Continuous monitoring, compositions analysis and the implication of regional transport for submicron and fine aerosols in Beijing, China

A comprehensive observation was conducted to reveal the characteristics of submicron and fine aerosols and their chemical components in Beijing in autumn. The potential source contribution function (PSCF) and concentration weighted trajectory (CWT) model, Weather Research & Forecasting (WRF) and Comprehensive Air Quality Model with Extension (CAMx) were applied to identify the impacts of emissions from surrounding areas on PM2.5 concentrations in Beijing for different study episodes. These results showed that the monthly average concentration of non-refractory submicron aerosols (NR-PM1) was 60.20 ± 53.48 μg/m3, with organics being the major fraction (42.33%), followed by NO3− (22.87%), SO42− (18.70%), NH4+ (12.24%) and Cl− (3.86%). The PSCF and CWT analysis indicated common high source regions was located in the western Shandong, northern Henan and southern Hebei, and the distribution of the source areas usually varied with species during different episodes. The regional source apportionment results revealed that 52.36% of surface PM2.5 in Beijing was contributed by surrounding sources in terms of monthly mean and the outside contribution increased to 64.56% during the haze days, indicating the strong influence of regional transport on Beijing haze pollution. Note that the PM2.5 inflows for Beijing mainly came from Baoding and Langfang, and outflows towards Chengde and Zhangjiakou, identifying two key transport pathways: the southwest-northeast pathway and the southeast-northwest pathway. The PM2.5 inflow fluxes from surroundings were much higher than outflows from Beijing, leading to the peaking PM2.5 pollution, with the highest concentrations occurring from 00:00 to 11:00 LT, 14 October 2016. Based on the net PM2.5 fluxes and their vertical distributions, we discovered the PM2.5 transport mainly occurred at 400-800 m during the entire episode, while at 600-1000 m for a heavy pollution episode, both followed by the southwest-northeast pathway. However, the PM2.5 fluxes were far lower for a clean episode, and followed by the northwest-southeast pathway.