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The Sichuan Basin (SCB), to the east of the Tibetan Plateau (TP), experiences severe ozone (O 3 ) pollution. Unfavorable atmospheric diffusion conditions are considered the main causes of heavy air pollution over the basin. However, the meteorological impact of thermally driven mountain-plains solenoid (MPS) between the TP and SCB on O 3 pollution has not been reported. Here we show the MPS driving the diurnal O 3 changes in the atmospheric boundary layer over the SCB based on surface and high-resolution vertical observations, ERA5 reanalysis data, and the WRF-Chem model. The MPS shifts between upslope and easterly flows along the eastern slope of the TP and SCB during the day and downslope westerly flows to the western SCB at night. The daytime MPS flows drive the westward transport of O 3 -rich air mass in the atmospheric boundary layer from the polluted SCB and accumulate high O 3 levels from the western edge of the SCB to the eastern slope of TP, subsequently aggravating O 3 pollution in this region. After sunset, the MPS drainage flows carry air containing elevated O 3 eastward downslope along the eastern slope of the TP into the nocturnal residual layer, enhancing the O 3 concentrations aloft over the western SCB. The high-level O 3 in the residual layer is transported downstream by nocturnal prevailing winds and contributes significantly to the next-day surface O 3 buildup in the downwind region through daytime vertical mixing (~30 μg m -3 h -1 ). The present study reveals a transport mechanism driven by the MPS with coupling diurnal changes in the atmospheric boundary layer, which redistributes O 3 over the basin and exacerbates O 3 pollution along the western edge of the basin. This study has important implications for understanding meteorological drivers on atmospheric environment underlying the complex terrain., Competing Interests: Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Exploring the ozone pollution over the western Sichuan Basin, Southwest China: the impact of diurnal change in mountain-plains solenoid”., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In recent years, ozone pollution becomes a serious environmental issue in China. A good understanding of source-receptor relationships of ozone transport from aboard and inside China is beneficial to mitigating ozone pollution there. To date, these issues have not been comprehensively assessed, especially for highly polluted regions in the central and eastern China (CEC), including the North China Plain (NCP), Twain-Hu region (THR), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB). Here, based on simulations over 2013-2020 from a well-validated chemical transport model, GEOS-Chem, we show that foreign ozone accounts for a large portion of surface ozone over CEC, ranging from 25.0% in THR to 39.4% in NCP. Focusing on transport of domestic ozone between the five regions in CEC, we find that atmospheric transport can largely modulate regional interactions of ozone pollution in China. At the surface, THR receives the largest amount of ozone from the other four regions (54.2% of domestic ozone in the receptor region, the same in below), followed by PRD (32.3%), SCB (26.7%), YRD (21.1%), and NCP (18.0%). Meanwhile, YRD exports largest amount of ozone to the other regions, ranging from 8.9% in SCB to 28.4% in THR. Although SCB is relatively isolated and thus impacts NCP, YRD, and PRD weakly (< 2.2%), export of SCB ozone to THR reaches 9.3%. The regional ozone transport over CEC, occurring mostly in the lower troposphere, is mainly modulated by the East Asian monsoon circulations, proximity between source and receptor regions, seasonal changes of ozone production, and topography., Competing Interests: Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Atmospheric transport drives regional interactions of ozone pollution in China”., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Single short-term exposures to ozone are known to cause acute changes in pulmonary function and neutrophilic airway inflammation. The respiratory health effects of repeated exposures are not as well studied. Pulmonary function decrements are known to attenuate, but it is less clear how injury and inflammation are affected. Using sputum induction (SI) to sample respiratory tract lining fluid after single- and multiday exposures, we designed a study to test the hypothesis that neutrophils would increase after multiday exposure compared with single-day exposure. In a randomized, crossover design, 15 normal healthy subjects were exposed to O3 (0.2 ppm) under two conditions: for 4 hr for 1 day (1D) and for 4 hr for 4 consecutive days (4D). Pulmonary function testing was performed immediately before and after each 4-hr exposure. The SI was performed 18 hr after the end of the 1D and 4D conditions. The symptom and pulmonary function data followed a pattern seen in other multiday O3 exposure studies, with the greatest changes occurring on the second day. In contrast to previous studies using bronchoalveolar lavage, however, there was a significant increase in the percentage of neutrophils and a significant decrease in the percentage of macrophages after the 4D condition compared with the 1D condition. Given that SI likely samples proximal airways better than distal lung, these results add to the body of evidence that differential airway compartmental responses to O3 occur in humans and other species.

Events of stratospheric intrusions to the surface (SITS) can lead to severe ozone (O3) pollution. Still, to what extent SITS events impact surface O3 on a national scale over years remains a long-lasting question, mainly due to difficulty of resolving three key SITS metrics: frequency, duration and intensity. Here, we identify 27,616 SITS events over China during 2015-2022 based on spatiotemporally dense surface measurements of O3 and carbon monoxide, two effective indicators of SITS. An overview of the three metrics is presented, illustrating large influences of SITS on surface O3 in China. We find that SITS events occur preferentially in high-elevation regions, while those in plain regions are more intense. SITS enhances surface O3 by 20 ppbv on average, contributing to 30-45% of O3 during SITS periods. Nationally, SITS-induced O3 peaks in spring and autumn, while over 70% of SITS events during the warm months exacerbate O3 pollution. Over 2015-2022, SITS-induced O3 shows a declining trend. Our observation-based results can have implications for O3 mitigation policies in short and long terms. The authors analyze the frequency, duration and intensity of stratospheric intrusions to the surface in China over 2015-2022 and find that such intrusions enhance surface ozone pollution, especially in spring and autumn, followed by summer. [ABSTRACT FROM AUTHOR]

The ozone (O3) variations in southeast China are largely different between mountainous forest areas located inland, and lowland urban areas located near the coast. Here, we selected these two kinds of areas to compare their similarities and differences in surface O3 variability from diurnal to seasonal scales. Our results show that in comparison with the lowland urban areas (coastal areas), the mountainous forest areas (inland areas) are characterized with less human activates, lower precursor emissions, wetter and colder meteorological conditions, and denser vegetation covers. This can lead to lower chemical O3 production and higher O3 deposition rates in the inland areas. The annual mean of 8-h O3 maximum concentrations (MDA8 O3) in the inland areas are ~15 μg·m−3 (i.e. ~15%) lower than that in the coastal areas. The day-to-day variation in surface O3 in the two types of the areas is rather similar, with a correlation coefficient of 0.75 between them, suggesting similar influences on large scales, such as weather patterns, regional O3 transport, and background O3. Over 2016–2020, O3 concentrations in all the areas shows a trend of "rising and then falling", with a peak in 2017 and 2018. Daily MDA8 O3 correlates with solar radiation most in the coastal areas, while in the inland areas, it is correlated with relative humidity most. Diurnally, during the morning, O3 concentrations in the inland areas increase faster than in the coastal areas in most seasons, mainly due to a faster increase in temperature and decrease in humidity. While in the evening, O3 concentrations decrease faster in the inland areas than in the coastal areas, mostly attributable to a higher titration effect in the inland areas. Seasonally, both areas share a double-peak variation in O3 concentrations, with two peaks in spring and autumn and two valleys in summer and winter. We found that the valley in summer is related to the summer Asian monsoon that induces large-scale convections bringing local O3 upward but blocking inflow of O3 downward, while the one in winter is due to low O3 production. The coastal areas experienced more exceedance days (~30 days per year) than inland areas (~5-10 days per year), with O3 sources largely from the northeast. Overall, the similarities and differences in O3 concentrations between inland and coastal areas in southeastern China are rather unique, reflecting the collective impact of geographic-related meteorology, O3 precursor emissions, and vegetation on surface O3 concentrations. [ABSTRACT FROM AUTHOR]

A global-scale horizontally- and vertically-resolved ozone climatology can provide a detailed assessment of ozone variability. Here, the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST) ozone climatology is improved and updated to the recent decade (1970s–2010s) on a grid of 5° × 5° × 1 km (latitude, longitude, and altitude) from the surface to 26 km altitude, with the most recent ozonesonde data re-evaluated following the ASOPOS-2 guidelines (GAW Report No. 268, 2021). Comparison between independent ozonesonde and trajectory-derived ozone shows good agreement in each decade, altitude, and station, with relative differences (RD) of 2–4 % in the troposphere and 0.5 % in the stratosphere. Comparisons of TOST with aircraft and two satellite datasets, the Satellite Aerosol and Gas Experiment (SAGE) and the Microwave Limb Sounder (MLS), show comparable overall agreement. The updated TOST outperforms the previous version with higher data coverage in all latitude bands and altitudes and 14–17 % lower RD compared to independent ozonesondes, employing twice as many ozonesonde profiles and an updated trajectory simulation model. Higher uncertainties in TOST are where data are sparse, i.e., over the southern high latitudes and the tropics, and before the 1980s, and where variability is high, i.e., at the surface and upper troposphere and lower stratosphere (UTLS). Caution should therefore be taken when using TOST in these spaces and times. TOST captures global ozone distributions and temporal variations, showing an overall insignificant change of stratospheric ozone after 1998. TOST offers users a long record, global coverage, and high vertical resolution. [ABSTRACT FROM AUTHOR]

Urban air pollution continues to pose a significant health threat, despite regulations to control emissions. Here we present a comparative analysis of urban ozone (O3) responses to nitrogen oxide (NOx) changes in China and the United States (US) over 2015–2020 by integrating various data‐ and model‐based methods. The data‐based deep learning (DL) model exhibited good performance in simulating urban air quality: the correlation coefficients (R) of O3 daily variabilities with respect to independent O3 observations are 0.88 and 0.79 over N. China, 0.87 and 0.90 over S. China, and 0.87 and 0.49 over E. United States by the DL and GEOS‐Chem chemical transport models, respectively. Furthermore, the data‐based methods suggest volatile organic compound (VOC)‐limited regimes in urban areas over northern inland China and transitional regimes over eastern US urban areas; in contrast, GEOS‐Chem model suggests strong NOx‐limited regimes. Sensitivity analysis indicates that the inconsistent O3 responses are partially caused by the inaccurate representation of O3 precursor concentrations at the locations of urban air quality stations in the simulations, while the data‐based methods are driven by the variabilities in local O3 precursor concentrations and meteorological conditions. The O3 responses to NOx changes reported here provide a better understanding of urban O3 pollution; for example, reductions in NOx emissions are suggested to have resulted in an increase in surface O3 by approximately 7 ppb in the Sichuan Basin in 2014–2020. Plain Language Summary: Ozone (O3) at the surface level is an important pollutant with complicated photochemistry. Here we analyze the responses of urban O3 concentrations to nitrogen oxide (NOx) changes in China and the United States (US) over 2015–2020. The data‐based methods suggest that NOx emission declines have resulted in increases in urban O3 concentrations over northern inland China, while urban O3 concentrations over eastern US are less sensitive to NOx changes. In contrast, GEOS‐Chem simulation suggests that NOx emission declines can result in dramatic decreases in urban O3 concentrations over both China and the US. The derived O3 responses in this work are helpful for making effective regulatory policies to control urban O3 pollution. Key Points: Data‐based methods suggest VOC‐limited regimes over northern inland China and transitional regimes over eastern United States urban areasGEOS‐Chem simulation suggests strong NOx‐limited regimesData‐based methods have the potential to provide new insights for air quality regulations as a supplement to Chemical transport models [ABSTRACT FROM AUTHOR]

Surface ozone increased unexpectedly over northern China during the COVID-19 lockdown (CLD) period (23 January–29 February 2020), which was characterized by vigorous emission reduction. The reasons for this ozone enhancement have been speculated from perspectives of chemical responses to the emissions and meteorology. As known, the processes of natural stratospheric ozone injecting to the troposphere are most active in winter and spring. Yet, little attention was paid to stratospheric influences on this ozone enhancement. Here we report a stratospheric intrusion (SI) that reached the surface over northern China on 15–17 February during the CLD. The coevolution of enhanced ozone and sharply declined carbon monoxide and relative humidity (RH) was indicative of the SI occurrence. We show that the SI was facilitated by a cutoff low system that led to abnormally high surface ozone in most part of northern China. We estimate that over the SI period, the injected stratospheric ozone constituted up to 40–45% of the surface ozone over northern China. If the stratospheric ozone inputs were scaled over the entire CLD period, these inputs would account for 4–8% of the surface ozone. In view of the unexpected ozone increase during the CLD, this SI event could explain up to 18% of the ozone increase in some cities, and average 5–10% over larger areas that were affected. Hence, the nonnegligible stratospheric influences urge extra consideration of natural ozone sources in disentangling the role of emission reduction and meteorological conditions during the CLD in China and elsewhere in the world. [ABSTRACT FROM AUTHOR]

Despite the implementation of the Clean Air Action Plan by the Chinese government in 2013, the issue of increasing surface ozone (O 3) concentrations remains a significant environmental concern in China. In this study, we used an improved regional climate–chemistry–ecology model (RegCM-Chem-YIBs) to investigate the impact of anthropogenic emissions, meteorological factors, and CO 2 changes on summer surface O 3 levels in China from 2008 to 2018. Compared to its predecessor, the model has been enhanced concerning the photolysis of O 3 and the radiative impacts of CO 2 and O 3. The investigations showed anthropogenic emissions were the primary contributor to the O 3 increase in China, responsible for 4.08–18.51 ppb in the North China Plain. However, changed meteorological conditions played a crucial role in decreasing O 3 in China and may have a more significant impact than anthropogenic emissions in some regions. Changed CO 2 played a critical role in the variability of O 3 through radiative forcing and isoprene emissions, particularly in southern China, inducing an increase in O 3 on the southeast coast of China (0.28–0.46 ppb) and a decrease in southwest and central China (- 0.51 to - 0.11 ppb). Our study comprehensively analyzed O 3 variation across China from various perspectives and highlighted the importance of considering CO 2 variations when designing long-term O 3 control policies, especially in high-vegetation-coverage areas. [ABSTRACT FROM AUTHOR]

Figures 6(a) and 6(b) in the study by L2022 used total ozone data from different sources (Total Ozone Mapping Spectrometer, Ozone Monitoring Instrument, and Ozone Mapping and Profiler Suite) without removing potential biases and drifts between the datasets, a step that is essential for accurate diagnosis of trends. [37] (hereafter L2022) used the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST) to argue that there has been very substantial ozone depletion (>80%) in the tropical (30°S-30°N) lower stratosphere (LS) since the 1960s. OZONE IN THE TROPICAL LOWER STRATOSPHERE A. TOST data used by L2022 The World Ozone and Ultraviolet Radiation Data Center (WOUDC) website (https://woudc.org/data.php) hosts the TOST data in two formats: (1) ozonesonde data at 1 km intervals in altitude expanded with 4-day forward and backward trajectories to increase spatial sampling (hereafter labeled TOST RAW) and (2) a smoothed, gap-filled version of TOST RAW using linear interpolation of maps (hereafter labeled TOST SM). The stronger decline reported by L2022 is caused by inappropriate use of the gap-filled version of the TOST ozone dataset, which is based on sparse tropical ozone sondes before the 1990s. [Extracted from the article]

Tropospheric ozone (O3) is an important greenhouse gas and a surface pollutant. The future evolution of O3 abundances and chemical processing are uncertain due to a changing climate, socioeconomic developments, and missing chemistry in global models. Here, we use an Earth System Model with natural halogen chemistry to investigate the changes in the O3 budget over the 21st century following Representative Concentration Pathway (RCP)6.0 and RCP8.5 climate scenarios. Our results indicate that the global tropospheric O3 net chemical change (NCC, chemical gross production minus destruction) will decrease ∼50%, notwithstanding increasing or decreasing trends in ozone production and loss. However, a wide range of surface NCC variations (from −60% to 150%) are projected over polluted regions with stringent abatements in O3 precursor emissions. Water vapor and iodine are found to be key drivers of future tropospheric O3 destruction, while the largest changes in O3 production are determined by the future evolution of peroxy radicals. We show that natural halogens, currently not considered in climate models, significantly impact on the present‐day and future global O3 burden reducing ∼30–35 Tg (11–15%) of tropospheric ozone throughout the 21st century regardless of the RCP scenario considered. This highlights the importance of including natural halogen chemistry in climate model projections of future tropospheric ozone. Key Points: Water vapor and iodine are key drivers of future tropospheric ozone destructionNatural halogens reduce 30–35 Tg of tropospheric ozone burden throughout the 21st century regardless of the Representative Concentration Pathway scenario consideredEnhanced halogen‐driven surface ozone loss will determine the effectiveness of future policies on air quality [ABSTRACT FROM AUTHOR]

Observational data indicate increasing trends of surface ozone (O3) in China, despite emission controls that have resulted in reductions of precursor emissions. Here, we explore the cause of this contradiction, through analysis of surface observations (2014–2019) in China and historical observation record in the United States (US, 1990–2019). Our observation‐based analysis indicates that the reductions of nitrogen oxides (NOx) emissions led to increase of surface O3 in North China Plain (NCP) and Yangtze River Delta (YRD) of around 8 ppb. However, NOx controls resulted in shift of O3 chemical regimes over NCP and YRD, with turning points between NOx‐ and volatile organic compound (VOC)‐limited regimes around 2019, while model simulations suggest transitional or NOx‐limited regimes over the rest of China. The impacts of high fine particles (PM2.5) on O3 formation has declined because of the reduction of PM2.5 concentrations. Stricter NOx controls can mitigate O3 pollutions over industrialized areas in China. Plain Language Summary: Ozone (O3) in surface air is an important pollutant with adverse effects on human health and vegetation growth. Here, we explore the sustainable pathway to control O3 pollution in China through analysis of observations in China (2014–2019) and the United States (US, 1990–2019). We find that the reductions of nitrogen oxides (NOx) emissions have led to increase of surface O3 in North China Plain (NCP) and Yangtze River Delta (YRD) by about 8 ppb. However, the severe O3 pollution in China can be mitigated because NOx controls led to the shift of O3 chemical regimes over NCP and YRD to transitional regime around 2019. In addition, the impacts of high fine particles (PM2.5) on O3 formation has declined because of the reduction of PM2.5 concentrations. Key Points: Reductions of nitrogen oxides (NOx) led to increase of surface ozone (O3) in North China Plain and Yangtze River DeltaShift of O3 chemical regimes with turning points between NOx‐ and volatile organic compound‐limited regimes around 2019The impacts of high fine particles (PM2.5) on O3 formation has declined with reduction of PM2.5 concentrations [ABSTRACT FROM AUTHOR]

Ozone (O3) is of great importance to air quality in the Yangtze River Delta (YRD) region, China. Focusing on surface O3 in the YRD, its spatiotemporal characteristics, meteorological driving factors, and health impacts are investigated based on surface O3 observations in this study. We find that from 2015 to 2019, most cities failed to meet the national standards and O3 pollution mainly occurred in warm seasons (April‐September). Surface O3 in the YRD increased in the recent years, so did tropospheric column O3. Spatially, surface shows a significant positive autocorrelation over the YRD except for 2019, and high O3 values mainly gather in the central of the YRD. The first two empirical orthogonal functions (EOFs) of O3 accounted for 37.4% and 20.0% of the total variance in O3, respectively, with their spatial patterns characterized by the same phase and west and east contrast, respectively. The EOF1 is related to the change in radiation and the EOF2 is attributed to regional transport by prevailing westerly wind. Downward UV radiation, temperature and u wind are crucial factors. The premature mortality caused by O3 for respiratory disease in the YRD is estimated to be 5,889 cases per year. Furthermore, we find that premature mortality is more sensitive to O3 concentration than population, suggesting that controlling peak O3 concentration can bring great health benefits. Key Points: Spatiotemporal characteristics of ozone were investigated based on satellite data and observation network dataThe main meteorological driving factors of ozone were captured using EOF analysisThe premature mortality attributed to ozone was estimated with epidemiological exposure response functions [ABSTRACT FROM AUTHOR]

The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled atmosphere–ocean chemistry–climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 ± 36 Tg in 1850 to a mean value of 356 ± 31 Tg for the period 2005–2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 ± 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 ± 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3 - LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere–troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850–2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden. [ABSTRACT FROM AUTHOR]

Landfall typhoons can significantly affect O 3 in the Yangtze River Delta (YRD) region. In this study, we investigate a unique case characterized by two multiday regional O 3 pollution episodes related to four successive landfall typhoons in the summer of 2018 in the YRD. The results show that O 3 pollution episodes mainly occurred during the period from the end of a typhoon to the arrival of the next typhoon. The time when a typhoon reached the 24 h warning line and the time when the typhoon dies away in mainland China can be roughly regarded as time nodes. Meanwhile, the variations of O 3 were related to the track, duration and landing intensity of the typhoons. The impact of typhoons on O 3 was like a wave superimposed on the background of high O 3 concentration in the YRD in summer. When a typhoon was near the 24 h warning line before it landed on the coastline of the YRD, the prevailing wind originally from the ocean changed to be from inland, and it transported lots of precursors from the polluted areas to the YRD. Under influences of the typhoon, the low temperature, strong upward airflows, more precipitation and wild wind hindered occurrences of high O 3 episodes. After the passing of the typhoon, the air below the 700 hPa atmospheric layer was warm and dry, and the downward airflows resumed. The low troposphere was filed with high concentration of O 3 due to O 3 -rich air transported from the low stratosphere and strong photochemical reactions. It is noteworthy that O 3 was mainly generated in the middle of the boundary layer (∼ 1000 m) instead of at the surface. High O 3 levels remained in the residual layer at night, and would be transported to the surface by downward airflows or turbulence by the second day. Moreover, O 3 can be accumulated and trapped on the ground due to the poor diffusion conditions because the vertical diffusion and horizontal diffusion were suppressed by downward airflows and light wind, respectively. The premature deaths attributed to O 3 exposure in the YRD during the study period were 194.0, more than the casualties caused directly by the typhoons. This work has enhanced our understanding of how landfall typhoons affect O 3 in the YRD and thus can be useful in forecasting O 3 pollution in regions strongly influenced by typhoon activities. [ABSTRACT FROM AUTHOR]

Ozone pollution in China is influenced by meteorological processes on multiple scales. Using regression analysis and weather classification, we statistically assess the impacts of local and synoptic meteorology on daily variability in surface ozone in eastern China in summer during 2013–2018. In this period, summertime surface ozone in eastern China (20–42 ∘ N, 110–130 ∘ E) is among the highest in the world, with regional means of 73.1 and 114.7 µ g m -3 , respectively, in daily mean and daily maximum 8 h average. Through developing a multiple linear regression (MLR) model driven by local and synoptic weather factors, we establish a quantitative linkage between the daily mean ozone concentrations and meteorology in the study region. The meteorology described by the MLR can explain ∼43 % of the daily variability in summertime surface ozone across eastern China. Among local meteorological factors, relative humidity is the most influential variable in the center and south of eastern China, including the Yangtze River Delta and the Pearl River Delta regions, while temperature is the most influential variable in the north, covering the Beijing–Tianjin–Hebei region. To further examine the synoptic influence of weather conditions explicitly, six predominant synoptic weather patterns (SWPs) over eastern China in summer are objectively identified using the self-organizing map clustering technique. The six SWPs are formed under the integral influence of the East Asian summer monsoon, the western Pacific subtropical high, the Meiyu front, and the typhoon activities. On average, regionally, two SWPs bring about positive ozone anomalies (1.1 µ g m -3 or 1.7 % and 2.7 µ g m -3 or 4.6 %), when eastern China is under a weak cyclone system or under the prevailing southerly wind. The impact of SWPs on the daily variability in surface ozone varies largely within eastern China. The maximum impact can reach ±8 µ g m -3 or ±16 % of the daily mean in some areas. A combination of the regression and the clustering approaches suggests a strong performance of the MLR in predicting the sensitivity of surface ozone in eastern China to the variation of synoptic weather. Our assessment highlights the importance of meteorology in modulating ozone pollution over China. [ABSTRACT FROM AUTHOR]

Tropospheric ozone in East Asia is influenced by the transport of ozone from foreign regions around the world. However, the magnitudes and variations in such influences remain unclear. This study was performed to investigate the influences using a global chemical transport model, GEOS-Chem, through the tagged ozone and emission perturbation simulations. The results show that foreign ozone is transported to East Asia (20–60 ∘ N, 95–150 ∘ E) mainly through the middle and upper troposphere. In East Asia, the influence of foreign ozone increases rapidly with altitude. In the middle and upper troposphere, the regional mean concentrations of foreign ozone range from 32 to 65 ppbv, being 0.8–4.8 times higher than its native counterpart (11–18 ppbv). Annually, ∼60 % of foreign ozone in the East Asian middle and upper troposphere comes from North America (5–13 ppbv) and Europe (5–7 ppbv), as well as from foreign oceanic regions (9–21 ppbv). Over the East Asian tropospheric columns, foreign ozone appears most in spring when ozone concentrations in the foreign regions are high and the westerlies are strong and least in summer when the South Asian High blocks eastward foreign ozone from reaching East Asia south of 35 ∘ N. At the East Asian surface, the annual mean of foreign ozone concentrations is ∼22.2 ppbv, which is comparable to its native counterpart of ∼20.4 ppbv. In the meantime, the annual mean of anthropogenic ozone concentrations from foreign regions is ∼4.7 ppbv, half of which comes from North America (1.3 ppbv) and Europe (1.0 ppbv). Seasonally, foreign ozone concentrations at the East Asian surface are highest in winter (27.1 ppbv) and lowest in summer (16.5 ppbv). This strong seasonality is largely modulated by the East Asian monsoon (EAM) via its influence on vertical motion. The large-scale subsidence prevailing during the East Asian winter monsoon (EAWM) favours the downdraft of foreign ozone to the surface, while widespread convection in the East Asian summer monsoon (EASM) blocks such transport. Interannually, the variation in foreign ozone at the East Asian surface is found to be closely related to the intensity of the EAM. Specifically, the stronger the EAWM is in a winter, the more ozone from North America and Europe reaches the East Asian surface because of the stronger subsidence behind the East Asian trough. In summer, ozone from South and South-east Asia is reduced in strong EASM years due to weakened south-westerly monsoon winds. This study suggests substantial foreign influences on ozone at the East Asian surface and in its tropospheric columns. It also underscores the importance of the EAM in the seasonal and interannual variations in foreign influences on surface ozone in East Asia. [ABSTRACT FROM AUTHOR]

Aerosols and ozone have significant influences on air qualities, human health, and climate changes. To further understand the characteristics and interactions among different urban air pollutants in the west Yangtze River Delta (YRD) region, continuous measurements of low-layer atmospheric particles and trace gases have been performed at an urban site in Nanjing from September 2016 to February 2017 in this study. In the urban area of the west YRD, the mean PM 10 and O3 concentrations are 86.3 µ g m -3 and 37.7 ppb, respectively, with significant seasonal and diurnal variations. Particles, which are dominated by fine aerosols, are relatively scattering. And most of their optical properties have similar variations to the aerosol concentrations. Results also show that the west YRD could still suffer severe air pollution, although the seasonal mean aerosol concentrations have decreased in recent years. Even in cold seasons, O3 could exceed the National Ambient Air Quality Standards for about 40 days during the sampling period. Most of polluted episodes are caused by local and subregional emissions. A case study for a typical O3 and PM 2.5 episode in December 2016 demonstrates that the episode was generally associated with regional transport and a stable weather system. Air pollutants were mostly transported from the western areas with high emissions, as well as with an anticyclone and high-pressure system in this region. Correlation analysis reveals that the interactions between O3 and PM are complex, with a combination of inhibition and promotion under different conditions. The inhibition effect might result from the reduction of photolysis frequency near the surface due to aerosols in addition to their positive correlations with precursors, while the promotion effect is from the formation of secondary aerosols under high concentrations of oxidants and solar radiation. However, the interaction between O3 and BC shows an inhibiting effect due to its chemical stability. This also indicated a VOC-sensitive regime for photochemical production of O3 in this region. This study further improves insight into the characteristics and interactions of main pollutants and may contribute to the improvement of the simulation and prediction of aerosols and gases in the urban area of the YRD. [ABSTRACT FROM AUTHOR]

The transport of North American (NA) ozone to East Asia is investigated through the analysis of a 20 year simulation (1987-2006) using a global chemical transport model (GEOS-Chem) and forward trajectories during the 1990s at three NA sites. NA ozone mainly influences northern East Asia (> 30°N), where NA ozone in the free troposphere peaks in spring and fall (~12 ppbv). At the surface, NA ozone ranges from 2 to 7 ppbv and peaks in winter, ~50% of which is from the NA boundary layer. The seasonality of the imported NA ozone reflects the combined effects of meteorology and chemistry. In summer, NA ozone can be diverted from reaching East Asia by strong downdrafts behind the European trough. In winter, the prevailing monsoon climate in East Asia can boost downdrafts of NA ozone to the surface. In spring and fall, the westerlies are stronger and shift farther south than in summer, bring more NA ozone to the East Asian (EA) free troposphere than in summer. The imported NA ozone at the EA surface also varies with interannual meteorology. This interannual variation is found to closely correlate to the East Asian winter monsoon (EAWM). The stronger the EAWM in a winter is, the stronger are the downdrafts bringing more NA ozone to the EA surface in that winter and the subsequent spring. Because the anthropogenic NA emissions have decreased since 1999, the year an emission inventory was used in the simulation, the simulated NA influence may serve as an upper limit. [ABSTRACT FROM AUTHOR]

Accuracy of global tropopause altitude products from reanalyses is important to applications of the products, including the derivation of tropospheric column ozone (TCO). Here, monthly biases in lapse-rate tropopause pressure (PLRT) in two reanalyses, NCEP/NCAR and MERRA-2, and associated implications for estimating TCO are examined, based on global radiosonde observations over 1980–2017 at 689 stations. Our analysis suggests that the global mean PLRT is underestimated by −2.3 hPa in NCEP/NCAR and by −0.9 hPa in MERRA-2, mainly attributable to large negative biases around the subtropics (~20°–50°) in both hemispheres, with generally positive biases at other latitudes. Overall, NCEP/NCAR outperforms MERRA-2 in the Northern Hemisphere but underperforms MERRA-2 in the Southern Hemisphere. PLRT biases in the two reanalyses vary more evidently with latitude than with longitude. From winter to summer, the peaks of negative PLRT biases around the subtropics shift poleward by ~10°. Approximately, 70% of the reanalysis PLRT biases are within −10–10 hPa. Consequently, a negative (positive) PLRT bias induces a positive (negative) TCO bias. In absolute magnitude, the mean ozonesonde TCO bias attributable to PLRT biases is ~0.2, ~0.8 and ~1.2 Dobson Units (DU) if a PLRT bias is within 0–5, 10–15, and 10–15 hPa. Using a global ozone climatology, we estimate that the global mean bias in TCO induced by the PLRT biases in both reanalyses is positive, being 0.64 DU (or 2.2%) for NCEP/NCAR and 0.28 DU (or 1.1%) for MERRA-2. [ABSTRACT FROM AUTHOR]

Black carbon aerosol (BC) has significant impacts on regional climate changes due to its absorption of solar radiation, which might lead to substantial variations in surface ozone (O 3) pollution. Therefore, the potential role of the BC warming effect on O 3 responses during summertime in China is investigated and quantified based on online-coupled regional climate-chemistry model (RegCM-Chem) simulations and multiple linear regression (MLR) analysis. The results suggest that the absorption of solar radiation can lead to considerable decreases in cloud amount, precipitation and relative humidity in most regions of China in summer, which further cause a warming of the low-layer atmosphere and an enhancement in the southerly East Asian summer monsoon. The variations in O 3 concentrations in response to the BC warming effect are almost as large as the observed O 3 growth in China from 2013 to 2017 (7.2%). They have significant regional specificity over China. Due to the warming effect of BC, the near-surface daily O 3 concentrations are increased in most areas of Central to East China, with an increase of 6.2%, while they are decreased in Southwest China, with a reduction exceeding 5.8%, implying that BC emission reductions favor the mitigation of ozone pollution in East China in summer. Further investigations via MLR analysis indicate that the BC-induced O 3 increases in Central and East China and O 3 decreases in Southwest China are strongly related to changes in relative humidity, while the BC-induced O 3 increases in North and Northeast China are likely linked to the changes in transportation. These results may help us better understand the mechanism of O 3 variation and can provide a scientific reference for BC and O 3 reduction strategies over China. [Display omitted] • BC and surface O 3 is found to be significantly and positively correlated at over 80% sites of China. • BC warming effect has a contribution to surface O 3 growth in Eastern China (EC) in summer. • Humidity and transportation changed by BC are mostly responsible for O 3 increase in EC. [ABSTRACT FROM AUTHOR]

In coastal areas where many highly developed metropolis and city agglomeration are located, high ozone (O 3) concentrations are one of the most severe environmental problems, which is gravely impacted by the sea-land breeze (SLB) circulation. The complex terrain, such as coastal mountain, complicates SLB circulation and its impact on coastal O 3 pollution. Previous studies have shown that coastal mountain intensifies SLB circulation in various regions worldwide, such as the southern California coastal region, the Mediterranean coastal region, and the Beijing-Tianjin-Hebei region. However, the specific impacts of the underlying mechanism on coastal O 3 levels, along with a quantified contribution analysis, remain insufficiently explored. Here we present that the coastal mountain terrain in the Western Taiwan Strait of South China induces a three-dimensional O 3 transport involving SLB circulation through integrating observational data from 2015 to 2022 and employing WRF-Chem modeling, specifically focusing on the thermally driven sea breeze during the day combined with the up-slope flow generated by the coastal mountain. This flow carries O 3 -rich air mass, which are caused by both photochemical reactions and cross-boundary transport originating from land and offshore sources. As the air moves inland and up the mountainside, it ultimately forming a layer of high-level O 3 aloft. O 3 trapped in the layer acts as a reservoir of elevated O 3 for the coastal plain and aggravates O 3 pollution on the following day via vertical mixing by over 40 μg m−3 h−1. Based on the sensitivity simulation, this work quantifies that the coastal mountain contributes 23.4 % of surface O 3 concentrations increase in the coastal urban area. This recirculated transport of O 3 has important implications for understanding meteorological drivers on atmospheric environment in complex terrain. • The mountain-plain/urban-sea terrain induces a recirculated ozone transport involving SLB circulation. • The recirculated transport mechanism aggravates O 3 pollution over the mountainous coastal region by increasing vertical mixing contribution. • The coastal mountain contributes 23.4 % of surface O 3 concentrations increase in the coastal urban area. [ABSTRACT FROM AUTHOR]

With the enactment of Air Pollution Action Plan in 2013, the air quality improved in most Chinese cities, except that surface ozone (O 3) increased markedly. Some recent studies have examined this issue and presented controversial opinions, but only focus on summertime ozone increase. This study extends a comprehensive analysis of the influencing factors on China's ozone changes from 2013 to 2017 out of the summer season, combining satellite data, ground measurements and model analyses. The annual trends of air pollutants, e.g., increase in 95th percentile O 3 concentration (+1.4–8.7 μg m−3 yr−1), and decreases in fine particulate matter (PM 2.5 ; −4.0~−7.5 μg m−3 yr−1) and sulfur dioxide (−2.6~−9.7 μg m−3 yr−1) are uncovered by satellite and observational data. Model results show that the attributions of surface O 3 changes from 2013 to 2017 vary spatially and seasonally, and most regions are more affected by emission changes (−9.5–47.0 μg m−3) rather than meteorological changes (−8.1–21.3 μg m−3). In specific regions and seasons, e.g., south/southwestern and eastern China south of 35°N in May and July, the surface O 3 responses to climate variability could have an equal or even greater importance than emission changes. In these major pollution control regions, e.g. northern and mid-eastern China, the precursor emissions control (11–35%) contributes in the same degree as the changes in aerosol effects (35–38%) to surface ozone enhancement in the warm seasons. More scientific emission controls and climate adaptation strategies are required to attain the synergetic control of atmospheric particulate matter and ozone in China. Image 1 • Air pollution trends are uncovered by satellite data and observations. • The attributions of surface O 3 changes are more affected by emission changes. • O 3 response to atmospheric warming is dominant south of 30°N in warm season. • NO x and VOC s control contributed equally as aerosol effect in NCP and East China. [ABSTRACT FROM AUTHOR]

Ozone pollution is of great concern in the Yangtze River Delta (YRD) region of China. The regional air pollution is significantly associated with dominant weather systems. In this study, the relationship between O 3 concentration and weather patterns over the YRD region is investigated. Specifically, based on the National Center for Environmental Prediction reanalysis data and the sums-of-squares technique, six typical weather patterns in 2017 are identified. It is found that higher O 3 concentrations tend to occur under the weather patterns 3, 4 and 5. These high mean O 3 concentrations are associated with the southerly flow introduced by the western Pacific subtropical high, the low pressure in the saddle field and the warm ocean current introduced by typhoon. Whereas the low mean O 3 concentrations under the weather patterns 1, 2 and 6 are linked to the northwesterly flow brought from the Siberian high. Moreover, when the light pollution appears, main synoptic weather systems get weakened and the wind speed is reduced at 850 hPa and surface. At the same time, the temperature increases due to the weakening of the northerly wind introduced by the Siberian high and the North China warm high, and the strengthening of the southerly wind introduced by a southwest vortex. The occurrence of the heavy pollution is highly related to these further enhanced synoptic processes. The downward motion is obviously enhanced due to the trough or ridge deepening at 500 hPa and the strong downdraft around the typhoon. Higher temperature, lower wind and stronger downward motion due to the minor changes of atmospheric circulation establish conditions conducive to the formation and accumulation of O 3. • Weather patterns and their effects on different O 3 pollution levels in the YRD is investigated. • Six typical weather patterns are identified, and three pollution levels under them are further discussed. • O 3 pollution is related to the high-pressure systems, the southwest vortex, and the tropical cyclones. • Weakened northerly flow, low wind speed and strengthened downward motion are primarily factors to severe O 3 pollution. [ABSTRACT FROM AUTHOR]