Impact of source and atmospheric processing on Fe solubility in aerosols over the Yellow Sea, China

Thirty four atmospheric total suspended particulate (TSP) samples collected over the Yellow Sea between February and August, 2009 were used to study Fe solubility. Samples were classified into two groups based on calculated air mass back trajectories, i.e., one originating from a northern continental source (NCS, in which Fe solubility was 5.6±3.9%) and the remaining one from other regions (non-NCS, in which Fe solubility was 3.1±5.3%). The calculated enrichment factor (Al is used as a reference) indicated that Fe in the NCS samples was probably derived from natural sources dominated by mineral dust. A deep analysis of soluble Fe and Fe solubility suggested that cloud processing played an important role in the increase of both. No correlation between soluble Fe and secondary inorganic ions (sulfate and nitrate) was observed in NCS samples. However, in non-NCS samples, a good correlation between soluble Fe and secondary inorganic ions was observed, probably due to acidification reactions. Enrichment factors indicated that the Fe in the non-NCS samples was probably from anthropogenic sources. Moreover, a logarithmic curve between Fe solubility and ([SO₄^2-]+[NO₃^-]) together with correlations between soluble Fe and K⁺ obtained in the non-NCS samples further indicated that soluble Fe was more likely derived from aged ambient aerosols rather than from fresh aerosols. The lower solubility of Fe in non-NCS samples was comparable to Fe solubility in coal fly ash recently reported in the literature, while it was lower than Fe solubility in mineral dust.