Different effects of anthropogenic emissions and aging processes on the mixing state of soot particles in the nucleation and accumulation modes

Abstract

In this study, the mixing state of size-resolved soot particles and their influencing factors were investigated based on a 5-month aerosol volatility measurement at a suburban site (Xingtai, XT) in the central North China Plain (NCP). The volatility and mixing state of soot-containing particles at XT were complex, caused by multiple pollution sources and various aging processes. The results suggest that anthropogenic emissions can weaken the mean volatility of soot-containing particles and enhance their degree of external mixing. There were fewer externally mixed soot particles in warm months (June, July, and August) than in cold months (May, September, and October). Monthly variations in the mean coating depth (Dc,mean) of volatile matter on soot particles showed that the coating effect was stronger in warm months than in cold months, even though aerosol pollution was heavier in cold months. Moreover, the volatility was stronger, and the degree of internal mixing was higher in nucleation-mode soot-containing particles than in accumulation-mode soot-containing particles. Relationships between Dc,mean and possible influencing factors (temperature (T), relative humidity (RH), and particulate matter, with diameters ranging from 10 to 400 nm) further suggest that high ambient T and RH in a polluted environment could promote the coating growth of accumulation-mode soot particles. However, high ambient T but low RH in a clean environment was beneficial to the coating growth of nucleation-mode soot particles. Our results highlight the diverse impact of anthropogenic emissions and aging processes on the mixing state of soot particles in different modes, which should be considered separately in models to improve the simulation accuracy of aerosol absorption.