Investigation of a haze-to-dust and dust swing process at a coastal city in northern China part II: A study on the solubility of iron and manganese across aerosol sources and secondary processes

The soluble fractions of iron (FeS) and manganese (MnS) in atmospheric aerosols potentially impact human health and ocean ecosystems. This study investigated a persistent haze-to-dust process observed in winter Qingdao, a coastal city in Northern China, focusing on the aerosol sources and acidification process affecting the dissolution of Fe and Mn. It was found that FeS in PM2.5 peaked during the intermingling of dust plumes with local haze, while other species of Fe and Mn reached their maximal levels coinciding with PM10. Positive Matrix Factorization (PMF) analysis determined that aged dust was the main source of FeS in PM2.5, accounting 44.0%, while fresh dust contributed just 1.9%. Conversely, fresh dust played a more significant role in PM10, contributing 8.9% to FeS. Similar trends were observed in the initial dust and dust swing periods, aged dust was the dominant contributor to FeS, but with the fresh dust accounting for a higher fraction in PM10 (11.5%) than in PM2.5 (4.9%). In contrast, MnS levels, with high solubility (%MnS), were mainly from fresh dust (27.9% and 63.2% in PM2.5 and PM10, respectively). The PMF estimated %MnS in fresh dust at 17.0% for PM2.5 and 19.4% for PM10, considerably higher than Fe solubility (%FeS) at 0.3% and 0.1% in the corresponding particulate sizes. Additionally, %FeS correlated more strongly with sulfate and nitrate per unit Fe mass (R2=0.68 and 0.79 in PM2.5 and PM10) than %MnS (R2=0.49 and 0.50), suggesting a more profound impact of acidification on Fe dissolution. Further analysis suggests liquid water content rather than pH played a more critical role during the acidification process within fine particles. This study successfully estimated the solubility of Fe and Mn within various aerosols, providing valuable information and advancing our understanding of their dissolution and potential health and ecosystem impacts.