Characterization and sources of water-soluble inorganic ions during sulfate-driven and nitrate-driven haze on the largest loess accumulation plateau.

With the rapid reduction of anthropogenic SO emissions, the critical driver of haze in China has shifted from being dominated by sulfate to alternating sulfate and nitrate. Haze induced by different driver species may differ in the chemical forms of water-soluble inorganic ions (WSIIs). The unique topography and high-emission industrial agglomeration of the Loess Plateau determine its severe local PM pollution and influence global weather patterns through the outward export of pollutants. PM samples were conducted in Pingyao, on the eastern Loess Plateau of China, in autumn and winter. The average mass of PM was 88.82 ± 57.37 μg/m; sulfate, nitrate, and ammonium were the dominant component. The chemical form of the ion was dominated by (NH)SO, NHNO, NaNO and KNO during the nitrate-driven (ND) haze, while (NH)SO, NHHSO, NHNO, NaNO and KNO were predominant species during the sulfate-driven (SD) haze. Heterogeneous oxidation reactions dominated the mechanism of sulfate formation. Primary sulfate emissions or other generation pathways contributed to sulfate formation during the SD haze. The gas-phase homogeneous reaction of NO and NH dominates the nitrate generation during the ND haze. The heterogeneous reactions also played an essential role during the SD haze. Nitrate aerosol (42.30%) and coal and biomass combustion (23.23%) were the dominant sources of WSIIs during the ND haze. In comparison, nitrate aerosol (31.80%) and sulfate aerosol (25.08%) were considered the primary control direction during the SD haze. The chemical characteristics and sources of aerosols under various types of haze differ significantly, and knowledge gained from this investigation provides insight into the causes of heavy haze.