Simulated O3 Concentrations Research Articles

Assessment of tropospheric ozone simulations in a regional chemical transport model using GEOS-Chem outputs as chemical boundary conditions

Regional chemical transport models (e.g., Community Multiscale Air Quality (CMAQ) Modeling System) are widely used to simulate the physical and chemical process of regional ozone (O3) pollution and its variation trend in recent years. However, chemical boundary condition (CBC) is an important input of these models and contributes to the model bias against observations. In this study, we develop a tool named GC2CMAQ that provides the CMAQ model with the CBCs from the GEOS-Chem simulation. Two experiments using different CBCs were conducted to evaluate their effect on seasonal O3 simulation in China. The Default experiment utilized the model-default static condition (the relatively clean atmosphere in the eastern United States), and the GC experiment employed the GEOS-Chem simulation results. Compared with the observation, the GC experiment has a much better performance in reproducing elevated O3 levels in the higher troposphere and lower stratosphere during different seasons. Near the earth's surface, the simulated concentrations of pollutants O3 (and PM2.5) in the GC experiment were also closer to the observation in April and July. The accuracy of simulation results in provinces close to the boundary was improved by approximately 20 %–30 % relative to the Default experiment. The CBCs provided by GEOS-Chem enabled a better simulation of stratosphere-troposphere O3 exchange in late spring and early summer, which then affected the pollutant concentration near surfaces through vertical transport. This finding was confirmed by a case study in southwestern Tibet on April 28, 2017, in which we quantified the contributions of different physical and chemical processes to O3 variations at different altitudes using the process analysis method. This study highlights the importance of using a reliable CBC for the regional chemical transport model to derive a better performance of O3 simulation.

Development of an ecophysiology module in the GEOS-Chem chemical transport model version 12.2.0 to represent biosphere–atmosphere fluxes relevant for ozone air quality

Abstract. Ground-level ozone (O3) is a major air pollutant that adversely affects human health and ecosystem productivity. Removal of tropospheric O3 by plant stomatal uptake can in turn cause damage to plant tissues with ramifications for ecosystem and crop health. In many atmospheric and land surface models, the functionality of stomata opening is represented by a bulk stomatal conductance, which is often semi-empirically parameterized and highly fitted to historical observations. A lack of mechanistic linkage to ecophysiological processes such as photosynthesis may render models inadequate to represent plant-mediated responses of atmospheric chemistry to long-term changes in CO2, climate, and short-lived air pollutant concentrations. A new ecophysiology module was thus developed to mechanistically simulate land−atmosphere exchange of important gas species in GEOS-Chem, a chemical transport model widely used in atmospheric chemistry studies. The implementation not only allows for dry deposition to be coupled with plant ecophysiology but also enables plant and crop productivity and functions to respond dynamically to atmospheric chemical changes. We conduct simulations to evaluate the effects of the ecophysiology module on simulated dry deposition velocity and concentration of surface O3 against an observation-derived dataset known as SynFlux. Our estimated stomatal conductance and dry deposition velocity of O3 are close to SynFlux with root-mean-squared errors (RMSEs) below 0.3 cm s−1 across different plant functional types (PFTs), despite an overall positive bias in surface O3 concentration (by up to 16 ppbv). Representing ecophysiology was found to reduce the simulated biases in deposition fluxes from the prior model but worsen the positive biases in simulated O3 concentrations. The increase in positive concentration biases is mostly attributable to the ecophysiology-based stomatal conductance being generally smaller (and closer to SynFlux values) than that estimated by the prior semi-empirical formulation, calling for further improvements in non-stomatal depositional and non-depositional processes relevant for O3 simulations. The estimated global O3 deposition flux is 864 Tg O3 yr−1 with GEOS-Chem, and the new module decreases this estimate by 92 Tg O3 yr−1. Estimated global gross primary production (GPP) without O3 damage is 119 Pg C yr−1. O3-induced reduction in GPP is 4.2 Pg C yr−1 (3.5 %). An elevated CO2 scenario (580 ppm) yields higher global GPP (+16.8 %) and lower global O3 depositional sink (−3.3 %). Global isoprene emission simulated with a photosynthesis-based scheme is 317.9 Tg C yr−1, which is 31.2 Tg C yr−1 (−8.9 %) less than that calculated using the MEGAN (Model of Emissions of Gases and Aerosols from Nature) emission algorithm. This new model development dynamically represents the two-way interactions between vegetation and air pollutants and thus provides a unique capability in evaluating vegetation-mediated processes and feedbacks that can shape atmospheric chemistry and air quality, as well as pollutant impacts on vegetation health, especially for any timescales shorter than the multidecadal timescale.

Biogenic isoprene emissions, dry deposition velocity, and surface ozone concentration during summer droughts, heatwaves, and normal conditions in southwestern Europe

Abstract. At high concentrations, tropospheric ozone (O3) deteriorates air quality, inducing adverse effects on human and ecosystem health. Meteorological conditions are key to understanding the variability in O3 concentration, especially during extreme weather events. In addition to modifying photochemistry and atmospheric transport, droughts and heatwaves affect the state of vegetation and thus the biosphere–troposphere interactions that control atmospheric chemistry, namely biogenic emissions of precursors and gas dry deposition. A major source of uncertainty and inaccuracy in the simulation of surface O3 during droughts and heatwaves is the poor representation of such interactions. This publication aims at quantifying the isolated and combined impacts of both extremes on biogenic isoprene (C5H8) emissions, O3 dry deposition, and surface O3 in southwestern Europe. First, the sensitivity of biogenic C5H8 emissions, O3 dry deposition, and surface O3 to two specific effects of droughts, the decrease in soil moisture and in biomass, is analysed for the extremely dry summer 2012 using the biogenic emission model MEGANv2.1 and the chemistry transport model CHIMEREv2020r1. Despite a significant decrease in biogenic C5H8 emissions and O3 dry deposition velocity, characterized by a large spatial variability, the combined effect on surface O3 concentration remains limited (between +0.5 % and +3 % over the continent). The variations in simulated biogenic C5H8 emissions, O3 dry deposition, and surface O3 during the heatwaves and agricultural droughts are then analysed for summer 2012 (warm and dry), 2013 (warm), and 2014 (relatively wet and cool). We compare the results with large observational data sets, namely O3 concentrations from Air Quality (AQ) e-Reporting (2000–2016) and total columns of formaldehyde (HCHO, which is used as a proxy for biogenic emissions of volatile organic compounds) from the Ozone Monitoring Instrument (OMI) of the Aura satellite (2005–2016). Based on a cluster approach using the percentile limit anomalies indicator, we find that C5H8 emissions increase by +33 % during heatwaves compared to normal conditions, do not vary significantly during all droughts (either accompanied or not by a heatwave), and decrease by −16 % during isolated droughts. OMI data confirm an average increase in HCHO during heatwaves (between +15 % and +31 % depending on the product used) and decrease in HCHO (between −2 % and −6 %) during isolated droughts over the 2005–2016 summers. Simulated O3 dry deposition velocity decreases by −25 % during heatwaves and −35 % during all droughts. Simulated O3 concentrations increase by +7 % during heatwaves and by +3 % during all droughts. Compared to observations, CHIMERE tends to underestimate the daily maximum O3. However, similar sensitivity to droughts and heatwaves are obtained. The analysis of the AQ e-Reporting data set shows an average increase of +14 % during heatwaves and +7 % during all droughts over the 2000–2016 summers (for an average daily concentration value of 69 µg m−3 under normal conditions). This suggests that identifying the presence of combined heatwaves is fundamental to the study of droughts on surface–atmosphere interactions and O3 concentration.

Effects of chemical boundary conditions on simulated O3 concentrations in China and their chemical mechanisms

Chemical boundary conditions (BCs) are important inputs for regional chemical transport models. In this study, we use the brute-force method (BFM), process analysis (PA) and response surface model (RSM) to quantify the effects of BCs on simulated O3 concentrations in different regions of China by the weather research and forecasting with chemistry (WRF–Chem) model. We combine the model with an integrated gas-phase reaction rate (IRR) tool to further analyze the changes in the O3 chemical mechanisms. Our results show that the simulated O3 concentrations in western cities are significantly affected by the O3 in the BCs (BC-O3), which can increase the maximum simulated O3 concentration, such as in Lanzhou (36.6 μg/m3, 26.3 %), Wuhai (30.1 μg/m3, 25.5 %) and Urumqi (50.7 μg/m3, 41.2 %). In contrast, O3 generation in the eastern region is dominated by emissions. Subsequently, we compare the reaction rate changes in O3 generation and consumption under the effects of BC-O3 in the western city of Urumqi and the eastern city of Beijing. The results show that in Beijing, the O3 concentration and the related chemical reaction rates undergo little change, while in Urumqi, the concentration and reaction rates have significant differences. The BC-O3 significantly accelerates the O3 photochemical reaction process in Urumqi, resulting in increased O3 generation and consumption reaction rates; additionally, there may be a chemical reaction pathway for the formation of O3: BC-O3 + NO → NO2 + hv → O + O2 → O3. BC-O3 transmission is the main pathway of changes in the simulated O3 concentration in the study area, and the chemical reactions between BC-O3 and local pollutants are primarily characterized by O3 consumption. In conclusion, the study shows the importance of BCs for regional model simulation while providing supporting information for O3 formation in model studies.

Contrasting chemical environments in summertime for atmospheric ozone across major Chinese industrial regions: the effectiveness of emission control strategies

Abstract. The United Kingdom Chemistry and Aerosols (UKCA) chemistry–climate model is used to quantify the differences in chemical environment for surface O3 for six major industrial regions across China in summer 2016. We first enhance the UKCA gas-phase chemistry scheme by incorporating reactive volatile organic compound (VOC) tracers that are necessary to represent urban and regional-scale O3 photochemistry. We demonstrate that the model with the improved chemistry scheme captures the observed magnitudes and diurnal patterns of surface O3 concentrations across these regions well. Simulated O3 concentrations are highest in Beijing and Shijiazhuang on the North China Plain and in Chongqing, lower in Shanghai and Nanjing in the Yangtze River Delta, and lowest in Guangzhou in the Pearl River Delta despite the highest daytime O3 production rates in Guangzhou. NOx / VOC and H2O2 / HNO3 ratios indicate that O3 production across all regions except Chongqing is VOC limited. We confirm this by constructing O3 response surfaces for each region changing NOx and VOC emissions and further contrast the effectiveness of measures to reduce surface O3 concentrations. In VOC-limited regions, reducing NOx emissions by 20 % leads to a substantial O3 increase (11 %) in Shanghai. We find that reductions in NOx emissions alone of more than 70 % are required to decrease O3 concentrations across all regions. Reductions in VOC emissions alone of 20 % produce the largest decrease (−11 %) in O3 levels in Shanghai and Guangzhou and the smallest decrease (−1 %) in Chongqing. These responses are substantially different from those currently found in highly populated regions in other parts of the world, likely due to higher NOx emission levels in these Chinese regions. Our work provides an assessment of the effectiveness of emission control strategies to mitigate surface O3 pollution in these major industrial regions and emphasises that combined NOx and VOC emission controls play a pivotal role in effectively offsetting high O3 levels. It also demonstrates new capabilities in capturing regional air pollution that will permit this model to be used for future studies of regional air-quality–climate interactions.

Impacts of Ozone‐Vegetation Interactions on Ozone Pollution Episodes in North China and the Yangtze River Delta

AbstractPersistent ozone (O3) pollution episodes (OPEs, with regionally averaged maximum daily 8‐h average (MDA8) O3 concentration exceeding 80 ppbv and lasting for 5 days or longer) occurred frequently in megalopolis over eastern China in recent years. Here, we apply a newly developed chemistry‐biosphere model (GEOS‐Chem‐YIBs) to comprehensively quantify the vegetation contributions to OPEs in North China and Yangtze River Delta (YRD) over May to October in 2014–2017. The simulated MDA8 O3 concentrations increases 16.7 and 16.6 ppbv) during OPEs in North China and YRD, of which about 1.4 ppbv (8.4%) and 3.8 ppbv (22.9%) are caused by the processes including increased BVOC emissions and reduced stomatal dry deposition. Furthermore, the O3 damages to vegetation stomata slightly increase seasonal‐mean O3 concentrations by <1 ppbv, but such effects are not exacerbated during OPEs despite the high O3 exposure.

Smog Chamber Study on the Ozone Formation Potential of Acetaldehyde

Acetaldehyde is one of the important VOC species of O3 precursors in the atmospheric environment. The influences of relative humidity (RH) and initial VOC/NOx ratio (RCN) on the formation of O3 are studied in smog chamber experiments, and the MCM v3.3.1 mechanism of acetaldehyde is modified based on the experimental results. In low-RH conditions (RH = 11.6%±1.1%), the O3 concentration at 6 h increases first and then decreases with the increase of RCN, and the RCN at the inflection point of O3 concentrations is 3.2. In high-RH experiments (RH = 78.8%±1.0%), variation of the O3 concentration at 6 h with RCN is similar to that in low-RH experiments, but the RCN at the inflection point is 2.8. RH has no significant effect on the O3 concentrations under low RCN ( 3). Compared with the experimental results, original MCM v3.3.1 greatly underestimates the O3 concentrations. Addition of both the photolysis process of peroxyacetyl nitrate and the photolysis process of HNO3 on the reactor surface into the original MCM can reduce the difference between the simulated O3 concentrations and the experimental results at 6 h from 24%−35% and 17%−49% to 6%−26% and 10%−42% under low- and high-RH conditions, respectively. The maximum incremental reactivity (MIR) of acetaldehyde simulated with the modified MCM is 4.0 ppb ppb−1 without considering the effect of other VOCs.

Description of the NASA GEOS Composition Forecast Modeling System GEOS-CF v1.0.

The Goddard Earth Observing System composition forecast (GEOS‐CF) system is a high‐resolution (0.25°) global constituent prediction system from NASA's Global Modeling and Assimilation Office (GMAO). GEOS‐CF offers a new tool for atmospheric chemistry research, with the goal to supplement NASA's broad range of space‐based and in‐situ observations. GEOS‐CF expands on the GEOS weather and aerosol modeling system by introducing the GEOS‐Chem chemistry module to provide hindcasts and 5‐days forecasts of atmospheric constituents including ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and fine particulate matter (PM2.5). The chemistry module integrated in GEOS‐CF is identical to the offline GEOS‐Chem model and readily benefits from the innovations provided by the GEOS‐Chem community. Evaluation of GEOS‐CF against satellite, ozonesonde and surface observations for years 2018–2019 show realistic simulated concentrations of O3, NO2, and CO, with normalized mean biases of −0.1 to 0.3, normalized root mean square errors between 0.1–0.4, and correlations between 0.3–0.8. Comparisons against surface observations highlight the successful representation of air pollutants in many regions of the world and during all seasons, yet also highlight current limitations, such as a global high bias in SO2 and an overprediction of summertime O3 over the Southeast United States. GEOS‐CF v1.0 generally overestimates aerosols by 20%–50% due to known issues in GEOS‐Chem v12.0.1 that have been addressed in later versions. The 5‐days forecasts have skill scores comparable to the 1‐day hindcast. Model skills can be improved significantly by applying a bias‐correction to the surface model output using a machine‐learning approach.

The impact of ship emissions on air quality and human health in the Gothenburg area – Part II: Scenarios for 2040

Abstract. Shipping is an important source of air pollutants, from the global to the local scale. Ships emit substantial amounts of sulfur dioxides, nitrogen dioxides, and particulate matter in the vicinity of coasts, threatening the health of the coastal population, especially in harbour cities. Reductions in emissions due to shipping have been targeted by several regulations. Nevertheless, effects of these regulations come into force with temporal delays, global ship traffic is expected to grow in the future, and other land-based anthropogenic emissions might decrease. Thus, it is necessary to investigate combined impacts to identify the impact of shipping activities on air quality, population exposure, and health effects in the future. We investigated the future effect of shipping emissions on air quality and related health effects considering different scenarios of the development of shipping under current regional trends of economic growth and already decided regulations in the Gothenburg urban area in 2040. Additionally, we investigated the impact of a large-scale implementation of shore electricity in the Port of Gothenburg. For this purpose, we established a one-way nested chemistry transport modelling (CTM) system from the global to the urban scale, to calculate pollutant concentrations, population-weighted concentrations, and health effects related to NO2, PM2.5, and O3. The simulated concentrations of NO2 and PM2.5 in future scenarios for the year 2040 are in general very low with up to 4 ppb for NO2 and up to 3.5 µg m−3 PM2.5 in the urban areas which are not close to the port area. From 2012 the simulated overall exposure to PM2.5 decreased by approximately 30 % in simulated future scenarios; for NO2 the decrease was over 60 %. The simulated concentrations of O3 increased from the year 2012 to 2040 by about 20 %. In general, the contributions of local shipping emissions in 2040 focus on the harbour area but to some extent also influence the rest of the city domain. The simulated impact of onshore electricity implementation for shipping in 2040 shows reductions for NO2 in the port of up to 30 %, while increasing O3 of up to 3 %. Implementation of onshore electricity for ships at berth leads to additional local reduction potentials of up to 3 % for PM2.5 and 12 % for SO2 in the port area. All future scenarios show substantial decreases in population-weighted exposure and health-effect impacts.

Urban population exposure to NO<sub><i>x</i></sub> emissions from local shipping in three Baltic Sea harbour cities – a generic approach

Abstract. Ship emissions in ports can have a significant impact on local air quality (AQ), population exposure and therefore human health in harbour cities. We determined the impact of shipping emissions in harbours on local AQ and population exposure in the Baltic Sea harbour cities Rostock (Germany), Riga (Latvia) and the urban agglomeration of Gdańsk–Gdynia (Poland) for 2012. An urban AQ study was performed using a global-to-local chemistry transport model chain with the EPISODE-CityChem model for the urban scale. We simulated NO2, O3 and PM concentrations in 2012 with the aim of determining the impact of local shipping activities on population exposure in Baltic Sea harbour cities. Based on simulated concentrations, dynamic population exposure to outdoor NO2 concentrations for all urban domains was calculated. We developed and used a novel generic approach to model dynamic population activity in different microenvironments based on publicly available data. The results of the new approach are hourly microenvironment-specific population grids with a spatial resolution of 100 m × 100 m. We multiplied these grids with surface pollutant concentration fields of the same resolution to calculate total population exposure. We found that the local shipping impact on NO2 concentrations is significant, contributing 22 %, 11 % and 16 % to the total annually averaged grid mean concentration for Rostock, Riga and Gdańsk–Gdynia, respectively. For PM2.5, the contribution of shipping is substantially lower, at 1 %–3 %. When it comes to microenvironment-specific exposure to annual NO2, the highest exposure to NO2 from all emission sources was found in the home environment (54 %–59 %). Emissions from shipping have a high impact on NO2 exposure in the port area (50 %–80 %), while the influence in home, work and other environments is lower on average (3 %–14 %) but still has high impacts close to the port areas and downwind of them. Besides this, the newly developed generic approach allows for dynamic population-weighted outdoor exposure calculations in European cities without the necessity of individually measured data or large-scale surveys on population data.

Global and local sensitivity analysis of urban background ozone modelled with a simplified photochemical scheme

The Generic Reaction Set (GRS) is a simplified photochemical scheme that allows estimation of ozone (O3) concentrations resulting from nitrogen oxides (NOx) and reactive organic compounds (ROC) emissions in an urban area. Due to its simplicity and relative good performance, it has been included in the algorithms of many atmospheric dispersion models. However, few works assess quantitatively the sensitivity of simulated O3 concentrations to the uncertainty in the GRS input variables. In order to do so, in this work we carry out two sensitivity analyses: global (applying an atmospheric dispersion model to a representative real case) and local (using a box-GRS model under a wide range of hypothetical conditions). In the global study, a Monte Carlo analysis is performed to estimate the uncertainty of maximum O3 concentrations that is caused by possible errors in the GRS input variables, applying the DAUMOD-GRS model in the Metropolitan Area of Buenos Aires. Results show that the initial concentration of O3 dominates at all analysed receptors while that of NOx can make a non-negligible contribution if its error is relatively large. In order to further analyse the effect of each parameter individually, a local sensitivity analysis is performed using a box-GRS model under a wide range of conditions. Except for the small NOx-limited region, sensitivity indexes computed for ±1 ppb changes in the initial concentrations of ROC, NOx and O3 are in the ranges 0.00–0.35, 0.00–0.81 and 0.05–0.96, respectively, with that of ozone dominating over most of the isopleth diagram space. In turn, its relative impact increases with decreasing ROC initial concentrations. Reaction rate coefficients have also different effects on O3 peak concentrations depending on the initial conditions of the system. Our results show quantitatively the change of dominant variables under different environments.

Uncertainties in O3 concentrations simulated by CMAQ over Japan using four chemical mechanisms

Uncertainty was evaluated in four chemical mechanisms pertaining to O3 concentrations predicted over Japan by the Community Multiscale Air Quality Model (CMAQ) to investigate factors contributing to model overestimation of O3 concentration. The model setting and meteorological and emissions input data were obtained from a Japanese model inter-comparison project, Japan's Study for Reference Air Quality Modeling (J-STREAM). The compared gas-phase chemical mechanisms included the Carbon Bond Mechanism (CB05TUCL), Regional Atmospheric Chemical Mechanism (RACM2), and two mechanisms developed by the State Air Pollution Research Center (SAPRC), namely SAPRC07TC and SAPRC99. The O3 concentrations produced by CB05TUCL were low compared to those from SAPRC07TC. The RACM2 concentrations were similar to those from SAPRC07TC over inland Japan and lower over the sea. The concentrations from SAPRC99 were higher than those from SAPRC07TC in urban areas and lower in other areas. At most of the monitoring sites in Japan, the modeled O3 concentrations were higher than those from observations. Module overestimation can be ranked in the order of SAPRC99 > SAPRC07TC > RACM2 > CB05TUCL for urban sites and SAPRC07TC > SAPRC99 > RACM2 > CB05TUCL for rural sites. The concentration differences between the chemical mechanisms were within 10 ppb, whereas those between the observed and simulated O3 concentrations reached 40 ppb. Differences in O3 concentrations between the chemical mechanisms accounted for only a part of the model overestimation, while the rest remained unexplained. To investigate factors influencing the differences in O3 concentration between the chemical mechanisms, domain- and 10-vertical-layer-average hourly integrated process rates (IPRs) and integrated reaction rates (IRRs) were calculated using process analysis in CMAQ. The O3 chemical IPRs from SAPRC07TC were higher than those from CB05TUCL and RACM2. The SAPRC99 IPRs were higher than those from SAPRC07TC in urban areas and lower in other areas. The IRR differences in the chemical mechanisms showed that IRRs for the O3 and NO reactions were responsible for the differences in the O3 chemical IPR. The coefficients of determination between the O3 chemical process IPR and IRR differences in the chemical mechanisms were highest for the HO2-NO reaction in CB05TUCL and SAPRC99 and the RO2-NO reaction in RACM2. Differences in reaction rate constants and lumped volatile organic compounds may have caused some of the differences in O3 production between the chemical mechanisms.

The influence of model spatial resolution on simulated ozone and fine particulate matter for Europe: implications for health impact assessments

Abstract. We examine the impact of model horizontal resolution on simulated concentrations of surface ozone (O3) and particulate matter less than 2.5 µm in diameter (PM2.5), and the associated health impacts over Europe, using the HadGEM3–UKCA chemistry–climate model to simulate pollutant concentrations at a coarse (∼ 140 km) and a finer (∼ 50 km) resolution. The attributable fraction (AF) of total mortality due to long-term exposure to warm season daily maximum 8 h running mean (MDA8) O3 and annual-average PM2.5 concentrations is then calculated for each European country using pollutant concentrations simulated at each resolution. Our results highlight a seasonal variation in simulated O3 and PM2.5 differences between the two model resolutions in Europe. Compared to the finer resolution results, simulated European O3 concentrations at the coarse resolution are higher on average in winter and spring (∼ 10 and ∼ 6 %, respectively). In contrast, simulated O3 concentrations at the coarse resolution are lower in summer and autumn (∼ −1 and ∼ −4 %, respectively). These differences may be partly explained by differences in nitrogen dioxide (NO2) concentrations simulated at the two resolutions. Compared to O3, we find the opposite seasonality in simulated PM2.5 differences between the two resolutions. In winter and spring, simulated PM2.5 concentrations are lower at the coarse compared to the finer resolution (∼ −8 and ∼ −6 %, respectively) but higher in summer and autumn (∼ 29 and ∼ 8 %, respectively). Simulated PM2.5 values are also mostly related to differences in convective rainfall between the two resolutions for all seasons. These differences between the two resolutions exhibit clear spatial patterns for both pollutants that vary by season, and exert a strong influence on country to country variations in estimated AF for the two resolutions. Warm season MDA8 O3 levels are higher in most of southern Europe, but lower in areas of northern and eastern Europe when simulated at the coarse resolution compared to the finer resolution. Annual-average PM2.5 concentrations are higher across most of northern and eastern Europe but lower over parts of southwest Europe at the coarse compared to the finer resolution. Across Europe, differences in the AF associated with long-term exposure to population-weighted MDA8 O3 range between −0.9 and +2.6 % (largest positive differences in southern Europe), while differences in the AF associated with long-term exposure to population-weighted annual mean PM2.5 range from −4.7 to +2.8 % (largest positive differences in eastern Europe) of the total mortality. Therefore this study, with its unique focus on Europe, demonstrates that health impact assessments calculated using modelled pollutant concentrations, are sensitive to a change in model resolution by up to ∼ ±5 % of the total mortality across Europe.

The chemical effects on the summertime ozone in the upper troposphere and lower stratosphere over the Tibetan Plateau and the South Asian monsoon region

We use the global three-dimensional Goddard Earth Observing System chemical transport model with the Universal tropospheric–stratospheric Chemistry eXtension mechanism to examine the contributions of the chemical processes to summertime O3 in the upper troposphere and lower stratosphere (UTLS) over the Tibetan Plateau and the South Asian monsoon region (TP/SASM). Simulated UTLS O3 concentrations are evaluated by comparisons with Microwave Limb Sounder products and net chemical production of O3 (NPO3) are evaluated by comparisons with model results in previous studies. Simulations show that the chemical processes lead to an increase in O3 concentration, which is opposite to the effect of O3 transport in the UTLS over the TP/SASM region throughout the boreal summer. NPO3 in UTLS over the TP/SASM region is the largest in summer. Elevated values (0.016–0.020 Tg year−1) of the seasonal mean NPO3 are simulated to locate at 100 hPa in the TP/SASM region, where the mixing ratios of O3 are low and those of O3 precursors (NOx, VOCs, and CO) are high. The high concentrations of O3 precursors (NOx, VOCs, and CO) together with the active photochemical reactions of NO2 in the UTLS over the TP/SASM region during summertime could be important reasons for the enhancement of $${\text{NP}}_{{{\text{O}}_{3} }}$$ over the studied region.

Influence of an urban canopy model and PBL schemes on vertical mixing for air quality modeling over Greater Paris

Impacts of meteorological modeling in the planetary boundary layer (PBL) and urban canopy model (UCM) on the vertical mixing of pollutants are studied. Concentrations of gaseous chemical species, including ozone (O3) and nitrogen dioxide (NO2), and particulate matter over Paris and the near suburbs are simulated using the 3-dimensional chemistry-transport model Polair3D of the Polyphemus platform. Simulated concentrations of O3, NO2 and PM10/PM2.5 (particulate matter of aerodynamic diameter lower than 10 μm/2.5 μm, respectively) are first evaluated using ground measurements. Higher surface concentrations are obtained for PM10, PM2.5 and NO2 with the MYNN PBL scheme than the YSU PBL scheme because of lower PBL heights in the MYNN scheme. Differences between simulations using different PBL schemes are lower than differences between simulations with and without the UCM and the Corine land-use over urban areas. Regarding the root mean square error, the simulations using the UCM and the Corine land-use tend to perform better than the simulations without it. At urban stations, the PM10 and PM2.5 concentrations are over-estimated and the over-estimation is reduced using the UCM and the Corine land-use. The ability of the model to reproduce vertical mixing is evaluated using NO2 measurement data at the upper air observation station of the Eiffel Tower, and measurement data at a ground station near the Eiffel Tower. Although NO2 is under-estimated in all simulations, vertical mixing is greatly improved when using the UCM and the Corine land-use. Comparisons of the modeled PM10 vertical distributions to distributions deduced from surface and mobile lidar measurements are performed. The use of the UCM and the Corine land-use is crucial to accurately model PM10 concentrations during nighttime in the center of Paris. In the nocturnal stable boundary layer, PM10 is relatively well modeled, although it is over-estimated on 24 May and under-estimated on 25 May. However, PM10 is under-estimated on both days in the residual layer, and over-estimated on both days over the residual layer. The under-estimations in the residual layer are partly due to difficulties to estimate the PBL height, to an over-estimation of vertical mixing during nighttime at high altitudes and to uncertainties in PM10 emissions. The PBL schemes and the UCM influence the PM vertical distributions not only because they influence vertical mixing (PBL height and eddy–diffusion coefficient), but also horizontal wind fields and humidity. However, for the UCM, it is the influence on vertical mixing that impacts the most the PM10 vertical distribution below 1.5 km.

Influence of regional climate change on meteorological characteristics and their subsequent effect on ozone dispersion in Taiwan

The objective of this study is to understand the influence of regional climate change on local meteorological conditions and their subsequent effect on local ozone (O3) dispersion in Taiwan. The 33-year NCEP-DOE Reanalysis 2 (NNR2) data set (1979–2011) was analyzed to understand the variations in regional-scale atmospheric conditions in East Asia and the western North Pacific. To save computational processing time, two scenarios representative of past (1979–86) and current (2004–11) atmospheric conditions were selected but only targeting the autumn season (September, October and November) when the O3 concentrations were at high levels. Numerical simulations were performed using weather research and forecasting (WRF) model and Community Multiscale Air Quality (CMAQ) model for the past and current scenarios individually but only for the month of October because of limited computational resources.Analysis of NNR2 data exhibited increased air temperature, weakened Asian continental anticyclone, enhanced northeasterly monsoonal flow, and a deepened low-pressure system forming near Taiwan. With enhanced evaporation from oceans along with a deepened low-pressure system, precipitation amounts increased in Taiwan in the current scenario. As demonstrated in the WRF simulation, the land surface physical process responded to the enhanced precipitation resulting in damper soil conditions, and reduced ground temperatures that in turn restricted the development of boundary layer height. The weakened land–sea breeze flow was simulated in the current scenario. With reduced dispersion capability, air pollutants would tend to accumulate near the emission source leading to a degradation of air quality in this region. The conditions would be even worse in southwestern Taiwan due to the fact that stagnant wind fields would occur more frequently in the current scenario. On the other hand, in northern Taiwan, the simulated O3 concentrations are lower during the day in the current scenario due to the enhanced cloud conditions and reduced solar radiation.

Aviation 2006 NO<sub>x</sub>-induced effects on atmospheric ozone and HO<sub>x</sub> in Community Earth System Model (CESM)

Abstract. The interaction between atmospheric chemistry and ozone (O3) in the upper troposphere–lower stratosphere (UTLS) presents a major uncertainty in understanding the effects of aviation on climate. In this study, two configurations of the atmospheric model from the Community Earth System Model (CESM), Community Atmosphere Model with Chemistry, Version 4 (CAM4) and Version 5 (CAM5), are used to evaluate the effects of aircraft nitrogen oxide (NOx = NO + NO2) emissions on ozone and the background chemistry in the UTLS. CAM4 and CAM5 simulations were both performed with extensive tropospheric and stratospheric chemistry including 133 species and 330 photochemical reactions. CAM5 includes direct and indirect aerosol effects on clouds using a modal aerosol module (MAM), whereby CAM4 uses a bulk aerosol module, which can only simulate the direct effect. To examine the accuracy of the aviation NOx-induced ozone distribution in the two models, results from the CAM5 and CAM4 simulations are compared to ozonesonde data. Aviation NOx emissions for 2006 were obtained from the AEDT (Aviation Environmental Design Tool) global commercial aircraft emissions inventory. Differences between simulated O3 concentrations and ozonesonde measurements averaged at representative levels in the troposphere and different regions are 13% in CAM5 and 18% in CAM4. Results show a localized increase in aviation-induced O3 concentrations at aviation cruise altitudes that stretches from 40° N to the North Pole. The results indicate a greater and more disperse production of aviation NOx-induced ozone in CAM5, with the annual tropospheric mean O3 perturbation of 1.2 ppb (2.4%) for CAM5 and 1.0 ppb (1.9%) for CAM4. The annual mean O3 perturbation peaks at about 8.2 ppb (6.4%) and 8.8 ppb (5.2%) in CAM5 and CAM4, respectively. Aviation emissions also result in increased hydroxyl radical (OH) concentrations and methane (CH4) loss rates, reducing the tropospheric methane lifetime in CAM5 and CAM4 by 1.69 and 1.40%, respectively. Aviation NOx emissions are associated with an instantaneous change in global mean short-term O3 radiative forcing (RF) of 40.3 and 36.5 mWm−2 in CAM5 and CAM4, respectively.

The impact of satellite-adjusted NO<sub>x</sub> emissions on simulated NO<sub>x</sub> and O<sub>3</sub> discrepancies in the urban and outflow areas of the Pacific and Lower Middle US

Abstract. We analyze the simulation results from a CMAQ model and GOME-2 NO2 retrievals over the United States for August 2009 to estimate the model-simulated biases of NOx concentrations over six geological regions (Pacific Coast = PC, Rocky Mountains = RM, Lower Middle = LM, Upper Middle = UM, Southeast = SE, Northeast = NE). By comparing GOME-2 NO2 columns to corresponding CMAQ NO2 columns, we produced satellite-adjusted NOx emission ("GOME2009") and compared baseline emission ("BASE2009") CMAQ simulations with GOME2009 CMAQ runs. We found that the latter exhibited decreases of −5.6%, −12.3%, −21.3%, and −15.9 % over the PC, RM, LM, and SE regions, respectively, and increases of +2.3% and +10.0% over the UM and NE regions. In addition, we found that changes in NOx emissions generally mitigate discrepancies between the surface NOx concentrations of baseline CMAQ and those of AQS at EPA AQS stations (mean bias of +19.8% to −13.7% over PC, −13.8% to −36.7% over RM, +149.7% to −1.8% over LM, +22.5% to −7.8% over UM, +31.3% to −7.9% over SE, and +11.6% to +0.7% over NE). The relatively high simulated NOx biases from baseline CMAQ over LM (+149.7%) are likely the results of over-predictions of simulated NOx emissions, which could shed light on those from global/regional Chemical Transport Models. We also perform more detailed investigations on surface NOx and O3 concentrations in two urban and outflow areas, PC (e.g., Los Angeles, South Pasadena, Anaheim, La Habra and Riverside) and LM (e.g., Houston, Beaumont and Sulphur). From two case studies, we found that the GOME2009 emissions decreased surface NOx concentrations significantly in the urban areas of PC (up to 30 ppbv) and in those of LM (up to 10 ppbv) during the daytime and that simulated NOx concentrations from CMAQ with GOME2009 compare well to those of in-situ AQS observations. A significant reduction in NOx concentrations resulted in a comparable increase in surface O3 concentrations in the urban areas of PC (up to 30 ppbv) and the resulting simulated O3 concentrations compare well with in-situ surface O3 observations over South Pasadena, Anaheim, and Riverside. Over Houston, Beaumont, and Sulphur, large reductions in NOx emissions from CMAQ with GOME2009 coincides with large reduced concentrations of simulated NOx. These concentrations are similar to those of the EPA AQS NOx observations. However, the resulting simulated increase in surface O3 at the urban stations in Houston and Sulphur exacerbated preexisting high O3 over-predictions of the baseline CMAQ. This study implies that simulated low O3 biases in the urban areas of PC are likely caused by simulated high NOx biases, but high O3 biases in the urban areas of LM cannot be explained by simulated high NOx biases over the region. This study also suggests that both in-situ surface NOx and O3 observations should be used simultaneously to resolve issues pertaining to simulated high/low O3 bias and that remote-sensing data could be used as a constraint for bottom-up emissions. In addition, we also found that daytime O3 reductions over the outflow regions of LM following large reductions in NOx emissions in the urban areas are significantly larger than they are over outflow regions of PC. These findings provide policymakers in the two regions with information critical to establishing strategies for mitigating air pollution.

Impacts of aerosols on surface-layer ozone concentrations in China through heterogeneous reactions and changes in photolysis rates

We quantify the impacts of aerosols on distributions and concentrations of O3 over China through heterogeneous reactions and changes in photolysis rates using the global chemical transport model GEOS-Chem. Aerosols considered include sulfate, nitrate, ammonium, organic carbon, and black carbon. Consideration of the impacts of aerosols improves the simulated O3 concentrations in China; the averaged biases in simulated O3 concentrations in China are +9% and +33% with and without the impacts of aerosols, respectively. The impacts of heterogeneous reactions on O3 are simulated to exhibit large spatial and temporal variations, and those of aerosols through altering photolysis rates are simulated to be small. Accounting for hydrolysis of N2O5, irreversible absorption of NO2 and NO3 on wet aerosols, and the uptake of HO2 by aerosols, O3 concentrations are simulated to decrease by 8–12 ppbv in northern China and to increase by 3–6 ppbv in southern China in winter, and reductions in O3 of exceeding 6 ppbv are simulated in a large fraction of China in other seasons. With the assumed uptake coefficients in this work, the hydrolysis of N2O5 is simulated to have a dominant role in reducing O3 concentrations all over China, as a result of the large reductions in NOx in the lower to middle troposphere in the northern mid-latitudes. On the contrary, the absorption of NO2 and NO3 is found to increase O3 concentrations by 3–10 ppbv in eastern China in winter because of the increases in chemical production of O3 in the VOC-limited regions. The impact of aerosols on O3 concentration through heterogeneous reactions is characterized in this work by the ratio of change in O3 concentration to local PM2.5 level (ROP = Δ[O3]/[PM2.5]). The locations of maximum reductions in O3 are not necessarily the places of maximum aerosol concentrations; the annual mean values of ROP are calculated to be −0.14, –0.17, –0.27, and −0.16 ppbv (μg m−3)−1 over the heavily polluted regions of Beijing–Tianjin–Tanggu, Yangtze River Delta, Pearl River Delta, and Sichuan Basin, respectively. Values of ROP are determined by both local heterogeneous reactions and transport of O3 from surrounding areas.

Simulation of the interannual variations of biogenic emissions of volatile organic compounds in China: Impacts on tropospheric ozone and secondary organic aerosol

We use the MEGAN (Model of emissions of Gases and Aerosols from Nature) module embedded within the global three-dimensional Goddard Earth Observing System chemical transport model (GEOS-Chem) to simulate the interannual variations in biogenic volatile organic compound (BVOC) emissions and concentrations of ozone and secondary organic aerosols (SOA) in China over years 2001–2006. To have better representation of biogenic emissions, we have updated in the model the land cover and leaf area index in China using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite measurements, and we have developed a new classification of vegetation with 21 plant functional types. Estimated annual BVOC emission in China averaged over 2001–2006 is 18.85 Tg C yr−1, in which emissions of isoprene, monoterpenes, and other reactive volatile organic compounds account for 50.9%, 15.0%, and 34.1%, respectively. The simulated BVOC emissions in China have large interannual variations. The values of regionally averaged absolute percent departure from the mean (APDM) of isoprene emissions are in the range of 21–42% in January and 15–28% in July. The APDM values of monoterpene emissions are 14–32% in January and 10–21% in July, which are generally smaller than those of isoprene emissions. Model results indicate that the interannual variations in isoprene emissions are more dependent on variations in meteorological fields, whereas the interannual variations in monoterpene emissions are more sensitive to changes in vegetation parameters. With fixed anthropogenic emissions, as a result of the variations in both meteorological parameters and vegetation, simulated O3 concentrations show interannual variations of 0.8–5 ppbv (or largest APDM values of 4–15%), and simulated SOA shows APDM values of 5–15% in southwestern China in January as well as 10–25% in southeastern and 20–35% in northeastern China in July. On a regional mean basis, the interannual variations in BVOCs alone can lead to 2–5% differences in simulated O3 and SOA in summer.