LAUSR

Abstract Beijing has experienced increasingly frequent heavy air pollution caused by high loadings of fine particulate matter (PM2.5) in recent decades, especially in autumn and winter. In this study, the formation mechanisms of air pollution episodes between late September and early October were investigated on the basis of observations and reanalysis data from 2006 to 2015. During the transitional period from the East Asian summer monsoon (EASM) to the East Asian winter monsoon (EAWM), an abrupt change in meteorological conditions resulting in heavy PM2.5 pollution episodes was regularly observed in autumn (around October 4–5). Thereafter, pollution episodes with higher PM2.5 concentrations and more frequent and longer duration occurred after October 4–5. In particular, another marked increase in the PM2.5 concentration occurred around mid-November when city heat supply began. We applied the moving t-test (MTT) and Mann–Kendall (MK) methods to investigate the role of meteorological factors affected by the Asian monsoon. The abrupt change in the PM2.5 concentration in autumn (around October 5) was closely related to the monsoon transition, during which the atmospheric circulation and diffusion conditions were undergoing a transition from summer to winter type. This could be favorable for the transport, accumulation and secondary formation of PM2.5. The PM2.5 concentration increased obviously around November 15, and it could be caused by the combination of more emissions from city heat supply and the more stable winter atmosphere. This study revealed significant differences between the two mechanisms that produced increases in the PM2.5 concentration in autumn and winter, and indicated close connections between the variation of monsoon type and air pollution characteristics to some extent. When considering emission reductions, attention could also be given to the monsoon transition period when the atmospheric circulation pattern and diffusion conditions begin to change.