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Photocatalytic reactions during the daytime, alongside aqueous-phase reactions occurring during both daytime and nighttime, are identified as the two primary processes facilitating the conversion of aerosol iron (Fe) from the insoluble state to the soluble state within the atmospheric environment. This study investigated the levels of total Fe (Fe T) and soluble Fe (Fe S) in PM 2.5 samples collected during daytime and nighttime in Qingdao, a coastal city in eastern China, evaluating the distinctive roles of these two pathways in enhancing aerosol Fe solubility (%Fe S , defined as the ratio of Fe S to Fe T). Under clean and humid conditions, characterized by prevailing sea breezes and a relative humidity (RH) typically above 80 %, an average daytime %Fe S of 8.7 % was observed, which systematically exceeded the nighttime %Fe S (6.3 %). Photochemical conversions involving oxalate contributed to the higher %Fe S observed during daytime. Conversely, in scenarios where air masses originated from inland areas and exhibited slightly polluted, daytime %Fe S (3.7 %) was noted to be lower than the nighttime %Fe S (5.8 %). This discrepancy was attributable to the variations in RH, with nighttime RH averaging around 77 %, conducive to the more efficient generation of acidic compounds, thereby accelerating Fe S production compared to the daytime, when RH was only about 62 %. Furthermore, the oxidation rates of sulfur (SOR) displayed a strong correlation with RH, particularly when RH fell below 75 %. A 10 % increase in RH corresponded to a 7.6 % rise in SOR, which served as the primary driver of the higher aerosol acidity and %Fe S at night. These findings highlight the RH-dependent activation of aqueous-phase reactions and the augmentation of daytime photocatalysis in the formation of Fe S in the coastal moist atmosphere. [ABSTRACT FROM AUTHOR]

Photocatalysis reactions occurring during daytime and aqueous-phase reactions during both daytime and nighttime constitute the two primary processes responsible for converting aerosol iron (Fe) from insoluble to soluble forms within the atmosphere. This study investigated the composition of total Fe (FeT) and soluble Fe (FeS) in daytime and nighttime PM2.5 in Qingdao, a coastal city in eastern China, and evaluated the distinctive roles of the two pathways in enhancing the solubility of aerosol Fe (%FeS, the ratio of FeS to FeT). In clean and humid conditions characterized by sea breezes, with relative humidity (RH) prevalently exceeding 80 %, an average daytime %FeS of 8.7 % was observed, which systematically surpassed its nighttime %FeS (6.3 %). In contrast, when the air originated from land regions and was slightly polluted, the daytime %FeS (3.7 %) was noted to be lower than the nighttime %FeS (5.8 %). This discrepancy was attributable to the variations in RH, as the nighttime RH approximated to be 77 %, facilitating the more efficient formation of acidic substances and resulting in faster FeS production than during daytime, when RH was about 62 %. Furthermore, the oxidation rates of sulfur (SOR) and nitrogen (NOR) displayed strong correlations with RH, particularly when RH was below 75 %. A 10 % increase in RH resulted in a 7.6 % increase in SOR and a 7.2 % elevation in NOR, which served as the primary reason for the differences in aerosol acidity and %FeS between daytime and nighttime. These findings highlight the RH-dependent activation of aqueous-phase reactions and the augmentation of daytime photocatalysis in the formation of FeS in the coastal moisture atmosphere. [ABSTRACT FROM AUTHOR]

The soluble fractions of iron (Fe S) and manganese (Mn S) 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 Fe S in PM 2.5 peaked during the intermingling of dust plumes with local haze, while other species of Fe and Mn reached their maximal levels coinciding with PM 10. Positive Matrix Factorization (PMF) analysis determined that aged dust was the main source of Fe S in PM 2.5 , accounting for 44.0%, while fresh dust contributed just 1.9%. Conversely, fresh dust played a more significant role in PM 10 , contributing 8.9% to Fe S. Similar trends were observed in the initial dust and dust swing periods, aged dust was the dominant contributor to Fe S , but fresh dust accounted for a higher fraction in PM 10 (11.5%) than in PM 2.5 (4.9%). In contrast, Mn S levels, with high solubility (%Mn S), were mainly from fresh dust (27.9% and 63.2% in PM 2.5 and PM 10 , respectively). The PMF estimated %Mn S in fresh dust at 17.0% for PM 2.5 and 19.4% for PM 10 , considerably higher than Fe solubility (%Fe S) at 0.3% and 0.1% in the corresponding particulate sizes. Additionally, %Fe S correlated more strongly with sulfate and nitrate per unit Fe mass (R2 = 0.68 and 0.79 in PM 2.5 and PM 10) than %Mn S (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. [Display omitted] • The fresh and aged dust were successfully distinguished during a haze-to-dust event via a refined PMF model. • Soluble Fe in both PM 2.5 and PM 10 mainly from aged dust, with more fresh dust in PM 10 ; soluble Mn mainly from fresh dust. • Solubilities of Fe and Mn from fresh and aged dust aerosols estimated via PMF analysis generally resemble the observed data. • The acidification process exerts a more pronounced impact on the dissolution of Fe than on Mn. • Liquid water content rather than pH played a more critical role during the acidification process within fine particles. [ABSTRACT FROM AUTHOR]

Sulfate and nitrate formed on dust aerosols can considerably affect particles' physicochemical properties due to their high hygroscopicity. Dust reverse-transport (DRT) events can occur in eastern China when dust plumes undergo a distinct turn caused by Asian highs or cyclones, resulting in the re-appearance of the dust plume in one place but with anthropogenic pollutants mixed in. In this study, three DRT events were identified at a coastal city, Qingdao. The secondary sulfate and nitrate in PM 2.5 collected during the DRT events were estimated according to the mass concentration of water-soluble ions and dust tracer metal. Several key factors, including heterogeneous reactions on aerosols, photochemical conversions in the air, and precursors' abundance in dust plumes, affected the aging of dust plumes. In the presence of adequate ammonium and high-level oxidants in the air, sulfate and nitrate were efficiently produced under moderate/high humidity conditions when DRT events occurred to the south of Qingdao. The proportions of the secondary sulfate/nitrate in total concentrations exceeded 90%, in which 20.8%–43.1% of secondary nitrate was formed on dust particles. In contrast, when the DRT occurred to the north of Qingdao with low relative humidity and low [NH 4 +]/[SO 4 2−] ratio, the growth of sulfate and nitrate was small. This study reveals a manner of efficient sulfate and nitrate formation in dust plumes in the continental atmosphere. • Dust reverse-transport phenomena were used to study the aging process of dust plumes. • East Asian dust plumes were prone to aging when transported to the south of Qingdao. • Secondary sulfate/nitrate formation was strongly influenced by the precursors' concentrations on dust transport pathways. [ABSTRACT FROM AUTHOR]