Elucidating roles of near-surface vertical layer structure in different stages of PM2.5 pollution episodes over urban Beijing during 2004-2016

Local weather conditions play important roles in surface fine particulate matter (PM2.5) pollution episodes in metropolitan areas. Relative to surface meteorological factors, the roles of near-surface vertical structure of urban boundary layer in air pollution has yet to be fully understood, due to limitation of vertical observation. By using surface PM2.5 observations and multi-layer (15 levels) vertical meteorological elements from a 325-m meteorological tower during 2004-2016, we investigated the different stages of 456 PM2.5 Pollution Episodes (PPEs) and their associations with vertical urban boundary layer structure in Beijing. The average PM2.5 concentration during PPEs is 141.2 mu g m(-3), twice the 13-year average (70.3 mu g m(-3)). PPEs are more frequent and severe in autumn and winter, showing significant seasonal variations. For vertical wind, southerly (southwesterly) winds prevailed from 80 m to 320 m in the early stage of pollution episodes, transporting pollutants to Beijing. While strong northerly (northwesterly) winds prevailed in near-surface layer in the dissipation stages (DSs) and after PPEs, effectively removing pollutants. Owing to different thermal dynamic conditions, the starting hours of PPEs are various, showing that PPEs with stable atmospheric stratification occur most frequently at 1800 LST. Particularly, for long-lasting PPEs, shortwave (longwave) radiation at near-surface layer is weakened (enhanced) due to the existence of clouds and pollutant aerosols, attenuating the momentum flux and sensible heat flux, which further aggravates the accumulation of pollutants. Our findings provide a better understanding of the physical formation mechanism of high concentrations of the PM2.5 pollution episodes and may aid in designing effective mitigation strategies for the urban environment and health.