: By analyzing the data of urban air pollutant measurements from 2013 to 2015 in Nanjing, East China, we found that the correlation coefficients between major atmospheric compound pollutants PM_2.5 and O₃ were respectively 0.40 in hot season (June, July and August) and −0.16 in cold season (December, January and February) with both passing the confidence level of 99%. This provides evidence for the inverse relations of ambient PM_2.5 and O₃ between cold and hot seasons in an urban area of East China. To understand the interaction of PM_2.5 and O₃ in air compound pollution, the underlying mechanisms on the inversion relations between cold and hot seasons were investigated from the seasonal variations in atmospheric oxidation and radiative forcing of PM_2.5 based on three-year environmental and meteorological data. The analyses showed that the augmentation of atmospheric oxidation could strengthen the production of secondary particles with the contribution up to 26.76% to ambient PM_2.5 levels. High O₃ concentrations in a strong oxidative air condition during hot season promoted the formation of secondary particles, which could result in a positive correlation between PM_2.5 and O₃ in hot season. In cold season with weak atmospheric oxidation, the enhanced PM_2.5 levels suppressed surface solar radiation, which could weaken O₃ production for decreasing ambient O₃ level with the low diurnal peaks. Under the high PM_2.5 level exceeding 115 μg·m⁻³, the surface O₃ concentration dropped to 12.7 μg·m⁻³ at noon with a significant inhibitory effect, leading to a negative correlation between PM_2.5 and O₃ in cold season. This observational study revealed the interaction of PM_2.5 and O₃ in air compound pollution for understanding the seasonal change of atmospheric environment.

A high O₃ episode with the large increases in surface ozone by 21–42 ppbv and the nocturnal surface O₃ levels exceeding 70 ppbv was observed in the region between Xiamen and Quanzhou over the southeastern coast of China during 12–14 June 2014, before the Typhoon Hagibis landing. Variations in the surface O₃, NO₂, CO and meteorology during the Typhoon Hagibis event clearly suggest a substantial impact of the peripheral downdrafts in the large-scale typhoon circulation on such an O₃ episode excluding the contributions of photochemical production and the horizontal transport. The influence of vertical O₃ transport from the upper troposphere and lower stratosphere (UTLS) region on high surface O₃ levels is further confirmed by a negative correlation between surface O₃ and CO concentrations as well as dry surface air observed during the O₃ episode. This study provides observational evidence of typhoon-driven intrusion of O₃ from the UTLS region to surface air, revealing a significant effect of such a process of stratosphere–troposphere exchange (STE) of O₃ on tropospheric O₃ and ambient air quality.

While the elevated ambient levels of particulate matters with aerodynamic diameter of 2.5 µm or less (PM_2.5) are alleviated largely with the implementation of effective emission control measures, an opposite trend with a rapid increase has been seen in surface ozone (O₃) in the North China Plain (NCP) region over the past several years. It is critical to determine the real culprit causing such a large increase in surface O₃. In this study, 7-year surface observations and satellite retrieval data are analyzed to determine the long-term change in surface O₃ as well as driving factors. Results indicate that anthropogenic emission control strategies and changes in aerosol concentrations as well as aerosol optical properties such as single-scattering albedo (SSA) are the most important factors driving such a large increase in surface O₃. Numerical simulations with the National Center for Atmospheric Research (NCAR) Master Mechanism (MM) model suggest that reduction of O₃ precursor emissions and aerosol radiative effect accounted for 45 % and 23 % of the total change in surface O₃ in summertime during 2013–2019, respectively. Planetary boundary layer (PBL) height with an increase of 0.21 km and surface air temperature with an increase of 2.1 ^∘C contributed 18 % and 12 % to the total change in surface O₃, respectively. The combined effect of these factors was responsible for the rest of the change. Decrease in SSA or strengthened absorption property of aerosols may offset the impact of aerosol optical depth (AOD) reduction on surface O₃ substantially. While the MM model enables quantification of an individual factor's percentage contributions, it requires further refinement with aerosol chemistry included in the future investigation. The study indicates an important role of aerosol radiative effect in development of more effective emission control strategies on reduction of ambient levels of O₃ as well as alleviation of national air quality standard exceedance events.