Characterization, mixing state, and evolution of single particles in a megacity of Sichuan Basin, southwest China

A Single Particle Aerosol Mass Spectrometer (SPAMS) was deployed in the urban area of Chengdu to investigate the size-resolved chemical composition and mixing state of single particles from 22 September to 26 October 2016. Ten major single particle types were resolved with the ART-2a algorithm and these types were classified into carbonaceous particles, secondary inorganic particles, and other particles. The mixing state analysis showed that carbonaceous and other particles were commonly internally mixed with sulfate and nitrate by atmospheric processes. Carbonaceous particles were mainly internally mixed with sulfate, while other particle types were mixed with nitrate. The abatement of NOx concentrations is essential for the reduction of PM2.5 (particles with the aerodynamic diameter smaller than 2.5 μm) pollution in Chengdu. The complete process of a pollution event can be summarized as regional (clean), local emission (transition), and accumulation of pollutants (polluted). At high relative humidity levels, the aqueous-phase reaction is a key pathway for the formation of PM2.5 species. The diverse emission characteristics of different types of days (weekdays, weekends, and the National Day holiday) resulted in significantly different diurnal patterns in the PM2.5 mass and total number concentrations. The preliminarily formed particles experienced a variety of atmospheric processes (such as coagulation, condensation, and accumulation), which were responsible for the decrease in the number concentration and subsequent high PM2.5 mass concentration values. All these findings provide new insights into, and improved understanding of, the behavior of single particles and the pollution formation mechanisms in the Sichuan Basin.