Chemical characteristics of size-resolved fog water at an urban site in Nanjing and the summit of Mt. Lu, East China

Abstract

The chemical characteristics of fog water are closely related to the droplet size. This study used a three-stage Caltech Active Strand Cloud Collector (CASCC) to collect 170 size-resolved fog samples from eight fog events in the winter of 2013–2017 at an urban site in Nanjing and the summit of Mt. Lu. The pH, electrical conductivity (EC), and inorganic ion concentration were analyzed, and the sources of pollutants in size-dependent droplets were discussed according to the local emission source type. The pH, EC, total ion concentration (TIC), and ion composition of fog water at Nanjing were dependent on the droplet size. The TIC of fog water at Nanjing was approximately 10 times that measured at Mt. Lu. The concentration of different chemical species in the droplet was also size-dependent: Ca2+ concentration was higher in larger droplets with the diameter (D) ≥ 22 μm, whereas SO42−, NO3−, and NH4+ concentrations were higher in smaller droplets with D ranging 4–16 μm. The ion concentration of fog water at Mt. Lu first decreased then increased as the fog developed. Owing to the faster drop rate of larger droplets, coarse particles (2.5–10 μm) exhibited a faster response to fog removal, and the final mass concentration of fine particles (<2.5 μm) decreased the most due to competition between deposition and gravity. Different pollution sources exhibited different contribution rates to the chemical components of size-dependent droplets. Soil sources mainly affected the Ca2+ content of large droplets, whereas industrial sources and vehicles affected the NO3− and SO42− concentrations of medium and small droplets. Soil sources, industrial sources, and vehicles were the main sources of fog water at the urban site in Nanjing, whereas industrial sources, vehicles, soil sources, biomass combustion, and secondary aerosols were the main sources of fog water at the summit of Mt. Lu.