Attenuation of mountain-valley winds on wintertime PM2.5 change over the western Sichuan Basin, Southwest China

Abstract

The Sichuan Basin (SCB) with complex mountain-basin topography, located immediately to the east of the Tibetan Plateau (TP) in southwest China, is identified as a region with severe PM2.5 pollution over China. To comprehensively understand the SCB terrain effect on the atmospheric environment change, we investigated the effect degree and meteorological mechanism of mountain-valley winds on wintertime PM2.5 in the western SCB, thermally driven mountain-valley winds between the mountains of eastern TP and the western edge of SCB based on near-surface observations of PM2.5 and the ERA5 reanalysis data of meteorology. The results show that the western SCB exhibits a significant diurnal change of mountain-valley winds, shifting between daytime easterly flows and nighttime westerly flows over the western edge of SCB. The frequency of the mountain-valley winds was accounted for 39% of the study’s duration, with the valley and mountain wind-controlling periods being from 2:00 pm to 5:00 pm and 01:00 am to 05:00 am in local time, respectively, and the remaining time being the transition period. Notably, a reduction of 4.5~20.7μg m-3 in near-surface PM2.5 concentrations averaged over the western edge of SCB, decreasing 6~74% in the PM2.5 pollution during the days of mountain-valley winds, which indicates that the mountain-valley winds could alleviate wintertime near-surface PM2.5 in improving air quality over the western SCB. The easterly wind speed was uplifted by 18% during the valley wind-controlling period, corresponding to a 22% reduction in the near-surface PM2.5 levels driven by the mountain-valley winds. The westerly wind speed increased by 50% at the mountain wind-controlling period with a 20% reduction in PM2.5, reflecting the attenuating effect of the favorable atmospheric diffusion conditions on PM2.5 pollution over the western SCB. The daytime noticeably enhanced easterly wind and the strong updraft flows over the eastern slope of TP promoted the dilution and diffusion of air pollutants over the western edge of SCB, and the nocturnal downhill flows along the slope in the westerly winds on the western edge of SCB brought clean TP air to the polluted SCB region in southwest China.