Origin of regional springtime ozone episodes in the Sichuan Basin, China: Role of synoptic forcing and regional transport.

The Sichuan Basin (SCB) located in southwestern China has long been considered the most polluted city cluster with exposure to unhealthy levels of ozone (O) at times. However, the features of O regional transport and source contributions in SCB are poorly understood. In this study, ambient measurements, ERA5 reanalysis dataset, IASI O column, and the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) modeling system coupled with the Integrated Source Apportionment Method (ISAM) module were used to investigate the formation mechanism and sources of a severe O episode in spring 2020 over the SCB. In the first stage of the O episode, a high-pressure system persisted over the western SCB and caused northeasterly wind fields, leading to enhanced regional transport from the northern boundary with the O contribution from the boundary exceeding 50% across the SCB. As the synoptic pattern evolved, southeasterly winds dominated the SCB and the stagnant zone over the Chengdu Plain confined O originating from the southern SCB and Chongqing city, leading to the accumulation of precursors and elevated O levels. During the O episode, transportation and industrial sources were major contributors to O formation especially for the Chengdu Plain and Chongqing city. In addition, the O-rich air mass in the nocturnal residual layer that formed over Chongqing city was transported to the Chengdu Plain through southeastern corridor at 400-1600m above ground-level under the prevailing southeasterly winds. With sunrise and the development of the atmospheric boundary layer, the O-rich air mass in the residual layer (RL) was entrained to the ground-level via vertical mixing, which further enhanced O pollution across the Chengdu Plain. Our results revealed the mechanism of regional transport via northeastern and southeastern corridors during an O episode and demonstrated the need for joint emission regulation across the SCB to mitigate O pollution.