Attenuation of mountain-valley circulations on PM2.5 pollution over the western Sichuan basin, southwest China

The Sichuan Basin (SCB), located immediately to the east of the Tibetan Plateau (TP) in southwest China, is identified as a region with severe PM2.5 pollution, especially in the western SCB region. To understand the terrain effect on the atmospheric environment change in detail, this study investigated the effect degree and meteorological mechanism of thermally driven mountain-valley breeze (MVB) circulations on wintertime PM2.5 in the western SCB region, based on the near-surface observations of PM2.5 and the ERA5 reanalysis data of meteorology. The results showed that the western SCB edge exhibited a significant diurnal change of MVB, shifting between daytime upslope easterly flows and nighttime downslope westerly flows. The frequency of the MVB circulations was accounted for 39% days in December 2017, with the mountain and valley breeze-controlling periods being from 01:00 to 05:00 and 14:00 to 17:00 local time, respectively. Notably, the hourly PM2.5 reductions of 13.9 +/- 4.6 mu g m(-3) was averaged during the MVB days in the western SCB edge, resulting in a decrease of 46.4 +/- 14.0% in PM2.5 pollution with the MVB, which indicates that the MVB could alleviate PM2.5 pollution in improving air quality over the western SCB region. The daytime MVB circulations drove the transport of PM2.5-rich air mass in the atmospheric boundary layer from the polluted western SCB edge to the surrounding regions, causing 22% attenuation in near-surface PM2.5 concentrations during the valley breeze-controlling period. The nocturnal MVB circulations carried clean TP air eastward downslope along the eastern slope of the TP into the polluted western SCB region, mitigating PM2.5 levels by 20% during the mountain breeze-controlling period.