Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective.

Sulfate (SO) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO (nss-SO) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM in Haikou. The S(IV)+HO pathway was the main formation pathway for secondary SO in wintertime in Haikou, contributing to 57% of secondary SO formation. By contrast, 54% of secondary SO was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+HO, S(IV)+Fe×Mn, and S(IV)+NO pathways contributed equally to secondary SO formation. The ionic strength was the controlling parameter for the S(IV)+NO pathway, while pH was identified as a key factor that mediates the S(IV)+HO and S(IV)+Fe×Mn pathways to produce secondary SO. This study contributes to our understanding of secondary SO production under low PM concentrations but high SO percentages.