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 µg 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 µg m⁻³ or 1.7 % and 2.7 µg 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 µg 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.

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 (NO_x) emissions led to increase of surface O₃ in North China Plain (NCP) and Yangtze River Delta (YRD) of around 8 ppb. However, NO_x controls resulted in shift of O₃ chemical regimes over NCP and YRD, with turning points between NO_x‐ and volatile organic compound (VOC)‐limited regimes around 2019, while model simulations suggest transitional or NO_x‐limited regimes over the rest of China. The impacts of high fine particles (PM_2.5) on O₃ formation has declined because of the reduction of PM_2.5 concentrations. Stricter NO_x controls can mitigate O₃ pollutions over industrialized areas in China.

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.

In this study, we characterize the transport of ozone from Africa to Asia through the analysis of the simulations of a global chemical transport model, GEOS-Chem, from 1987 to 2006. The receptor region Asia is defined within 5–60∘ N and 60–145∘ E, while the source region Africa is within 35∘ S–15∘ N and 20∘ W–55∘ E and within 15–35∘ N and 20∘ W–30∘ E. The ozone generated in the African troposphere from both natural and anthropogenic sources is tracked through tagged ozone simulation. Combining this with analysis of trajectory simulations using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we find that the upper branch of the Hadley cell connects with the subtropical westerlies in the Northern Hemisphere (NH) to form a primary transport pathway from Africa to Asia in the middle and upper troposphere throughout the year. The Somali jet that runs from eastern Africa near the equator to the Indian subcontinent in the lower troposphere is the second pathway that appears only in NH summer. The influence of African ozone mainly appears over Asia south of 40∘ N. The influence shows strong seasonality, varying with latitude, longitude, and altitude. In the Asian upper troposphere, imported African ozone is largest from March to May around 30∘ N (12–16 ppbv) and lowest during July–October around 10∘ N (∼ 2 ppbv). In the Asian middle and lower troposphere, imported African ozone peaks in NH winter between 20 and 25∘ N. Over 5–40∘ N, the mean fractional contribution of imported African ozone to the overall ozone concentrations in Asia is largest during NH winter in the middle troposphere (∼ 18 %) and lowest in NH summer throughout the tropospheric column (∼ 6 %). This seasonality mainly results from the collective effects of the ozone precursor emissions in Africa and meteorology and chemistry in Africa, in Asia and along the transport pathways. The seasonal swing of the Hadley circulation and subtropical westerlies along the primary transport pathway plays a dominant role in modulating the seasonality. There is more imported African ozone in the Asian upper troposphere in NH spring than in winter. This is likely due to more ozone in the NH African upper troposphere generated from biogenic and lightning NOx emissions in NH spring. The influence of African ozone on Asia appears larger in NH spring than in autumn. This can be attributed to both higher altitudes of the elevated ozone in Africa and stronger subtropical westerlies in NH spring. In NH summer, African ozone hardly reaches Asia because of the blocking by the Saharan High, Arabian High, and Tibetan High on the transport pathway in the middle and upper troposphere, in addition to the northward swing of the subtropical westerlies. The seasonal swings of the intertropical convergence zone (ITCZ) in Africa, coinciding with the geographic variations of the ozone precursor emissions, can further modulate the seasonality of the transport of African ozone, owing to the functions of the ITCZ in enhancing lightning NOx generation and uplifting ozone and ozone precursors to upper layers. The strength of the ITCZ in Africa is also found to be positively correlated with the interannual variation of the transport of African ozone to Asia in NH winter. Ozone from NH Africa makes up over 80 % of the total imported African ozone over Asia in most altitudes and seasons. The interhemispheric transport of ozone from southern hemispheric Africa (SHAF) is most evident in NH winter over the Asian upper troposphere and in NH summer over the Asian lower troposphere. The former case is associated with the primary transport pathway in NH winter, while the latter case is associated with the second transport pathway. The intensities of the ITCZ in Africa and the Somali jet can each explain ∼ 30 % of the interannual variations in the transport of ozone from SHAF to Asia in the two cases.