Wild
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Transport Model ◽  
Recent Model ◽  
Ozone Production ◽  
Chemistry Transport Model ◽  
Chemical Mechanisms ◽  
Model Studies ◽  
Coarse Model ◽  
Lightning Nox

Abstract. What are the largest uncertainties in modelling ozone in the troposphere, and how do they affect the calculated ozone budget? Published chemistry-transport model studies of tropospheric ozone differ significantly in their conclusions regarding the importance of the key processes controlling the ozone budget: influx from the stratosphere, chemical processing and surface deposition. This study surveys ozone budgets from previous studies and demonstrates that about two thirds of the increase in ozone production seen between early assessments and more recent model intercomparisons can be accounted for by increased precursor emissions. Model studies using recent estimates of emissions compare better with ozonesonde measurements than studies using older data, and the tropospheric burden of ozone is closer to that derived here from measurement climatologies, 335±10 Tg. However, differences between individual model studies remain large and cannot be explained by surface precursor emissions alone; cross-tropopause transport, wet and dry deposition, humidity, and lightning also make large contributions. The importance of these processes is examined here using a chemistry-transport model to investigate the sensitivity of the calculated ozone budget to different assumptions about emissions, physical processes, meteorology and model resolution. The budget is particularly sensitive to the magnitude and location of lightning NOx emissions, which remain poorly constrained; the 3–8 TgN/yr range in recent model studies may account for a 10% difference in tropospheric ozone burden and a 1.4 year difference in CH4 lifetime. Differences in humidity and dry deposition account for some of the variability in ozone abundance and loss seen in previous studies, with smaller contributions from wet deposition and stratospheric influx. At coarse model resolutions stratospheric influx is systematically overestimated and dry deposition is underestimated; these differences are 5–8% at the 300–600 km grid-scales investigated here, similar in magnitude to the changes induced by interannual variability in meteorology. However, a large proportion of the variability between models remains unexplained, suggesting that differences in chemical mechanisms and dynamical schemes have a large impact on the calculated ozone budget, and these should be the target of future model intercomparisons.

scholarly journals A revised dry deposition scheme for land–atmosphere exchange of trace gases in ECHAM/MESSy v2.54

2021 ◽  
Vol 14 (1) ◽  
pp. 495-519
Author(s):  
Tamara Emmerichs ◽  
Astrid Kerkweg ◽  
Huug Ouwersloot ◽  
Silvano Fares ◽  
Ivan Mammarella ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
General Circulation ◽  
Process Model ◽  
Trace Gases ◽  
Stomatal Closure ◽  
Circulation Model ◽  
Ground Level ◽  
Global Models ◽  
Ground Level Ozone

Abstract. Dry deposition to vegetation is a major sink of ground-level ozone and is responsible for about 20 % of the total tropospheric ozone loss. Its parameterization in atmospheric chemistry models represents a significant source of uncertainty for the global tropospheric ozone budget and might account for the mismatch with observations. The model used in this study, the Modular Earth Submodel System version 2 (MESSy2) linked to the fifth-generation European Centre Hamburg general circulation model (ECHAM5) as an atmospheric circulation model (EMAC), is no exception. Like many global models, EMAC employs a “resistance in series” scheme with the major surface deposition via plant stomata which is hardly sensitive to meteorology, depending only on solar radiation. Unlike many global models, however, EMAC uses a simplified high resistance for non-stomatal deposition which makes this pathway negligible in the model. However, several studies have shown this process to be comparable in magnitude to the stomatal uptake, especially during the night over moist surfaces. Hence, we present here a revised dry deposition in EMAC including meteorological adjustment factors for stomatal closure and an explicit cuticular pathway. These modifications for the three stomatal stress functions have been included in the newly developed MESSy VERTEX submodel, i.e. a process model describing the vertical exchange in the atmospheric boundary layer, which will be evaluated for the first time here. The scheme is limited by a small number of different surface types and generalized parameters. The MESSy submodel describing the dry deposition of trace gases and aerosols (DDEP) has been revised accordingly. The comparison of the simulation results with measurement data at four sites shows that the new scheme enables a more realistic representation of dry deposition. However, the representation is strongly limited by the local meteorology. In total, the changes increase the dry deposition velocity of ozone up to a factor of 2 globally, whereby the highest impact arises from the inclusion of cuticular uptake, especially over moist surfaces. This corresponds to a 6 % increase of global annual dry deposition loss of ozone resulting globally in a slight decrease of ground-level ozone but a regional decrease of up to 25 %. The change of ozone dry deposition is also reasoned by the altered loss of ozone precursors. Thus, the revision of the process parameterization as documented here has, among others, the potential to significantly reduce the overestimation of tropospheric ozone in global models.

scholarly journals A revised dry deposition scheme for land-atmosphere exchange of trace gases in ECHAM/MESSy v2.54

2020 ◽  
Author(s):  
Tamara Emmerichs ◽  
Astrid Kerkweg ◽  
Huug Ouwersloot ◽  
Silvano Fares ◽  
Ivan Mammarella ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Process Model ◽  
Trace Gases ◽  
Stomatal Closure ◽  
Deposition Velocity ◽  
Measurement Data ◽  
Ground Level ◽  
Global Models ◽  
Ground Level Ozone

Abstract. Dry deposition to vegetation is a major sink of ground-level ozone and is responsible for about 20 % of the total tropospheric ozone loss. Its parametrisation in atmospheric chemistry models represent a significant source of uncertainty for the global tropospheric ozone budget and might account for the mismatch with observations. The model used in this study, the Modular Earth Submodel System (MESSy2) linked to ECHAM5 as an atmospheric circulation model (EMAC), is no exception. Like many global models, EMAC employs a “resistance in series” scheme with the major surface deposition via plant stomata which is hardly sensitive to meteorology, depending only on solar radiation. Unlike many global models, however, EMAC uses a simplified high resistance for non-stomatal deposition which makes this pathway negligible in the model. However, several studies have shown this process to be comparable in magnitude to the stomatal uptake, especially during the night over moist surfaces. Hence, we present here a revised dry deposition in EMAC. The default dry deposition scheme has been extended with adjustment factors to predict stomatal responses to temperature and vapour pressure deficit. Furthermore, an explicit formulation of the non-stomatal deposition to the leaf surface (cuticle) dependent on humidity has been implemented based on established schemes. Finally, the soil moisture availability function for plants has been revised to be consistent with the simple hydrological model available in EMAC. This revision was necessary in order to avoid unrealistic stomatal closure where the model shows a strong soil dry bias, e.g. in the Amazon basin in the dry season. These modifications for the three stomatal stress functions have been included in the newly developed MESSy submodel VERTEX, i.e. a process model describing the vertical exchange in the atmospheric boundary layer, which will be evaluated for the first time here. The MESSy submodel describing the dry deposition of trace gases and aerosols (DDEP) has been revised accordingly. The comparison of the simulation results with measurement data at four sites shows that the new scheme enables a more realistic representation of dry deposition. However, the representation is strongly limited by the local meteorology. In total, the changes increase the dry deposition velocity of ozone up to a factor of 2 globally, whereby the highest impact arises from the inclusion of cuticular uptake, especially over moist surfaces. This corresponds to a 6 % increase of global annual dry deposition loss of ozone resulting globally in a slight decrease of ground-level ozone but a regional decrease of up to 25 %. Thus, the revision of the process parameterisation as documented here has the potential to significantly reduce the overestimation of tropospheric ozone in global models.

scholarly journals Modelling the global tropospheric ozone budget: exploring the variability in current models

2007 ◽  
Vol 7 (1) ◽  
pp. 1995-2035 ◽  
Author(s):  
O. Wild
Keyword(s):  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Transport Model ◽  
Recent Model ◽  
Ozone Production ◽  
Surface Deposition ◽  
Chemistry Transport Model ◽  
Model Studies ◽  
Coarse Model ◽  
Lightning Nox

Abstract. What are the largest uncertainties in modelling ozone in the troposphere, and how do they affect the calculated ozone budget? Published chemistry-transport model studies of tropospheric ozone differ significantly in their conclusions regarding the importance of the key processes controlling the ozone budget: influx from the stratosphere, chemical processing and surface deposition. This study surveys ozone budgets from previous studies and demonstrates that about two thirds of the increase in ozone production seen between early assessments and more recent model intercomparisons can be accounted for by increased precursor emissions. Model studies using recent estimates of emissions compare better with ozonesonde measurements than studies using older data, and the tropospheric burden of ozone is closer to that derived here from measurement climatologies, 335±10 Tg. However, differences between individual model studies remain large and cannot be explained by surface precursor emissions alone; cross-tropopause transport, wet and dry deposition, humidity, and lightning make large contributions to the differences seen between models. The importance of these processes is examined here using a chemistry-transport model to investigate the sensitivity of the calculated ozone budget to different assumptions about emissions, physical processes, meteorology and model resolution. The budget is particularly sensitive to the magnitude and location of lightning NOx emissions, which remain poorly constrained; the 3–8 TgN/yr range in recent model studies may account for a 10% difference in tropospheric ozone burden and a 1.4 year difference in CH4 lifetime. Differences in humidity and dry deposition account for some of the variability in ozone abundance and loss seen in previous studies, with smaller contributions from wet deposition and stratospheric influx. At coarse model resolutions stratospheric influx is systematically overestimated and dry deposition is underestimated; these differences are 5–8% at the 300–600 km grid-scales investigated here, similar in magnitude to the changes induced by interannual variability in meteorology. However, a large proportion of the variability between models remains unexplained, suggesting that differences in model chemistry and dynamics have a large impact on the calculated ozone budget, and these should be the target of future model intercomparisons.

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