Abstract. Water-soluble metals are known to produce greater adverse human health
outcomes than their water-insoluble forms. Although the concentrations of
water-soluble aerosol metals are usually limited by atmospheric processes
that convert water-insoluble metals to water-soluble forms, factors that
control the solubilities of aerosol metals in different environments remain
poorly understood. In this study, we investigated the abundance and fractional solubilities of different metals in size-fractionated aerosols
collected at an urban site in Hong Kong and identified the factors that
modulated metal solubilities in fine aerosols. The concentrations of total
and water-soluble metals in fine and coarse aerosols were the highest during the winter and spring seasons due to the long-range transport of air masses by northerly prevailing winds from emission sources located in continental areas north of Hong Kong. The study-averaged metal fractional solubilities spanned a wide range for both fine (7.8 % to 71.2 %) and coarse (0.4 % to 47.9 %) aerosols, but higher fractional solubilities were typically observed for fine aerosols. Sulfate was found to be strongly
associated with both the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn and their fractional solubilities in fine aerosols, which implied that sulfate-driven acid processing likely played an important role in the dissolution of the water-insoluble forms for these six metals. Further analyses revealed that these strong associations were due to sulfate providing both the acidic environment and liquid water reaction medium needed for the acid dissolution process. Thus, the variability in the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn and their
fractional solubilities were driven by both the aerosol acidity levels and
liquid water concentrations, which in turn were controlled by sulfate. These results highlight the roles that sulfate plays in the acid dissolution of metals in fine aerosols in Hong Kong. Our findings will likely also apply to other urban areas in South China, where sulfate is the dominant acidic and hygroscopic component in fine aerosols.