The effects of the "two-way feedback mechanism" on the maintenance of persistent heavy aerosol pollution over areas with relatively light aerosol pollution in northwest China.

Persistent heavy aerosol pollution episodes (HPEs) often occur in the central and eastern regions of China during the wintertime. When PM2.5 mass concentration cumulates to a certain extent that normally exceeds 100 µg m-3, the PM2.5 pollution will then further worsen the meteorological conditions in boundary layer (BL), resulting in significant two-way feedback between the cumulative PM2.5 pollution and worsened meteorological conditions. In the less polluted northwest area of China, whether there is significant two-way feedback phenomenon mentioned above after the accumulation of PM2.5 mass concentration has always been an issue of concern for understanding the full picture of meteorological causes of HPEs in China. Here, measurements of PM2.5 concentration, meteorological data and an integrated pollution-linked meteorological index (PLAM) are used to characterize the relationship between meteorological elements and PM2.5 concentration in persistent HPEs from 1 December 2016 to 10 January 2017 in Lanzhou and Urumqi. In early stage of the HPEs formation, the general 500 hPa circulation situations are normally high-pressure ridge or zonal westerly airflow patterns, associating with worsened meteorological conditions. At this time, with the decrease of BL height, the mass concentration of PM2.5 also increases as the pollutants mix in a relatively small space as compared to the clean period. When PM2.5 cumulates to a certain extent (above 75 µg m-3 in Lanzhou and above 100 µg m-3 in Urumqi), there are often obvious inversion and humidification in the near-surface. The inversion is mainly caused by the accumulation of the PM2.5 concentration, which indicates that even in northwest China, where anthropogenic activities are less affected and PM2.5 pollution is less serious, the cumulative PM2.5 can nonetheless worsen meteorological conditions. The worsened meteorological conditions, which are quantified by the PLAM index, dominate the changes in PM2.5 in the explosive growth of persistent HPEs (accounted for 50-85%).