An integrated process rate analysis of a regional fine particulate matter episode over Yangtze River Delta in 2010

A high PM 2.5 pollution episode was detected in Shanghai in November 2010. The integrated process rate method, an advanced diagnostic tool, was applied to account for the contribution of different atmospheric processes during the high pollution episode in the Yangtze River Delta region (YRD). The PM 2.5 process analysis indicates that the emission of fine particles is the dominant source of high surface PM 2.5 concentrations in the major cities of the YRD like Shanghai, Nanjing, and Hangzhou, following horizontal transportation and aerosols. The PM 2.5 concentration could be reduced due to vertical advection and diffusion from lower levels to the upper air. The aerosols process such as homogeneous nucleation and condensation producing PM 2.5 occurs throughout the PBL layer in urban areas, causing vertical transport from upper levels down to the surface layer. The aerosols process is much more significant in a downwind rural and coastal site like Zhoushan than in the urban areas. The PM 2.5 change initiated by both horizontal transport and vertical transport is much stronger at 40–2000 m height than in the surface layer, while the PM 2.5 change caused by horizontal diffusion is very small. Dry deposition can significantly reduce concentration of the particulates in the surface level of the atmosphere, and wet deposition can remove the particles in the planetary boundary layer (PBL). The cloud processes can either increase PM 2.5 due to the aqueous-phase oxidation of SO 2 and NO 2 or remove PM 2.5 due to cloud scavenging. Solar radiation and humidity are more important to secondary pollution, and they are the significant external factors affecting the chemical reactions among sulfur dioxide, nitrogen oxides, ammonia, volatile compounds and fine particles.