Characteristics of chemical composition and role of meteorological factors during heavy aerosol pollution episodes in northern Beijing area in autumn and winter of 2015

Heavy aerosol pollution episodes (HPEs) usually start from late autumn and become more serious in winter in Beijing and its vicinity (BIV). In this study, we examine the reasons for the formation and change of HPEs in the areas of northern BIV. The size-resolved chemical components of PM₁ and meteorological conditions were investigated during HPEs in autumn and winter of 2015. Stable regional atmosphere and higher atmospheric condensation processes associated with southerly and lower speed wind led to the formation of HPEs. After the start of these HPEs, the concentration of fine particles increased more than twice in several hours. ~80% of the ‘explosive’ growth in PM mass can be considered as a positive feedback of meteorological factors that come from even more stable atmosphere and larger condensation rate of water vapour, which was derived from the interaction between formed aerosol pollution and the meteorological factors within boundary layer. Nitrate was the largest fraction of PM₁ in autumn, and the most significantly increased component during HPEs relative to clean period during both of autumn and winter. The proportion of organic aerosol (OA) was similar to that of nitrate in autumn, but its rise in HPE was much smaller, mainly because of the high concentration of OA existed during clean periods. Compared with the largest increase of nitrate, the similar uplift was found for ammonium production, showing that a large amount of ammonium was mainly formed by the combination of NO₃^- in HPEs, rather than SO₂₄-. In addition to the lower southerly wind carrying pollutants from southern part of BIV, westerly wind from central Inner Mongolia and north Shanxi can also bring air pollutants originating from coal combustion, contributing to the heavy pollution in the northern BIV area in winter, and resulting in higher sulphate, nitrate and OA masses.