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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 PM 10 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 PM 2.5 , and the sulfur oxidation ratios significantly increased in the sandy haze days. Heterogeneous reactions dominated the conversion of SO 2 during the haze and sandy haze days. Enhanced SO 2 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 O 3 at certain time points. Gas-phase NO 2 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 O 3 concentrations, which promoted the formation of N 2 O 5. Image 1 [ABSTRACT FROM AUTHOR]

The non-linear relationship between particle sulfate and SO 2 is still unclear. Continuous observations were conducted in a typical city in Northern China from 2017 to 2020. SO 2 concentration continually decreased, however, the SO 4 2− concentration and the sulfur oxidation ratio increased in 2018–2019. The major reason might be the variations of sulfate formation by the aqueous-phase reactions, especially for the O 2 + TMI oxidation (oxygen catalyzed by transition-metal ions). Particle pH, SO 2 , and Mn concentration were the main factors affecting the O 2 + TMI reaction rate. Sensitivity analysis indicated that the decrease of the O 2 + TMI reaction rate caused by SO 2 or Mn concentration dropping by half can only offset the increased rate caused by a 0.1 unit decrease in particle pH. Considering the actual atmospheric environment in Northern China, if the particle pH continues to drop, the slow decline of SO 2 and Mn concentration may not prevent the increase in the O 2 + TMI oxidation rate. In addition to meteorological parameters, the concentrations of SO 4 2−, crustal elements, and total ammonium were the primary factors for the decrease in particle pH in the past 3 years. Therefore, the impact of pH on sulfate formation must be considered when controlling dust and ammonia emissions. [Display omitted] • Changes in O 2 + TMI reaction rates partly explained the non-linear relationship between SO 4 2− and SO 2 ; • If the pH continues to drop, the slow decline of SO 2 and Mn may not offset the increase in the O 2 + TMI oxidation rate. • The concentrations of SO 4 2−, crustal elements and total ammonium were the primary factors for the decrease in particle pH. [ABSTRACT FROM AUTHOR]

Heterogeneous oxidation of sulfur dioxide (SO2) is suggested to be one of the most important pathways for sulfate formation during extreme haze events in China. Yet, the exact mechanism remains highly uncertain. We propose a much less explored pathway for aqueous-phase SO2 oxidation to form particulate sulfate by NO2 and OH radicals produced from photolysis of particulate nitrate. Reactive uptake experiments of SO2 by ammonium nitrate particles under UV irradiation show the measured SO2 uptake coefficients of ~10-5. Model calculations of sulfate production rates, comparing known oxidation mechanisms by O3, NO2, H2O2, and transition metal ions, and the nitrate photolysis mechanism suggest that the nitrate photolysis pathway could contribute significantly to the overall sulfate production at pH = 4 to 6. The present study provides a new insight into the current debate on sulfate production pathways under typical haze conditions in China. [ABSTRACT FROM AUTHOR]