Role of meteorology-driven regional transport on O3 pollution over the Chengdu Plain, southwestern China

Abstract

Owing to the complex mesoscale meteorological dynamics, deep basin topography and pollutants emissions, the Chengdu Plain (CDP) situated at west border of the Sichuan Basin (SCB) have suffered excessive ozone (O3) pollution in the warm season. However, the meteorology-driven mechanism and associated dominant processes underlying the elevated O3 levels are not well elucidated due to limited ambient measurements. Here, we adopted trace gases data measured by basin-wide network in combination with a high-resolution chemical transport model for characterizing a regional continuous O3 episode occurred from 10 to 18 August 2019. The extensive monitoring network shapes a much-detailed evolution map of O3 concentrations which depicts that O3 hotspots extend from metropolitan areas to suburban areas across the CDP. It is found that both advection of O3-rich air masses in the residual layer (RL) and regional transport of O3 and its precursors under prevailing winds exert profound influences on the elevation of ambient O3 across the CDP. As the convective boundary layer is destroyed after sunrise, vertical mixing effects cause the entrainment of O3-rich air masses in RL to the surface, affecting the redistribution of surface O3 concentration. In addition, process analysis (PA) and integrated source apportionment method (ISAM) results of the CMAQ model indicated that vertical transport and gas-phase chemistry were the main sources of O3 during the day, while dry deposition and horizontal advection were the main sinks of surface O3. O3 transport by boundary condition (BCO) was the largest contributor to all urban O3 concentrations in the CDP. Our findings highlight the regional transport mechanism of O3 in SCB under complex topographic conditions and point out the physical and chemical causes and sources of typical summer O3 pollution episodes in the SCB.