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Fire is an important source of ozone (O3) precursors. The formation of surface O3 can cause damage to vegetation and reduce stomatal conductance. Such processes can feed back to inhibit dry deposition and indirectly enhance surface O3. Here, we apply a fully coupled chemistry–vegetation model to estimate the indirect contributions of global fires to surface O3 through O3–vegetation feedback during 2005–2012. Fire emissions directly increase the global annual mean O3 by 1.2 ppbv (5.0 %) with a maximum of 5.9 ppbv (24.4 %) averaged over central Africa by emitting a substantial number of precursors. Considering O3–vegetation feedback, fires additionally increase surface O3 by 0.5 ppbv averaged over the Amazon in October, 0.3 ppbv averaged over southern Asia in April, and 0.2 ppbv averaged over central Africa in April. During extreme O3–vegetation interactions, such a feedback can rise to >0.6 ppbv in these fire-prone areas. Moreover, large ratios of indirect-to-direct fire O3 are found in eastern China (3.7 %) and the eastern US (2.0 %), where the high ambient O3 causes strong O3–vegetation interactions. With the likelihood of increasing fire risks in a warming climate, fires may promote surface O3 through both direct emissions and indirect chemistry–vegetation feedbacks. Such indirect enhancement will cause additional threats to public health and ecosystem productivity.

International audience; Rising air pollution by surface ozone (O 3) in China has induced extensive efforts to control ozone generation in major urban and industrial areas, yet mechanisms ruling the ozone production and loss are not well understood. In particular, ozone levels are strongly influenced by meteorological factors such as relative humidity, but this has been explored only in local situations, and the effect of relative humidity on ozone levels in warm seasons on a large scale in China is still unknown. Here we studied surface ozone, relative humidity, temperature, and other meteorological variables in 74 major cities in China during 2017-2018, focusing on the warm seasons in seven regions. Results show that ozone levels decrease with increasing relative humidity in all cities, with an average correlation coefficient of − 0.58, ranging from − 0.17 in Zhangjiakou to − 0.84 in Hengshui. At high relative humidity levels, above 75%, average ozone levels ranged from 44.6 to 122.5 μg m −3 , which is lower than Chinese quality threshold of hourly average ozone level of 200 μg m −3. The decreases of ozone with relative humidity were more pronounced at high temperature, above 30 °C, than below 25 °C. The increases of ozone with temperature were more pronounced at low relative humidity, below 40%. Overall, our findings reveal that mechanisms ruling surface ozone levels are similar on a large scale. This is promising to design common methods of climate engineering to protect human health.

In the present study, various solar and atmospheric parameters, namely solar radiation, surface ozone, relative humidity, air temperature, and wind speed, have been studied first time for the annular solar eclipse of June 21, 2020, at seven Indian stations simultaneously. All the seven different stations are selected along the solar eclipse path having the maximum eclipse magnitude of 92% and above. The various atmospheric parameters have shown the change in behavior during the solar eclipse duration. The incoming solar radiation has a maximum 95.97% decrease at Ludhiana at the time of maximum obscuration compared to the previous day. The surface ozone concentration has shown a decrease for all the seven stations during the maximum obscuration period. A maximum of 9.25% decrease in surface temperature has been observed nearly after 30 min of the time of maximum obscuration of the eclipse. Relative humidity started increasing at all the stations and showed an inverse trend from temperature. The wind speed was found to suppress during the maximum solar eclipse which may be attributed to stabilization of atmospheric boundary layer due to cooling.

Anomalously low ozone concentrations in the surface atmosphere in central Russia were recorded in spring of 2020. The increase in the ozone content usual for spring due to a seasonal increase in the temperature and light was followed by a monotonic decrease. The maximum daily values of surface ozone concentrations averaged over the month in April 2020 were three times less than the values recorded at the same time in 2019. Both a decrease in regional background ozone concentrations in the near-surface atmosphere and a decrease in the intensity of its photochemical formation were observed. The most probable cause of the observed phenomenon is a decrease in combustion product emissions into the atmosphere in China, Europe, and Russia due to the introduction of special regimes in connection with the COVID-19 pandemic.

Accurate nationwide spatiotemporal distribution of ambient ozone product is critical for environment & health departments and for researches to investigate the influence of ozone for epidemiological studies. Our hybrid method, the novel CAMS (The Copernicus Atmosphere Monitoring Service) ozone improvement (CAO3_I) method, is the first attempt to predict ambient ozone by improving CAMS ozone (CAO3) products. For this novel framework, the SVM (Support Vector Machine) has been adopted for classification through the most significant regional ozone patterns which were extracted through the REOF (Rotated Empirical Orthogonal Function) technique. For each classified region, meteorological data, geographical data, CAMS ozone and ground-sites ozone are fed into random forest for regional regulation training and prediction. The CAO3_I method has shown its great feasibility in daily ozone surface distribution prediction. Based on daily averaged ozone concentrations for each station (STO3), the performance of CAO3 products (R2 = 0.35, RMSE = 25.77 μg/m3, MAPE = 42.06) have significantly improved to CAO3_I (R2 = 0.81, RMSE = 14.10 μg/m3, MAPE = 22.37), which shows above 97.4% R2 and RMSE have been improved. Our model is also capable to predict high level ozone concentrations in summer where the R2 has improved from 0.37 (for CAO3) to 0.81. In comparison with ground monitoring stations, the CAMS ozone improvement results can excellently reflect the distribution of daily ground-level ozone concentration and outperform previous statistical models in predicting ambient O3 concentrations. Therefore, the prediction results and our proposed model can be used for future epidemiological studies and air pollution controlling programs.

Information on the ozone concentration in the surface air and troposphere in the first half of 2020 is presented. The data were acquired at 13 stations in different Russian regions; vertical distributions were obtained with the use of an aircraft laboratory. The excess over the Russian Federation hygienic standards is assessed; the daily average ozone concentration is found to be regularly higher the maximum permissible concentrations at most stations. At some stations, there are features the seasonal variations in the ozone concentration in the period under study different from previous years.

Statistical relationships between surface ozone (O-3) concentration, precursor species and meteorological conditions in continental South Africa were examined from data obtained from measurement stations in north-eastern South Africa. Three multivariate statistical methods were applied in the investigation, i.e. multiple linear regression (MLR), principal component analysis (PCA) and -regression (PCR), and generalised additive model (GAM) analysis. The daily maximum 8-h moving average O-3 concentrations were considered in these statistical models (dependent variable). MLR models indicated that meteorology and precursor species concentrations are able to explain similar to 50% of the variability in daily maximum O-3 levels. MLR analysis revealed that atmospheric carbon monoxide (CO), temperature and relative humidity were the strongest factors affecting the daily O-3 variability. In summer, daily O-3 variances were mostly associated with relative humidity, while winter O-3 levels were mostly linked to temperature and CO. PCA indicated that CO, temperature and relative humidity were not strongly collinear. GAM also identified CO, temperature and relative humidity as the strongest factors affecting the daily variation of O-3. Partial residual plots found that temperature, radiation and nitrogen oxides most likely have a non-linear relationship with O-3,while the relationship with relative humidity and CO is probably linear. An inter-comparison between O-3 levels modelled with the three statistical models compared to measured O-3 concentrations showed that the GAM model offered a slight improvement over the MLR model. These findings emphasise the critical role of regional-scale O-3 precursors coupled with meteorological conditions in daily variances of O-3 levels in continental South Africa.

Tropospheric NO2 and ozone simulations have large uncertainties, but their biases, seasonality, and trends can be improved with NO2 assimilations. We perform global top-down estimates of monthly NOx emissions using two Ozone Monitoring Instrument (OMI) NO2 retrievals (NASAv3 and DOMINOv2) from 2005 to 2016 through a hybrid 4D-Var/mass balance inversion. Discrepancy in NO2 retrieval products is a major source of uncertainties in the top-down NOx emission estimates. The different vertical sensitivities in the two NO2 retrievals affect both magnitude and seasonal variations of top-down NOx emissions. The 12-year averages of regional NOx budgets from the NASA posterior emissions are 37 % to 53 % smaller than the DOMINO posterior emissions. Consequently, the DOMINO posterior surface NO2 simulations greatly reduced the negative biases in China (by 15 %) and the US (by 22 %) compared to surface NO2 measurements. Posterior NOx emissions show consistent trends over China, the US, India, and Mexico constrained by the two retrievals. Emission trends are less robust over South America, Australia, western Europe, and Africa, where the two retrievals show less consistency. NO2 trends have more consistent decreases (by 26 %) with the measurements (by 32 %) in the US from 2006 to 2016 when using the NASA posterior emissions. The performance of posterior ozone simulations has spatial heterogeneities from region to region. On a global scale, ozone simulations using NASA-based emissions alleviate the double peak in the prior simulation of global ozone seasonality. The higher abundances of NO2 from the DOMINO posterior simulations increase the global background ozone concentrations and therefore reduce the negative biases more than the NASA posterior simulations using GEOS-Chem v12 at remote sites. Compared to surface ozone measurements, posterior simulations have more consistent magnitude and interannual variations than the prior estimates, but the performance from the NASA-based and DOMINO-based emissions varies across ozone metrics. The limited availability of remote-sensing data and the use of prior NOx diurnal variations hinder improvement of ozone diurnal variations from the assimilation, and therefore have mixed performance on improving different ozone metrics. Additional improvements in posterior NO2 and ozone simulations require more precise and consistent NO2 retrieval products, more accurate diurnal variations of NOx and VOC emissions, and improved simulations of ozone chemistry and depositions.

Previous studies have emphasized that the decrease in photolysis rate at the surface induced by the light extinction of aerosols could weaken ozone photochemistry and then reduce surface ozone. However, quantitative studies have shown that weakened photochemistry leads to a much greater reduction in the net chemical production of ozone, which does not match the reduction in surface ozone. This suggests that in addition to photochemistry, some other physical processes related to the variation of ozone should also be considered. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with the ozone source apportionment method was applied to determine the mechanism of ozone reduction induced by aerosols over central East China (CEC). Our results showed that weakened ozone photochemistry led to a significant reduction in ozone net chemical production, which occurred not only at the surface but also within the lowest several hundred meters in the planetary boundary layer (PBL). Meanwhile, a larger ozone gradient was formed in the vertical direction, which led to the high concentrations of ozone aloft being entrained by turbulence from the top of the PBL to the surface and partly counteracting the reduction in surface ozone. In addition, contribution from dry deposition was weakened due to the decrease in surface ozone concentration. The reduction in the ozone's sink also slowed down the rate of the decrease in surface ozone. Ozone in the upper layer of the PBL was also reduced, which was induced by much ozone aloft being entrained downward. Therefore, by affecting the photolysis rate, the impact of aerosols was a reduction in ozone not only at the surface but also throughout the entire PBL during the daytime over CEC in this study. The ozone source apportionment results showed that 41.4 %–66.3 % of the reduction in surface ozone came from local and adjacent source regions, which suggested that the impact of aerosols on ozone from local and adjacent regions was more significant than that from long-distance regions. The results also suggested that while controlling the concentration of aerosols, simultaneously controlling ozone precursors from local and adjacent source regions is an effective way to suppress the increase in surface ozone over CEC at present.

The earth’s surface ozone levels are becoming very significant due to their negative impact on human health, vegetation and climate. In this study, the methodology based on ensemble approach embodied linear and nonlinear behaviors was developed. It was applied for prediction of ozone concentration using dataset (2013–2016) of gaseous pollutants (O3, CO, NOx, MHC, TNMHCs) and meteorological variables as input variables. The daily O3 max/O3 min ratio of 10.9 marks the peculiar ozone pollution in the area. The fourteen prediction algorithms and their possible combinations of ensemble models were employed in this paper. Compared with individual models, the ensemble model approach showed an index of agreement of 0.91, the accuracy of 95.5% and mean absolute error of − 0.001 ppb between the predicted and observed diurnal cycle and daily averaged data of the year 2016 for benchmark analysis.

Air pollution caused by particulate matter has significantly improved in China in recent years since the implementation of a series of stringent clean-air regulations. However, surface ozone concentrations have increased, especially in developed city clusters, such as the Beijing–Tianjin–Hebei, Yangtze River Delta, Pearl River Delta, and Sichuan Basin regions. Due to the complexity and nonlinearity of the ozone formation, accurately locating major sources of ozone and its precursors is an important basis for the formulation of cost-effective pollution control strategies. In this paper, the authors systematically summarize the reported results and outcomes of the methods and main conclusions of ozone source apportionment (regions and categories) in China from the published literature, based on observation-based methods and emission-based methods, respectively. The authors aim to provide a comprehensive understanding of ozone pollution and reliable references for the formulation of air pollution prevention policies in China.

Urban ozone (O3) formation can be limited by NOx, VOCs, or both, complicating the design of effective O3 abatement plans. A satellite-retrieved ratio of formaldehyde to NO2 (HCHO/NO2), developed from theory and modeling, has previously been used to indicate O3 formation chemistry. Here, we connect this space-based indicator to spatiotemporal variations in O3 recorded by on-the-ground monitors over major U.S. cities. High-O3 events vary nonlinearly with OMI HCHO and NO2, and the transition from VOC-limited to NOx-limited O3 formation regimes occurs at higher HCHO/NO2 value (3 to 4) than previously determined from models, with slight intercity variations. To extend satellite records back to 1996, we develop an approach to harmonize observations from GOME and SCIAMACHY that accounts for differences in spatial resolution and overpass time. Two-decade (1996-2016) multisatellite HCHO/NO2 captures the timing and location of the transition from VOC-limited to NOx-limited O3 production regimes in major U.S. cities, which aligns with the observed long-term changes in urban-rural gradient of O3 and the reversal of O3 weekend effect. Our findings suggest promise for applying space-based HCHO/NO2 to interpret local O3 chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution.

China’s nationwide ozone monitoring network initiated in 2013 has observed severe surface ozone pollution. This network, combined with the recent Tropospheric Ozone Assessment Report (TOAR) data set, offers a more comprehensive view on global surface ozone distribution and trends. Here, we report quantitative estimates of the warm-season (April–September) surface ozone trends and resulting health impacts at Chinese cities in 2013–2019. Both the parametric and nonparametric linear trends for 12 ozone metrics relevant to human health and vegetation exposure are derived. We find that all ozone metrics averaged from Chinese urban sites have increased significantly since 2013. The warm-season daily maximum 8-h average (MDA8) ozone levels increased by 2.4 ppb (5.0%) year–¹, with over 90% of the sites showing positive trends and 30% with trends larger than 3.0 ppb year–¹. These rates are among the fastest trends, even faster in some Chinese cities, compared with the urban ozone trends in any other region worldwide reported in TOAR. Ozone metrics reflecting the cumulative exposure effect on human health and vegetation such as SOMO35 and AOT40 have increased at even faster rates (>10% year–¹). We estimate that the total premature respiratory mortalities attributable to ambient MDA8 ozone exposure in 69 Chinese cities are 64,370 in 2019, which has increased by 60% compared to 2013 levels and requires urgent attention.

The review is compiled on the basis of the results of operation of the total ozone (TO) monitoring system in Russia and neighboring countries functioning in operational mode at the Central Aerological Observatory. The monitoring system uses data from the national network equipped with M-124 filter ozonometers being under the methodological supervision of the Main Geophysical Observatory. The quality of the functioning of the entire system is operationally controlled using the OMI satellite equipment observations (NASA, USA). Basic TO observation data are generalized for each month of the fourth quarter of 2019, for the fourth quarter, and for the whole year. Data of routine observations of surface ozone values in the Moscow region, Crimea, and on the Kola Peninsula are also presented.

This study investigated the adverse effect of surface ozone on the maize yield using a unique panel from 880 counties in China. To identify the impact of elevated surface ozone concentrations, we constructed an econometric model by controlling the impact of climate variables and related economic variables. This study also considered the potential spatial correlation in the measurement of the impact of surface ozone on maize yield. Results confirmed that the increase of ozone concentration decreased the maize yield. Moreover, maize was found to be the most sensitive to ozone at the end of the second month of the growing season. The average annual loss of maize caused by ozone pollution is about 4.234 million tons in 2013–2015, accounting for 1.9% of the average output.

Although much attention has been paid to investigating and controlling air pollution in China, the trends of air-pollutant concentrations on a national scale have remained unclear. Here, we quantitatively investigated the variation of air pollutants in China using long-term comprehensive data sets from 2013 to 2017, during which Chinese government made major efforts to reduce anthropogenic emission in polluted regions. Our results show a significant decreasing trend in the PM2.5 concentration in heavily polluted regions of eastern China, with an annual decrease of ∼7% compared with measurements in 2013. The measured decreased concentrations of SO2, NO2 and CO (a proxy for anthropogenic volatile organic compounds) could explain a large fraction of the decreased PM2.5 concentrations in different regions. As a consequence, the heavily polluted days decreased significantly in corresponding regions. Concentrations of organic aerosol, nitrate, sulfate, ammonium and chloride measured in urban Beijing revealed a remarkable reduction from 2013 to 2017, connecting the decreases in aerosol precursors with corresponding chemical components closely. However, surface-ozone concentrations showed increasing trends in most urban stations from 2013 to 2017, which indicates stronger photochemical pollution. The boundary-layer height in capital cities of eastern China showed no significant trends over the Beijing–Tianjin–Hebei, Yangtze River Delta and Pearl River Delta regions from 2013 to 2017, which confirmed the reduction in anthropogenic emissions. Our results demonstrated that the Chinese government was successful in the reduction of particulate matter in urban areas from 2013 to 2017, although the ozone concentration has increased significantly, suggesting a more complex mechanism of improving Chinese air quality in the future.

Air pollutants can be transported to the pristine regions such as the Tibetan Plateau, by monsoon and stratospheric intrusion. The Tibetan Plateau region has limited local anthropogenic emissions, while this region is influenced strongly by transport of heavy emissions mainly from South Asia. We conducted a comprehensive study on various air pollutants (PM2.5, total gaseous mercury, and surface ozone) at Nam Co Station in the inland Tibetan Plateau. Monthly mean PM2.5 concentration at Nam Co peaked in April before monsoon season, and decreased during the whole monsoon season (June–September). Monthly mean total gaseous mercury concentrations at Nam Co peaked in July and were in high levels during monsoon season. The Indian summer monsoon acted as a facilitator for transporting gaseous pollutants (total gaseous mercury) but a suppressor for particulate pollutants (PM2.5) during the monsoon season. Different from both PM2.5 and total gaseous mercury variabilities, surface ozone concentrations at Nam Co are primarily attributed to stratospheric intrusion of ozone and peaked in May. The effects of the Indian summer monsoon and stratospheric intrusion on air pollutants in the inland Tibetan Plateau are complex and require further studies.

This study analyzed the increase in the surface ozone over Beijing-Tianjin-Hebei (BTH) and its surrounding areas during the period of 2005–2018 using satellite-retrieved data. First, a geographically weighted regression (GWR) model was developed to estimate the surface ozone concentration (SOC) between 2005 and 2018 based on Ozone Monitoring Instrument (OMI) ozone profiles. The resultant values and their growth rate were then analyzed. The SOC exhibited significant variation in the spatial distribution over the study area, with the maximum and the minimum values occurring in the southeast and the northeast, respectively. The latter region also displayed the highest SOC growth rate, however, whereas the southwest displayed the lowest one. Additionally, prominent seasonality was observed in the SOC: The concentration peaked during the warm season and troughed during the cold season, but the growth rate showed the opposite trend. The values during the cold season greatly affected the annual spatial distribution and the growth rate of the SOC, whereas those during the warm season significantly contributed to the annual concentration. From 2005 till 2018, the SOC showed an upward trend with an average growth rate of 3.4 µg m–3 y–1, with a greater increase in the second half (2012–2018) than the first half (2005–2011) of the study period because of the stronger photochemical reactions caused by the continual increase in HCHO during summer and the weaker NO titration effect caused by the rapid decrease in NO2 during winter. With constantly rising levels of HCHO and a VOCs-limited regime in the study area, we must formulate an effective reduction scheme for VOCs and NO2 co-emissions in order to mitigate the surface ozone pollution, despite the risk that decreasing the NO2 will lead to a certain increase in the SOC.

The review is based on the results of the operation of the total ozone (TO) monitoring system in Russia and adjoining territories that functions in the operational mode at the Central Aerological Observatory. The monitoring system uses data from the national network equipped with M-124 filter ozonometers being under the methodological supervision of the Main Geophysical Observatory. The quality of the system functioning is operationally controlled using OMI satellite equipment observations (NASA, the USA). Basic TO observation data are generalized for each month of the third quarter of 2019 and for the third quarter. Data of routine observations of surface ozone values in the Moscow region and Crimea are also considered.

Changes in ozone concentration during precipitation are studied using data of ozone monitoring in the surface air layer in Tomsk and at Karadag. It is found that these changes can be either positive or negative. The largest jumps in ozone content are observed during frontal precipitations. During air mass precipitation, the sign and magnitude of the changes are determined by the diurnal behavior of ozone concentration. Analysis showed that the increase in the ozone concentration during precipitation and ozone increase in the diurnal behavior are time-coincident in 59% of cases in Tomsk and in 63% of cases at the Karadag. The decreasing wave of ozone concentration in the diurnal behavior coincides in time with ozone decrease during precipitation even more often, in 85% of cases in Tomsk and in 79% of cases at Karadag. Based on data of aircraft sensing, it is shown that ozone descent from the boundary air layer occurs in a number cases when the temperature stratification during precipitation changes to neutral.

We present an Observing System Simulation Experiment (OSSE) dedicated to the evaluation of the added value of the Sentinel-4 and Sentinel-5P missions for tropospheric nitrogen dioxide (NO2). Sentinel-4 is a geostationary (GEO) mission covering the European continent, providing observations with high temporal resolution (hourly). Sentinel-5P is a low Earth orbit (LEO) mission providing daily observations with a global coverage. The OSSE experiment has been carefully designed, with separate models for the simulation of observations and for the assimilation experiments and with conservative estimates of the total observation uncertainties. In the experiment we simulate Sentinel-4 and Sentinel-5P tropospheric NO2 columns and surface ozone concentrations at 7 by 7 km resolution over Europe for two 3-month summer and winter periods. The synthetic observations are based on a nature run (NR) from a chemistry transport model (MOCAGE) and error estimates using instrument characteristics. We assimilate the simulated observations into a chemistry transport model (LOTOS-EUROS) independent of the NR to evaluate their impact on modelled NO2 tropospheric columns and surface concentrations. The results are compared to an operational system where only ground-based ozone observations are ingested. Both instruments have an added value to analysed NO2 columns and surface values, reflected in decreased biases and improved correlations. The Sentinel-4 NO2 observations with hourly temporal resolution benefit modelled NO2 analyses throughout the entire day where the daily Sentinel-5P NO2 observations have a slightly lower impact that lasts up to 3–6 h after overpass. The evaluated benefits may be even higher in reality as the applied error estimates were shown to be higher than actual errors in the now operational Sentinel-5P NO2 products. We show that an accurate representation of the NO2 profile is crucial for the benefit of the column observations on surface values. The results support the need for having a combination of GEO and LEO missions for NO2 analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel-4) and global coverage (LEO, Sentinel-5P).

Cold fronts are mesoscale meteorological phenomena that play important roles in chemical transport processes. Effects of a single cold front episode on surface ozone may vary since meteorological factors may alter ozone in different (increasing or decreasing) directions. We studied cold front events and their effects on surface maximum daily 8-h average (MDA8) ozone over the Contiguous United States (CONUS) during 2003–2017. The average number of cold front days in the CONUS was 50.02 days per year per site with a slightly increasing trend (1.06 per year) during 2003–2017. The Rocky Mountains blocked most cold fronts for the Northwest and the Southwest. Percentages of 24 h back trajectories from north side in 12 sample cities show that cold front effects exist ±12 h to cold fronts' arrival at least. Compared to all days’ average, MDA8 ozone differs from prefrontal days, cold front 1st days, cold front 2 + days, and post frontal days by 0.10, −0.94, −0.84, and −1.89 ppbv (−0.89 ppbv on average), respectively. By contrast, they overlapped 9.85%, 5.82%, 0.89%, and 5.04%, respectively (21.60% in total) with ozone exceedance days. Regional consistent long-term effects of cold fronts on surface MDA8 ozone have been found. Enhancements during prefrontal days can be seen in the Northeast and the Midwest during non-winter seasons. For cold front days and post frontal days, ozone differences in the Southwest, the Northwest, and the Northeast were negative in non-winter seasons then reversed to positive in winter. The areas along the coast of the Gulf of Mexico showed positive ozone difference in non-winter seasons and expanded to the inland area during summer then reversed to negative in winter. Synergy of cold fronts and other climatic and meteorological factors (e.g., prefrontal stagnation, southerly flow from the Gulf of Mexico, and winter stratospheric intrusions) can preliminarily explain these consistent long-term effects.

The trends per 5th percentile interval of European surface ozone concentrations over urban, suburban and rural sites in the period 1995–2012 have been estimated based on observational data from the European Environment Agency air quality database. Consistent with previous study results, we find a rapid decline of relatively high ozone concentrations after the year 1995, especially of rural summertime ozone. However, the decreasing rate has slowed, and even reversed, of low-level ozone, especially in the urban environment for all seasons. Although the ozone air pollution problem has been declared to be effectively resolved by achieving European air quality goals, our results indicate that ozone remains to be difficult to control for a long time into the future. Global and regional models poorly reproduce the measured differences in ozone and trends in different levels and regions. Thus policy guidelines from the model should be considered with care.

Ozone from a stratospheric intrusion (SI) reached sea level in association with a thunderstorm gust front during the predawn hours of 16 April 2018. The event caused surface ozone concentration increases of 30 to more than 50 ppbv in a matter of minutes in a band from approximately Richmond, Virginia, to Philadelphia, Pennsylvania. Peak hourly ozone concentrations reached 74 ppbv in northeastern Maryland despite absent photochemistry and ongoing convective activity. An intense jet stream with velocities >80 kt (41 m s−1) less than 1 km above ground level was observed associated with a deepening cyclone. Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), showed a filament of ozone with concentrations greater than 90 ppbv extending downward from the stratosphere to the lower troposphere. This SI filament became collocated with an ongoing severe squall line, and stratospheric ozone was transported directly to sea level when entrained into the squall-line gust front. Weather radar and in situ observations confirmed surface ozone increased with the thunderstorm gust front, while a concurrent reduction in carbon monoxide confirmed air within the gust front had stratospheric origins. While rare, such coupling events are important to troposphere–stratosphere exchanges and in overall atmospheric chemistry and climate. This may be the first event of its type and magnitude documented.

Vehicular evaporative emissions have been recognized as an important source of volatile organic compounds to the environment and are of high environmental concern since these compounds have been associated to the formation of surface ozone and secondary organic aerosols. Evaporative emissions occur during any vehicle operation. In Europe, a revised legislative test procedure has been recently introduced to better control evaporative emissions during parking. However, emissions related to normal driving conditions—the so-called running losses—have received less attention compared with the other categories. The current study aims at giving some insights to the prevailing temperature conditions in fuel tanks of typical European vehicles during normal driving operation. The effects of ambient air temperature, trip duration, vehicle speed, and fuel tank level on the temperature reached by the fuel inside the tank under different real-world operating conditions were studied. Tank temperature can exceed 40 °C depending on ambient and driving conditions. Ambient temperature was found to be the most important parameter affecting the tank temperature. Trip duration and driving pattern may also have an influence on the tank temperature particularly when long trips combined with high vehicle speed are examined. Additionally, the difference between tank and ambient temperature was examined during the individual trips and was found to vary between 1 and 10 °C depending on the testing conditions. The most important parameters affecting the delta temperature were found to be the trip duration and the maximum vehicle speed. Finally, the purging strategy of two of the test vehicles was monitored, and the parameters affecting the purging flow rate were investigated. No strong correlation between the canister flow rate with ambient temperature, vehicle speed, or fuel level was observed in either of the tested vehicles. Substantially different canister flow rate levels between the two vehicles point to different purging strategies. Electronic supplementary material The online version of this article (10.1007/s11356-019-04985-7) contains supplementary material, which is available to authorized users.

The California Baseline Ozone Transport Study (CABOTS) was conducted in the late spring and summer of 2016 to investigate the influence of long-range transport and stratospheric intrusions on surface ozone (O3) concentrations in California with emphasis on the San Joaquin Valley (SJV), one of two extreme ozone non-attainment areas in the US. One of the major objectives of CABOTS was to characterize the vertical distribution of O3 and aerosols above the SJV to aid in the identification of elevated transport layers and assess their surface impacts. To this end, the NOAA Earth System Research Laboratory (ESRL) deployed the Tunable Optical Profiler for Aerosol and oZone (TOPAZ) mobile lidar to the Visalia Municipal Airport (36.315∘ N, 119.392∘ E) in the central SJV between 27 May and 7 August 2016. Here we compare the TOPAZ ozone retrievals with co-located in situ surface measurements and nearby regulatory monitors and also with airborne in situ measurements from the University of California at Davis–Scientific Aviation (SciAv) Mooney and NASA Alpha Jet Atmospheric eXperiment (AJAX) research aircraft. Our analysis shows that the lidar and aircraft measurements agree, on average to within 5 ppbv, the sum of their stated uncertainties of 3 and 2 ppbv, respectively.

We have developed a new statistical approach (M3Fusion) for combining surface ozone observations from thousands of monitoring sites around the world with the output from multiple atmospheric chemistry models to produce a global surface ozone distribution with greater accuracy than can be provided by any individual model. The ozone observations from 4766 monitoring sites were provided by the Tropospheric Ozone Assessment Report (TOAR) surface ozone database, which contains the world's largest collection of surface ozone metrics. Output from six models was provided by the participants of the Chemistry-Climate Model Initiative (CCMI) and NASA's Global Modeling and Assimilation Office (GMAO). We analyze the 6-month maximum of the maximum daily 8 h average ozone value (DMA8) for relevance to ozone health impacts. We interpolate the irregularly spaced observations onto a fine-resolution grid by using integrated nested Laplace approximations and compare the ozone field to each model in each world region. This method allows us to produce a global surface ozone field based on TOAR observations, which we then use to select the combination of global models with the greatest skill in each of eight world regions; models with greater skill in a particular region are given higher weight. This blended model product is bias corrected within 2∘ of observation locations to produce the final fused surface ozone product. We show that our fused product has an improved mean squared error compared to the simple multi-model ensemble mean, which is biased high in most regions of the world.

The present paper reports a comparative study among two neurocomputing models in the form of Multilayer Perceptron (MLP) models and non-linear regression for the prediction of surface ozone (O3) during pre-monsoon season over Gangetic West Bengal (GWB), India considering NOx, SO2, PM10 and temperature as predictors. Learning the MLPs through gradient descent (GD) with tanhyperbolic and sigmoid nonlinearities, we found that all the models under consideration have almost the same degrees of prediction efficiency for O3 over GWB during pre-monsoon season with the said predictors. However, the MLP model with tanhyperbolic activation function is found to produce a significantly higher correlation and Willmott's index of agreement between actual and predicted O3 than the other models. Finally, MLP with GD learning characterized by tanhyperbolic nonlinearity is identified to have significant efficiency in surface ozone prediction over the region as mentioned above.

Recent studies have shown that surface ozone (O3) concentrations over central eastern China (CEC) have increased significantly during the past decade. We quantified the effects of changes in meteorological conditions and O3 precursor emissions on surface O3 levels over CEC between July 2003 and July 2015 using the GEOS-Chem model. The simulated monthly mean maximum daily 8 h average O3 concentration (MDA8 O3) in July increased by approximately 13.6 %, from 65.5±7.9 ppbv (2003) to 74.4±8.7 ppbv (2015), comparable to the observed results. The change in meteorology led to an increase in MDA8 O3 of 5.8±3.9 ppbv over the central part of CEC, in contrast to a decrease of about -0.8±3.5 ppbv over the eastern part of the region. In comparison, the MDA8 O3 over the central and eastern parts of CEC increased by 3.5±1.4 and 5.6±1.8 ppbv due to the increased emissions. The increase in averaged O3 in the CEC region resulting from the emission increase (4.0±1.9 ppbv) was higher than that caused by meteorological changes (3.1±4.9 ppbv) relative to the 2003 standard simulation, while the regions with larger O3 increases showed a higher sensitivity to meteorological conditions than to emission changes. Sensitivity tests indicate that increased levels of anthropogenic non-methane volatile organic compounds (NMVOCs) dominate the O3 increase over the eastern part of CEC, and anthropogenic nitrogen oxides (NOx) mainly increase MDA8 O3 over the central and western parts and decrease O3 in a few urban areas in the eastern part. Budget analysis showed that net photochemical production and meteorological conditions (transport in particular) are two important factors that influence O3 levels over the CEC. The results of this study suggest a need to further assess the effectiveness of control strategies for O3 pollution in the context of regional meteorology and anthropogenic emission changes.

A comprehensive analysis of the temporal evolution of tropospheric ozone in Antarctica using more than 25 years of surface ozone and ozonesonde measurements reveals significant changes in tropospheric ozone there. It shows a positive trend in ozone at the surface and lower and mid-troposphere, but a negative trend in the upper troposphere. We also find significant links between different climate modes and tropospheric ozone in Antarctica and observe that changes in residual overturning circulation, the strength of the polar vortex, and stratosphere-troposphere exchange make noticeable variability in tropospheric ozone. Therefore, this study alerts of increasing ozone concentration in Antarctica, which would have a profound impact on the future climate of the region as tropospheric ozone has warming feedback to the Earth's climate.

The air quality of Handan, a typical industrial city in China, has been significantly improved through atmospheric pollution control, except for ozone (O3) pollution. We found that, in summer, emissions of anthropogenic volatile organic compounds (VOCs) and NOx decreased yearly in Handan, but the O3 concentration significantly declined yearly during 2013–2015, whereas it experienced worsening O3 pollution after 2015. Therefore, we used the Weather Research and Forecasting Community Multiscale Air Quality (WRF–CMAQ) modeling system to simulate the influence of the meteorological conditions and emission changes in Handan during the typical period (June) of O3 pollution in 2013–2018. For benchmarked June 2013, the results showed that the reduction of the O3 concentration in June of 2014–2016 was mainly caused by emission reduction, while in June of 2017–2018, the combined effect of changes in emissions and meteorological conditions led to aggravated O3 pollution. Sensitivity analysis indicated that combined VOCs and NOx emission controls would effectively reduce incremental O3 formation when the abatement ratio of VOCs/NOx should be no less than 0.84, and we found that VOCs reduction would continusouly bring about O3 decreases under various NOx reductions, but its positive sensitivity to O3 would become smaller with NOx reduction. However, the positive influence of NOx reduction on O3 would happen until NOx reduction exceeding 45–60%. The findings will be helpful in formulating emission control strategies for coping with O3 pollution in an industrial city.

Estimates of ground-level ozone concentrations are necessary to determine the human health burden of ozone. To support the Global Burden of Disease Study, we produce yearly fine resolution global surface ozone estimates from 1990 to 2017 through a data fusion of observations and models. As ozone observations are sparse in many populated regions, we use a novel combination of the M3Fusion and Bayesian Maximum Entropy (BME) methods. With M3Fusion, we create a multimodel composite by bias-correcting and weighting nine global atmospheric chemistry models based on their ability to predict observations (8834 sites globally) in each region and year. BME is then used to integrate observations, such that estimates match observations at each monitoring site with the observational influence decreasing smoothly across space and time until the output matches the multimodel composite. After estimating at 0.5° resolution using BME, we add fine spatial detail from an additional model, yielding estimates at 0.1° resolution. Observed ozone is predicted more accurately (R2 = 0.81 at the test point, 0.63 at 0.1°, and 0.62 at 0.5°) than the multimodel mean (R2 = 0.28 at 0.5°). Global ozone exposure is estimated to be increasing, driven by highly populated regions of Asia and Africa, despite decreases in the United States and Russia.

Prior work suggests drought exacerbates US air quality by increasing surface ozone concentrations. We analyze 2005-2015 tropospheric column concentrations of two trace gases that serve as proxies for surface ozone precursors retrieved from the OMI/Aura satellite: Nitrogen dioxide (ΩNO

Ambient ozone (O3) pollution has become a big issue in China. Recent studies have linked long- and short-term O3 exposure to several public health risks. In this study, we (1) characterize the long-term and short-term O3-attributed health metric in China from 2015 to 2019; (2) estimate the surface O3 trends; and (3) quantify the long-term and short-term health impacts (i.e. all-cause, cardiovascular and respiratory mortality) in 350 urban Chinese cities. In these 5-years, the national annual average of daily maximum 8 h average (AVGDMA8) O3 concentrations and warm-season (April–September) 4th highest daily maximum 8 h average (4DMA8) O3 concentrations increased from 74.0 ± 15.5 μg/m3 (mean ± standard deviation) to 82.3 ± 12.0 μg/m3 and 167 ± 37.0 μg/m3 to 174 ± 30.0 μg/m3 respectively. During this period, the DMA8 O3 concentration increased by 1.9 ± 3.3 μg/m3/yr across China, with over 70% of the monitoring sites showing a positive upward trend and 19.4% with trends >5 μg/m3/yr. The estimated long-term all-cause, cardiovascular and respiratory premature mortalities attributable to AVGDMA8 O3 exposure in 350 Chinese cities were 181,000 (95% CI: 91,500–352,000), 112,000 (95% CI: 38,100–214,000) and 33,800 (95% CI: 0–71,400) in 2019, showing increases of 52.5%, 52.9% and 54.6% respectively compared to 2015 levels. Similarly, short-term all-cause, cardiovascular and respiratory premature mortalities attributed to ambient 4DMA8 O3 exposure were 156,000 (95% CI: 85,300–227,000), 73,500 (95% CI: 27,500–119,000) and 28,600 (95% CI: 14,500–42,800) in 2019, increases of 19.6%, 19.8% and 21.2% respectively compared to 2015. The results of this study are important in ascertaining the effectiveness of recent emission control measures and to identify the areas that require urgent attention.

Summertime ozone in the Western United States presents a unique public health challenge. Changes in population, background ozone, wildland fire, and local precursor emissions combined with terrain-induced meteorology can affect surface ozone levels and compliance with the National Ambient Air Quality Standards (NAAQS). While there is considerable research on ozone in the Northern Front Range Metropolitan Area of Colorado, United States, less is known about the Southern Front Range. In Colorado Springs, approximately 100 km south of Denver, summertime maximum daily 8-h average (MDA8) ozone shows no significant (p < .05) trend at the 5th, 50th, or 95th percentile over the past 20 years. However, the region is at risk of nonattainment with the NAAQS based on observations from 2018 to 2020. From June through September 2018, the Colorado Department of Public Health and Environment measured hourly ozone at eight sites to characterize the spatial distribution of ozone in Colorado Springs. Mean ozone (±1σ) ranged from 34 ± 19 to 60 ± 9 ppb. The 95th percentile of hourly ozone increased approximately 1.1 ppb per 100 m of elevation, while the amplitudes of mean diurnal profiles decreased with elevation and distance from the interstate. MDA8 ozone was also highly correlated across all sites, and there is little evidence of local photochemical production or ozone transport from Denver. Further, results from generalized additive modeling show that summertime MDA8 in this region is strongly influenced by regional background air and wildfire, with smoke contributing an average of 4–5 ppb to the MDA8. Enhanced MDA8 values due to wildfires were especially pronounced in 2018 and 2020. Lastly, we find that the permanent monitoring sites represent the lower end of observed ozone in the region, suggesting that additional long-term monitoring for public health may be warranted in populated, higher elevation areas.

Due to the strong temperature dependence of surface ozone concentrations (O3), future warmer conditions may worsen ozone pollution levels despite continued efforts on emission controls of ozone precursors. Using long-term measurements of hourly O3 concentrations co-located with NOx concentrations in stations distributed throughout Germany, we assess changes in the climate penalty in summertime, defined as the slope of ozone-temperature relationship during the period 1999–2018. We find a stronger temperature sensitivity in the urban stations over the southwestern regions, especially in the first period of the study (1999–2008). We show a decrease in the climate penalty in most of stations during the second period of the study (2009–2018), with some exceptions (e.g. Berlin) where the climate penalty did not show significant changes. A key motivation of this study is to provide further insights into the impacts of NOx reductions in the O3-temperature relationship. For that, we propose a statisti-cal approach based on Generalized Additive Models (GAMs) to describe ozone production rates, inferred from hourly observations, as a function of NOx and temperature, among other variables relevant during the O3 production. The GAMs confirm lower O3 production rates during the second period (2009–2018) at most of the stations and a decreasing sensitivity to temperature. We observe that a large number of stations are transitioning to NOx-limited chemistry, consistent with a decreasing temperature dependence of O3 at moderate-high temperatures as a result of sustained NOx reductions. Moreover, the GAMs results showed changes in the shape of the function representing the O3-temperature relationship when comparing the first and second period, which suggest changes in VOC influencing the temperature dependence of O3. From these results, we infer effective VOC reductions over time that have also contributed to the observed decrease of O3 production rates. Thus, our analysis indicates that emissions reductions have been effective in a number of stations, particularly in the southwestern regions. However, we notice that in a few stations (e.g. Berlin) additional emission reductions should be required to effectively mitigate the temperature dependence of O3.

We investigate the relationships among summertime ozone (O3), temperature, and humidity on daily timescales across the Northern Hemisphere using observations and model simulations. Temperature and ...

The concentration of surface ozone (O3) strongly depends on environmental and meteorological variables through a series of complex and non-linear functions. This study aims to explore the performances of an advanced machine learning (ML) method, the boosted regression trees (BRT) technique, in exploring the relationships between surface O3 and its driving factors, and in predicting the levels of O3 concentrations. To this end, a BRT model was trained on hourly data of air pollutants and meteorological parameters, acquired, over the 2016–2018 period, in a rural area affected by an anthropic source of air pollutants. The abilities of the BRT model in ranking, visualizing, and predicting the relationship between ground-level O3 concentrations and its driving factors were analyzed and illustrated. A comparison with a multiple linear regression (MLR) model was performed based on several statistical indicators. The results obtained indicated that the BRT model was able to account for 81% of changes in O3 concentrations, it slightly outperforms the MLR model in terms of the predictions accuracy and allows a better identification of the main factors influencing O3 variability on a local scale. This knowledge is expected to be useful in defining effective measures to prevent and/or mitigate the health damages associated with O3 exposure.

Air quality observations showed that surface ozone (O3) concentrations over Shandong increased significantly in springtime in recent years, especially 2017. The observed 90th percentile of hourly O3 concentrations (O3-h_90) in May increased from 148.4 μg/m3 in 2016 to 176.2 μg/m3 in 2017. To investigate the reasons of significant increase of O3 in spring of 2017, seven sensitivity cases were performed with the RAMS-CMAQ modeling system to identify the impacts of meteorological conditions (M) and emissions (E) on O3 concentrations in May of Shandong during the time period 2016–2018. The regional O3-h_90 in May of Shandong were 103.0, 120.3 and 86.3 μg/m3 in 2016, 2017 and 2018, respectively. It was found that the positive effects from favorable meteorology were the dominant reasons that resulted in the high O3 concentration in May 2017. When compared to 2017 standard simulation (17E17M), the differences of meteorological conditions led to the decrease of 17.5 and 33.8 μg/m3 in regional O3-h_90 of May in 2016 (17E16M) and 2018(17E18M), while small changes (0.6 and − 0.3 μg/m3) appeared in that of May 2016 (16E17M) and 2018(18E17M) due to emission differences. Since there were few differences in the wind speeds of May between three years, the higher temperature and lower relative humidity significantly contributed to O3 formation in May 2017 compared to May of 2016 and 2018. Besides, the amount of cloud fraction (CF), which has an indirect influence on the surface temperature and photochemical production of ozone by its impacts on the insolation, was the least in May 2017 compared to that in May 2016 and 2018 over Shandong. The distributions of changes in CF had obvious negative correlations with that of O3 vertical column concentrations and temperature. Thus, the fewer cloud fraction may play a key role in O3 formation of May 2017.

Surface ozone data from the Chinese Ministry of Ecology and Environment (MEE) network show sustained increases across the country over the 2013–2019 period. Despite Phase 2 of the Clean Air Action Plan targeting ozone pollution, ozone was higher in 2018–2019 than in previous years. The mean summer 2013–2019 trend in maximum 8 h average (MDA8) ozone was 1.9 ppb a−1 (p<0.01) across China and 3.3 ppb a−1 (p<0.01) over the North China Plain (NCP). Fitting ozone to meteorological variables with a multiple linear regression model shows that meteorology played a significant but not dominant role in the 2013–2019 ozone trend, contributing 0.70 ppb a−1 (p<0.01) across China and 1.4 ppb a−1 (p=0.02) over the NCP. Rising June–July temperatures over the NCP were the main meteorological driver, particularly in recent years (2017–2019), and were associated with increased foehn winds. NCP data for 2017–2019 show a 15 % decrease in fine particulate matter (PM2.5) that may be driving the continued anthropogenic increase in ozone, as well as unmitigated emissions of volatile organic compounds (VOCs). VOC emission reductions, as targeted by Phase 2 of the Chinese Clean Air Action Plan, are needed to reverse the increase in ozone.

Efforts to stem the spread of COVID‐19 in China hinged on severe restrictions to human movement starting 23 January 2020 in Wuhan and subsequently to other provinces. Here, we quantify the ancillary impacts on air pollution and human health using inverse emissions estimates based on multiple satellite observations. We find that Chinese NOx emissions were reduced by 36% from early January to mid‐February, with more than 80% of reductions occurring after their respective lockdown in most provinces. The reduced precursor emissions increased surface ozone by up to 16 ppb over northern China but decreased PM2.5 by up to 23 μg m−3 nationwide. Changes in human exposure are associated with about 2,100 more ozone‐related and at least 60,000 fewer PM2.5‐related morbidity incidences, primarily from asthma cases, thereby augmenting efforts to reduce hospital admissions and alleviate negative impacts from potential delayed treatments., Key Points We quantified the ancillary impacts of the COVID‐19 lockdown on air pollution and human health using multiple satellite observationsRapid reductions in Chinese NOx and SO2 emissions increased surface ozone by 16 ppb over northern China but decreased PM2.5 nationwideThese changes increased by about 60 ozone‐related but decreased by about 5,000 PM2.5‐related hospital admissions

We perform a source attribution for tropospheric and ground-level ozone using a novel technique that accounts separately for the contributions of the two chemically distinct emitted precursors (reactive carbon and oxides of nitrogen) to the chemical production of ozone in the troposphere. By tagging anthropogenic emissions of these precursors according to the geographical region from which they are emitted, we determine source–receptor relationships for ground-level ozone. Our methodology reproduces earlier results obtained via other techniques for ozone source attribution, and it also delivers additional information about the modelled processes responsible for the intercontinental transport of ozone, which is especially strong during the spring months. The current generation of chemical transport models used to support international negotiations aimed at reducing the intercontinental transport of ozone shows especially strong inter-model differences in simulated springtime ozone. Current models also simulate a large range of different responses of surface ozone to methane, which is one of the major precursors of ground-level ozone. Using our novel source attribution technique, we show that emissions of NOx (oxides of nitrogen) from international shipping over the high seas play a disproportionately strong role in our model system regarding the hemispheric-scale response of surface ozone to changes in methane, as well as to the springtime maximum in intercontinental transport of ozone and its precursors. We recommend a renewed focus on the improvement of the representation of the chemistry of ship NOx emissions in current-generation models. We demonstrate the utility of ozone source attribution as a powerful model diagnostic tool and recommend that similar source attribution techniques become a standard part of future model intercomparison studies.

The mutual relationship between surface atmospheric pollutants and Cloud-to-Ground (CG) lightning is studied in the Tehran area for the first time. The impact of surface pollutant concentration of PM10 on CG lightning initiation, as well as the production of NO2 and surface ozone derived by lightning occurrence is investigated. To proceed, the reports of thunderstorm occurrence, including CG lightning in the Tehran area have been collected for years of study from the Iran meteorological organization (IRIMO). The surface pollution data are obtained from the Air Quality Control Center (AQCC) for several stations in the area of interest. The number of lightning (NoL) associated with the selected dates is obtained using the World Wide Lightning Location Network (WWLLN). The correlation coefficients associated with the CG lightning and the concentrations of PM10, NO2, and surface ozone are calculated. The hourly variations of accumulated NoL and NO2 and surface ozone are also compared for 24 hours before and after the lightning activity for four days of study during the years of 2009-2013. The results show that there is a positive correlation between PM10 concentration and lightning flash number, obtained from observational data of WWLLN. Moreover, the comparison of NoL and surface pollutant concentration indicates a clear positive contribution from CG lightning in NO2 and ozone production. In days with a considerable number of lightning occurrences, the comparison of the hourly average of NO2 and O3 concentrations with the lightning flash number reveals that NO2 decreases, and O3 increases due to the significant increases of lightning strikes.

The outbreak of COVID-19, a global health challenge faced by countries worldwide, led to a lockdown in India, thereby bringing down the emissions of various air pollutants. Here, we discuss the behaviour of surface ozone (O3) concentrations and its precursors, oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOC) at two Indian megacities namely Mumbai and Pune, closely located yet vastly differing in meteorology due to their locations. Although levels of CO, NO2, and VOC declined sharply after the lockdown in both cities, with NO2 showing the highest reduction, ozone concentration in Pune remained unaffected, whereas Mumbai exhibited a mixed trend, touching even a maximum in between the lockdown. On a diurnal scale, the magnitude of O3 levels during the lockdown period is higher at almost all hours in Mumbai, and in Pune, it is almost identical except during night hours when it is marginally higher in the lockdown period as compared to the normal period. On a whole, the pollution levels were brought down significantly which can be used as a benchmark in the future for the implementation of policies related to air quality management and emission control in Indian megacities by the policymakers. These results also can pave a way for the scientific community for local air quality modelling.

In order to clarify the causes of variability among the model outputs for surface ozone in the Model Intercomparison Study Asia Phase III (MICS-Asia III), three regional models, CMAQ v.5.0.2, CMAQ v.4.7.1, and NAQPMS (abbreviated as NAQM in this paper), have been selected. Detailed analyses of monthly averaged diurnal variation have been performed for selected grids covering the metropolitan areas of Beijing and Tokyo and at a remote oceanic site, Oki. The chemical reaction mechanism, SAPRC99, used in the CMAQ models tended to give a higher net chemical ozone production than CBM-Z used in NAQM, agreeing with previous studies. Inclusion of the heterogeneous “renoxification” reaction of HNO3 (on soot surface)→NO+NO2 only in NAQM would give a higher NO concentration resulting in a better agreement with observational data for NO and nighttime O3 mixing ratios. In addition to chemical processes, the difference in the vertical transport of O3 was found to affect the simulated results significantly. Particularly, the increase in downward O3 flux from the upper layer to the surface after dawn was found to be substantially different among the models. Larger early morning vertical transport of O3 simulated by CMAQ 5.0.2 is thought to be the reason for higher daytime O3 in July in this model. All three models overestimated the daytime ozone by ca. 20 ppbv at the remote site Oki in July, where in situ photochemical activity is minimal.

The ozone (O3) concentrations in island cities are low due to the relatively low concentrations of O3 precursors. However, Pingtan, a typical island city along the southeast coast of China, suffers from frequent O3 pollution. In this study, one year of hourly O3 concentration data collected from three sites (rural, suburban, and urban) on Pingtan were used to investigate the characteristics and formation mechanism of O3 pollution. The results showed that the minimum O3 concentrations measured at the three sites were larger than 55.53 µg m–3, which was likely caused by low NO titration over Pingtan. The O3 concentrations in summer were low because of the East Asian monsoon. The daily maximum O3 concentrations with sea and land breezes (SLBs) were higher than those without SLBs at all three sites (8.52, 9.84, and 14.30 µg m–3 at the rural, suburban, and urban sites, respectively). In addition, SLBs amplified the diurnal variation in O3 concentrations. Cluster analysis and the analysis of an episode of high O3 concentration indicated that the developed Yangtze River Delta is the main source of O3 in Pingtan. This study helps reveal the characteristics and formation mechanism of O3 pollution in island cities.

Identifying the contributions of chemistry and transport to observed ozone pollution using regional-to-global models relies on accurate representation of ozone dry deposition. We use a recently developed configuration of the NOAA GFDL chemistry-climate model - in which the atmosphere and land are coupled through dry deposition-to investigate the influence of ozone dry deposition on ozone pollution over northern midlatitudes. In our model, deposition pathways are tied to dynamic terrestrial processes, such as photosynthesis and water cycling through the canopy and soil. Small increases in winter deposition due to more process-based representation of snow and deposition to surfaces reduce hemispheric-scale ozone throughout the lower troposphere by 5-12 ppb, improving agreement with observations relative to a simulation with the standard configuration for ozone dry deposition. Declining snow cover by the end of the 21st-century tempers the previously identified influence of rising methane on winter ozone. Dynamic dry deposition changes summer surface ozone by -4 to +7 ppb. While previous studies emphasize the importance of uptake by plant stomata, new diagnostic tracking of depositional pathways reveals a widespread impact of nonstomatal deposition on ozone pollution. Daily variability in both stomatal and nonstomatal deposition contribute to daily variability in ozone pollution. Twenty-first century changes in summer deposition result from a balance among changes in individual pathways, reflecting differing responses to both high carbon dioxide (through plant physiology versus biomass accumulation) and water availability. Our findings highlight a need for constraints on the processes driving ozone dry deposition to test representation in regional-to-global models.

Surface ozone data from the Chinese Ministry of Ecology and Environment (MEE) network show sustained increases across the country over the 2013–2019 period. Despite Phase 2 of Clean Air Action targeting ozone pollution, ozone was higher in 2018–2019 than in previous years. The mean summer 2013–2019 trend of maximum 8-h average (MDA8) ozone was 1.9 ppb a−1 across China and 3.3 ppb a−1 in the North China Plain (NCP). Fitting ozone to meteorological variables with a multiple linear regression model shows that meteorology played a significant but not dominant role in the 2013–2019 ozone trend, contributing 0.70 ppb a−1 across China and 1.4 ppb a−1 in the NCP. Higher June–July temperatures over the NCP were the main meteorological driver, particularly in recent years (2017–2019), and were associated with increased foehn winds. NCP data for 2017–2019 show a 15 % continuing decrease in fine particulate matter (PM2.5) and flat emissions of volatile organic compounds (VOCs), which would explain the continued anthropogenic increase in ozone. VOC emission controls, as targeted by Phase 2 of the Chinese Clean Air Action, are needed to reverse the increase of ozone.

Tropospheric ozone simulations have large uncertainties, but their biases, seasonality and trends can be improved with more accurate estimates of precursor gas emissions. We perform global top-down estimates of monthly NOx emissions using two OMI NO2 retrievals (NASAv3 and DOMINOv2) from 2005 to 2016 through a hybrid 4D-Var/mass balance inversion. The 12-year averages of regional NOx budgets from the NASA posterior emissions are 37 % to 53 % smaller than the DOMINO posterior. Compared to surface NO2 measurements, GEOS-Chem adjoint NO2 simulations using the DOMINO posterior NOx emissions have smaller biases in China (by 15 %) and the US (by 22 %), but NO2 trends have more consistent decreases (by 26 %) with the measurements (by 32 %) in the US from 2006 to 2016 when using the NASA posterior. The two posterior NOx emissions datasets have different strengths with respect to simulation of ozone concentrations. Simulations using NASA-based emissions provide better agreement for polluted conditions. Ozone from these shows the highest correlation (R2 = 0.88) with annual MDA8 ozone trends and alleviates the double peak in the prior simulation of global ozone seasonality. The DOMINO-based emissions, on the other hand, are better for simulating ozone at remote sites, making them well-suited to generation of boundary conditions for regional models, and in capturing the interannual variability of daytime-average (R2 = 0.72–0.81) and 24-hour average (R2 = 0.88–0.96) surface ozone. We recommend using NOx emission datasets that have the best performance in the corresponding spatial domain and temporal focus to improve ozone simulations.

The influence of climatically significant regional sources of NOx (=NO + NO2), CO, and biogenic volatile organic compounds (VOCs) on the photochemical generation of surface ozone (O3) in the lower troposphere over Europe and Siberia is studied. The sensitivity of the O3 field to the total emissions of ozone precursors is calculated using a global 3D chemical transport model (GEOS-Chem) based on the 2007–2012 databases for anthropogenic (EDGAR) and biogenic (MEGAN, GFED) emissions. The amount of photochemical ozone generated during the summer months is in good correlation with the air-mass age determined from the ratio between $${\text{N}}{{{\text{O}}}_{x}}$$ and (total reactive nitrogen) $${\text{N}}{{{\text{O}}}_{y}},$$ when the mean contribution of regional sources is $${\Delta\text{}}{{{\text{O}}}_{{\text{3}}}}$$ ~ 10–15 ppb, which is 20–30% of its background concentration in the middle latitudes ( $${{{\text{O}}}_{{\text{3}}}}$$ ~ 35–45 ppb). The quantitative estimates of the ozone production efficiency $${{{\Delta\text{}}{{{\text{O}}}_{{\text{3}}}}} \mathord{\left/ {\vphantom {{{\Delta\text{}}{{{\text{O}}}_{{\text{3}}}}} \Delta }} \right. \kern-0em} \Delta }{\text{(N}}{{{\text{O}}}_{y}} - {\text{N}}{{{\text{O}}}_{x}}{\text{)}}$$ ( $${\text{N}}{{{\text{O}}}_{y}}$$ is the total reactive nitrogen) for the summer months of the indicated period (~10–30 mol O3/mol NOx) are in good agreement with the theory of photochemical ozone generation under the conditions of slightly polluted air.