Observation of chemical components of PM2.5 and secondary inorganic aerosol formation during haze and sandy haze days in Zhengzhou, China

Abstract

Mineral dust particles play an important role in the formation of secondary inorganic aerosols, which largely contribute to haze pollution in China. During this study, a haze episode (haze days) and a typical haze process mixed with sandstorm (sandy haze days) were observed in Zhengzhou with a series of high-time-resolution monitoring instruments from November 22 to December 8, 2018. Concentrations of PM10 and crustal elements clearly increased in the sandy haze days. Concentrations of gaseous pollutants, metallic elements emitted from anthropogenic sources, nitrate, and ammonium during sandy haze days were slightly lower than those during the haze days but still obviously higher than those during the non-haze days. The sulfate concentrations, the sulfate fractions in PM2.5, and the sulfur oxidation ratios significantly increased in the sandy haze days. Heterogeneous reactions dominated the conversion of SO2 during the haze and sandy haze days. Enhanced SO2 conversion during the sandy haze days may be attributed to the high concentrations of transition metal ions from the sandstorm when the values of relative humidity (RH) were in 30%–70%, and high O3 at certain time points. Gas-phase NO2 oxidation reactions were the main pathways for nitrate formation. In the sandy haze days, higher nitrogen oxidation ratio (NOR) at daytime may be associated with higher RH and lower temperature than those in the haze days, which facilitate the gas-to-particle partitioning of nitrate; higher NOR values at night may be attributed to the higher O3 concentrations, which promoted the formation of N2O5.