A year-long study was performed to comprehensively investigate the mass concentration, water-soluble ions, organic and elemental carbons, and elemental components of atmospheric fine particles from March 2006 to February 2007 in Qingdao. The results showed that PM_2.5 concentration was over twice the National Ambient Air Quality Standards of 35 μg/m³ (GB3095-2012), with the highest value in winter (110 μg/m³) and the lowest in summer (42.8 μg/m³). Non-sea salt-SO₄²⁻ (nss-SO₄²⁻), NO₃⁻, and NH₄⁺ were the dominant water-soluble ions in PM_2.5, and showed the highest concentrations in autumn, winter and autumn, respectively, and all had the lowest concentrations in summer. Organic carbon (OC) and elemental carbon (EC) had the lowest levels in summer and the highest values in winter and autumn, respectively. The lower acetic/formic acid (A/F) ratio in summer suggested a predominant secondary formation of water-soluble organic acids. Cl-depletion mainly occurred in summer, along with the largest sea salt proportion in particles in a year when winds came from the southeastern sea. The enrichment factors (EFs) of elements followed an increasing order: Si, Al, Ti, Mg, Ca, Fe, Na, Sr, K, Mn, Ba, V, Cr, Ni, Co, Sc, Cu, As, Cl, Zn, Pb, S, Br, Cd, and Se. The higher EFs from 10.1 to 10,487 for Sr, K, Mn, Ba, V, Cr, Ni, Co, Sc, Cu, Cl, Zn, Pb, S, Br, Cd, and Se suggested the significant influence of anthropogenic sources in the area. Mass closure showed that (NH₄)₂SO₄, organic matter, and NH₄NO₃ were the main components, and contributed 31.0%, 25.2%, and 14.0% of PM_2.5 mass in Qingdao, respectively. Source apportionment results showed that secondary sulfate, nitrate, and coal combustion were the main sources. Based on this study and previous publications, the PM_2.5 characteristics—including seasonality in mass and chemical species concentrations and Cl-depletion in summer—were obtained across coastal cities in China.

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.