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 (PM_2.5) 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.

Severe surface ozone pollution over major Chinese cities has become an emerging air quality concern, raising a new challenge for emission control measures in China. In this study, we explore the source contributions to surface daily maximum 8 h average (MDA8) ozone over China in 2016 and 2017, the 2 years with the highest surface ozone averaged over Chinese cities in record. We estimate the contributions of anthropogenic, background, and individual natural sources to surface ozone over China using the GEOS-Chem chemical transport model at $\mathrm{0.25}{}^{\circ }\times \mathrm{0.3125}{}^{\circ }$ horizontal resolution with the most up-to-date Chinese anthropogenic emission inventory. Model results are evaluated with concurrent surface ozone measurements at 169 cities over China and show generally good agreement. We find that background ozone (defined as ozone that would be present in the absence of all Chinese anthropogenic emissions) accounts for 90 % (49.4 ppbv) of the national March–April mean surface MDA8 ozone over China and 80 % (44.5 ppbv) for May–August. It includes large contributions from natural sources (80 % in March–April and 72 % in May–August). Among them, biogenic volatile organic compound (BVOC) emissions enhance MDA8 ozone by more than 15 ppbv in eastern China during July–August, while lightning NO_x emissions and ozone transport from the stratosphere both lead to ozone enhancements of over 20 ppbv in western China during March–April. Over major Chinese city clusters, domestic anthropogenic sources account for about 30 % of the May–August mean surface MDA8 ozone and reach 39–73 ppbv (38 %–69 %) for days with simulated MDA8 ozone > 100 ppbv in the North China Plain, Fenwei Plain, Yangtze River Delta, and Pearl River Delta city clusters. These high ozone episodes are usually associated with high temperatures, which induce large BVOC emissions and enhance ozone chemical production. Our results indicate that there would be no days with MDA8 ozone > 80 ppbv in these major Chinese cities in the absence of domestic anthropogenic emissions. We find that the 2017 ozone increases relative to 2016 are largely due to higher background ozone driven by hotter and drier weather conditions, while changes in domestic anthropogenic emissions alone would have led to ozone decreases in 2017. Meteorological conditions in 2017 favor natural source contributions (particularly soil NO_x and BVOC ozone enhancements) and ozone chemical production, increase the thermal decomposition of peroxyacetyl nitrate (PAN), and further decrease ozone dry deposition velocity. More stringent emission control measures are thus required to offset the adverse effects of unfavorable meteorology, such as high temperature, on surface ozone air quality.

Tropospheric ozone is a key air pollutant and greenhouse gas. Its fate strongly depends on meteorological conditions and therefore subject to climate change influences. Such dependences through biogenic, chemical, and dynamic processes on different spatiotemporal scales have been unraveled from observations and modeling studies. In this process-oriented review, we summarize three dominant pathways of meteorological and climatic impacts on tropospheric ozone and present their recent progress. The three pathways are influences through changes in the natural precursor emissions, the kinetics and partitioning of chemistry and deposition, and the transport of ozone and its precursors. Tropospheric ozone levels have shown significant global or regional responses to meteorological/climatic changes (e.g., changes in the Brewer-Dobson Circulation, the Hadley Circulation, and El Niño–Southern Oscillation) and can be explained through the conjunction of these pathways. Most recent model projections predict that future climate will increase surface ozone in polluted regions and decrease ozone at a global scale due to stronger ozone chemical loss. However, uncertainties in climate-ozone responses and limitations in model capability still challenge the magnitude and even the sign of such projections. We highlight the rising importance of future increase of stratosphere-troposphere exchange in modulating tropospheric ozone that may largely compensate the predicted chemical loss of tropospheric ozone burden. We also highlight that uncertainties in isoprene chemistry, biogenic emissions in changing CO₂ levels and vegetation, and interactions between ozone and vegetation may largely affect the surface ozone response to climate change. Future research and model improvements are required to fill these gaps.

Significance: The North China Plain experiences severe summer ozone pollution, but ozone during winter haze (particulate) pollution events has been very low. Here, we show that the abrupt decrease in nitrogen oxide (NO _x ) emissions following the COVID-19 lockdown in January 2020 drove fast ozone production during winter haze events to levels approaching the air quality standard. This fast ozone production was driven by formaldehyde originating from high emissions of volatile organic compounds (VOCs). The COVID-19 experience highlights a general 2013 to 2019 trend of rapidly increasing ozone pollution in winter–spring in China as NO _x emissions have decreased. VOC emission controls would mitigate the spreading of ozone pollution into winter–spring with benefits for public health, crop production, and particulate pollution.