Analysis of aerosol particle number size distribution and source attribution at three megacities in China

Abstract

Air pollution has been a major concern in China, especially in densely populated megacities. Particulate pollutants, generally known as aerosols, have sizes that range widely and complex physical and chemical compositions with high spatiotemporal variabilities. We have conducted a series of experiments using the same comprehensive observation system, measuring various aerosol properties in the three megacities of Beijing, Shanghai, and Guangzhou. Although there are differences in the total particle number concentration and particle number size distribution among the three sites due to differences in emission sources and sampling seasons, a high proportion of ultrafine particles was observed in all cities, i.e., 86.33%, 88.34%, and 81.1%, respectively. The high particle number concentration at the Beijing and Guangzhou sites was caused by an increase in nucleation-mode particles at nighttime. The concentration of nucleation-mode particles at nighttime was significantly higher than during the daytime. We call this phenomenon a nocturnal nucleation event, and combined with analyses of factors involved, attribute this event to traffic emissions. To investigate the pollution sources at each location, we identified and quantified the pollution contributions from different sources by applying the k-means clustering analysis to observational datasets of environmental quantities. Clustering results showed that traffic emissions were the main contributor in Beijing and Guangzhou, with direct and significant impacts ranging from 37% to 45%, respectively.