Aerosol chemical compositions in the North China Plain and the impact on the visibility in Beijing and Tianjin

To better understand the characteristics of the chemical compositions of aerosols comprised of water soluble ions (WSIs) and carbonaceous aerosol (CA) and their impacts on the visibility throughout the North China Plain (NCP), four experimental sampling campaigns were carried out between June 2013 and May 2014. The sampling sites were located in Beijing, Xiangshan, Tianjin, Shijiazhuang, and Qinhuangdao. The air pollution episodes mainly occurred during the autumn and winter in the NCP. With regard to different particle size distributions, the ratio of anions to cations in the fine size (0.64) was greater than that in the coarse size (0.54) in the NCP. Coefficients of divergence indicate that aerosol pollution had similar characteristics in the five cities of the NCP and that the pollutants were characterized by mutual influences and regional transfer processes. There were different non-linear correlations between the visibility and mass concentrations of PM2.5 at different relative humidity (RH) conditions. When the RH was > 70%, the visibility was < 5 km; this shows that, due to hygroscopic growth in a higher RH, the increase in PM2.1 resulted in a decrease in the visibility. The Interagency Monitoring of Protected Visual Environments (IMPROVE) method was used to estimate the light extinction coefficients (LEC) from the measured concentrations of chemical species. Organic matter (OM), ammonium nitrate (AN), and ammonium sulfate (AS) were the three dominant species that contributed to the LEC in Beijing and had the highest proportions of total contributions to the LEC in the summer, accounting for 32.2%, 25.9%, and 24.4%, respectively. The LEC of sea salts did not change much throughout the year and accounted for 4.1-5.3% in Beijing and 4.8-7.4% in Tianjin. According to the Ambient Air Quality Standard, the days were divided into pollutional days (PD) and attained days (AD). The increasing concentrations of AN, AS and OM resulted in an increase in the LEC in the NCP, which led to the occurrences of low visibility events during the autumn and winter. NH4+, NO3-, and SO42- mainly existed as fine size particles (diameter < 2.1 pm) in Beijing (73.5%, 80.7%, and 78.0% on PD and 63.3%, 79.4%, and 72.5% on AD, respectively) and Tianjin (81.0%, 80.6%, and 82.1% on PD and 71.5%, 44.3%, and 69.7% on AD, respectively). However, the CA originating mainly from fuel combustion also played an important part in the visibility impairment in the coarse size (2.1 < diameter < 9.0 pm). According to a positive matrix factorization (PMF) model, 37.1% and 26.5% of the LEC came from secondary aerosols on PD in Beijing and in Tianjin, respectively. In addition, biomass burning, fuel combustion and fugitive dust were also important contributing sources of the LEC in the NCP.