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Under the influence of climate change, the increasing occurrence of extreme weather events, such as heatwaves, has led to an enhanced frequency of ozone (O 3) pollution issues. In August 2022, the Sichuan Basin (SCB), a typical large-scale geographical terrain located in southwestern China, experienced the most severe heatwave in the last 20 years. The heatwave led to substantial disparities in O 3 levels across the region. Here, by integrating observations, machine learning, and numerical simulations, we aim to understand the diverse O 3 formation mechanisms in two megacities, Chengdu (western location) and Chongqing (eastern location). Observational data showed that Chengdu experienced a consecutive 17 d period of O 3 exceedance, in contrast to Chongqing, where O 3 concentrations remained below the standard. Meteorological and precursor factors were assessed, highlighting high temperatures, intense solar radiation, and overnight accumulative pollutants as key contributors to O 3 concentrations. The interplay of isoprene, temperature, and O 3 , alongside the observation-based box model and MEGAN simulations, underscored the significant role of intensified biogenic volatile organic compounds (BVOCs) in O 3 formation. Interestingly, Chongqing exhibited nearly double the BVOC emissions of Chengdu, yet contributed less to O 3 concentrations. This discrepancy was addressed through CMAQ-DDM (Decoupled Direct Method) simulations and satellite diagnosis by investigating the O 3 –NO x –VOC sensitivity. Notably, Chengdu displayed a VOC-driven sensitivity, while Chongqing showed a transitional regime. Moreover, the regional transport also played a pivotal role in the spatial divergence of O 3 pollution. Cross-regional transport predominantly influenced Chongqing (contributing ∼ 80 %), whereas Chengdu was mainly affected by the emissions within the basin. The local accumulated pollutants gave rise to the atmospheric oxidizing capacity, resulting in a substantial photochemical contribution to O 3 levels (49.9 ppbv h -1) in Chengdu. This comparison of the difference provides insights into the complex interplay of meteorology, natural emissions, and anthropogenic sources during heatwaves, guiding the necessity of targeted pollution control measures on regional scales. [ABSTRACT FROM AUTHOR]

Nitrate, sulfate and ammonium (NSA) are the main secondary inorganic aerosols of PM2.5 and play an important role in air pollution. In this study, a 3-year observational experiment was conducted from 1 January 2015 to 31 December 2017, in Chengdu, southwest China. NSA pollution characteristics, chemical conversion generation, emission reduction control sensitivity and pollutant regional transport characteristics were analyzed. NSA are the most important chemical compositions of particles with aerodynamic equivalent diameter ≤2.5µm in ambient air (PM2.5), and the contribution of nitrate to the accumulation of PM2.5 concentration is greater than that of sulfate and ammonium. NSA also have obvious characteristics of annual, monthly, seasonal, diurnal and weekly variations. Through observation data and model simulation, it was also found that the existence of an aerosol aqueous environment plays an important role in the formation and existence of NSA. Sensitivity analysis between NSA found that controlling NO3- and SO42- plays an important role in reducing the contribution of NSA to PM2.5 , which also implies that the current control of NOx and SO2 is important for improving air pollution. Combined with meteorological conditions and potential source contribution function (PSCF) analysis, local emissions and regional emissions of pollutants are found to have important impacts on Chengdu's atmospheric environment. This research result not only provides an assessment of the current atmospheric emission reduction effect but also provides an important reference for atmospheric pollution control. [ABSTRACT FROM AUTHOR]