Chemical composition, water content and size distribution of aerosols during different development stages of regional haze episodes over the North China Plain

Aerosol size distribution and chemical composition are found to have significant effects on its hygroscopicity, acidity/alkalinity and light extinction. Heavy haze pollution occurred frequently in the wintertime of NCP, with long duration time and large impact area, which had important influences on air quality and human health. However, the study concerning the formation mechanism and physicochemical characteristics of aerosols in different haze stages have been rarely carried out. In this study, aerosol size distribution in the range of 10 nm 10 mu m, water soluble inorganic ions (WSIIs), PM (PM2.5, PM10), trace gases, organic carbon (OC), elemental carbon (EC) and meteorological elements were derived during a regional haze pollution episode from Nov. 9 to 16, 2018. The aerosol water content and pH were further calculated using the ISORROPIA model. We divided the whole observation period into segments of clean days, fog processes, and haze processes, including three stages of I: accumulation, II: growth, and III: explosion, and a dry haze (D-haze) process based on the PM2.5 concentration, visibility and RH. Given the shift of particle size to larger segment ascribed to the ageing process during fog/haze process, the size distribution peak in aerosol number concentration was located at 100 nm in the fog/ haze episode and was 70 nm larger than that on clean days. The concentration descended significantly in stage III, peaking at 160 nm, suggesting a strong aging process of aerosols. The PM2.5 increased sharply under high RH, static weather conditions and strengthening oxidation that favored liquid and heterogeneous reactions. The nitrate concentration was found to account for 20.5% (D-haze) - 29.0% (fog) of the total water soluble inorganic ions (WSIIs) during the fog/haze process, while sulfate constituted only 13.8% (stage III) - 21.3% (stage I), revealing dominant nitrate pollution. Hence, nitrate was considered to originate mainly from photochemical and heterogeneous reactions in haze episodes and was generated only by heterogeneous reactions. Higher (lower) aerosol water content made aerosols more acidic (alkaline) under similar chemical compositions. Therefore, the chemical reactions may differ under dry and wet haze pollution. Sources of carbonaceous aerosols changed in different haze stages with descending (enhancing) contributions from coal combustion (vehicle exhaust).

Automated summary

This study investigated a regional haze pollution episode from Nov. 9 to 16, 2018, and found that aerosol properties such as size distribution, composition, and sources of carbonaceous aerosols varied in different stages, with nitrate being the predominant pollutant. The study highlights the importance of understanding the physicochemical characteristics of aerosols in different haze stages for effective pollution control and management.