Iron solubility in fine particles associated with secondary acidic aerosols in east China

Soluble iron (Fe_S) in aerosols contributes to free oxygen radical generation with implications for human health, and potentially catalyzes sulfur dioxide oxidation. It is also an important external source of micronutrients for ocean ecosystems. However, factors controlling Fe_S concentration and its contribution to total iron (Fe_T) in aerosols remain poorly understand. Here, Fe_S and Fe_T in PM_2.5 was studied at four urban sites in eastern China from 21 to 31 December, 2017. Average Fe_T (869-1490 ng m^-3) and Fe_S (24-68 ng m^-3) concentrations were higher in northern than southern China cities, but Fe solubility (%Fe_S, 2.7-5.0%) showed no spatial pattern. Correlation analyses suggested %Fe_S was strongly correlated with Fe_S and PM_2.5 instead of Fe_T concentrations. Individual particle observations confirmed that more than 65% of nano-sized Fe-containing particles were internally mixed with sulfates and nitrates. Furthermore, there was a high correlation between sulfates or nitrates/Fe_T molar ratio and %Fe_S. We also found that the sulfates/nitrates had weaker effects on %Fe_S at RH < 50% than at RH > 50%, suggesting RH as indirect factor can influence %Fe_S in PM_2.5. These results suggest an important role of chemical processing in enhancing %Fe_S in the polluted atmosphere.