Sulfate Formation Driven by Wintertime Fog Processing and a Hydroxymethanesulfonate Complex With Iron: Observations From Single‐Particle Measurements in Hong Kong.

Fog processing has a significant impact on sulfur chemistry in the atmosphere. This study analyzed three winter fog events in Hong Kong using single‐particle aerosol mass spectrometry (SPAMS) and a Monitor for AeRosols and GAses in ambient air (MARGA). Black carbon (BC)‐related carbonaceous particles with substantial sulfate amounts comprised the largest particle number fraction (56.7%). Sulfate mass concentration decreased during fog due to the cloud's effective scavenging, but fog processing notably promoted sulfate formation at the single‐particle level (average peak area increases of 31.2%). Hydroxymethanesulfonate (HMS), an important S(IV) compound and fog tracer, was identified accounting for up to 12% by particle number fraction. Although pH showed a positive correlation (r = 0.53–0.69) with HMS particles in each fog scenario, a negative overall correlation (r = −0.51) was observed. Further analysis revealed that the higher aerosol acidity (pH 0.65–3.11), promoted Fe dissolution, leading to 49% of HMS particles being mixed with Fe, which potentially facilitated sulfate formation via the Fenton reaction. Additionally, around 40% of HMS‐Fe particles are mixed with oxalate, thereby warranting further attention for their potential to cause more intricate sulfur oxidation processes. This study reveals the initial identification of a high mixed‐state of HMS‐Fe, which could potentially serve as a crucial avenue for the formation of sulfate on individual particulate matter. Considering the persistent augmentation of aerosol acidity in the Asian region, this phenomenon necessitates further investigation and attention. Plain Language Summary: Cloud and fog processes are crucial for atmospheric chemistry and important pathways in sulfate formation. However, current models often underestimate sulfate production in the atmosphere. While several potential pathways have been proposed, the influence of organic compounds on hydroxymethanesulfonate (HMS) formation and its subsequent sulfate generation is yet to be fully explored. This study, utilizing a single‐particle perspective, reveals that higher aerosol acidity in coastal areas can lead to the dissolution of iron (Fe) in coastal aerosols, thereby promoting the formation of HMS‐Fe. Additionally, as a transition metal, Fe has the potential to catalyze the conversion of HMS to sulfates. This finding challenges the prevailing notion that HMS hinders sulfate production by being less susceptible to oxidation by hydrogen peroxide (H2O2) and other radicals. The significance of this process is particularly pronounced in coastal or remote marine atmospheres, where cloud and fog processes exhibit higher aerosol acidity and limited ammonia sources. Furthermore, considering the current global scenario of increased aerosol acidity, the impact of transition metals on sulfate formation in HMS is likely to be more prominent. Consequently, this phenomenon necessitates further investigation and attention. [ABSTRACT FROM AUTHOR]