Single-Particle Measurements Reveal Previously Overlooked Formation of Hydroxymethanesulfonate Particles under Nonfog Conditions: The Crucial Role of Iron

Hydroxymethanesulfonate (HMS), formed from liquid-phase reactions of formaldehyde (HCHO) and SO2, is a key organosulfur component. While its importance in northern China’s sulfur chemistry is recognized, HMS dynamics in southern coastal regions remain poorly understood. This study analyzes eight months of hourly single-particle measurements (5000 data sets, 76,515 HMS particles, 1.17% of total) from coastal Hong Kong, revealing a distinct winter-to-summer decline. Crucially, higher HMS particle numbers occurred during nonfog episodes versus fog, specifically under elevated iron (Fe) levels. Integrated statistical and machine learning models confirmed differing influences of temperature, liquid water content, pH, Fe, and precursor concentrations (SO2 and HCHO), with Fe-driven enhancement occurring under the characteristically low particle pH (0–3) of the coastal environment. This acidity promotes Fe dissolution into soluble forms that stabilize organosulfur complexes. The broader pH range of the Fe-sensitive zone under nonfog conditions (0–2.4, compared with 0–1.4 in fog) likely contributes to the elevated HMS number concentrations. These findings highlight Fe-mediated pathways, crucially modulated by particle acidity, as a potentially major but overlooked source of atmospheric organosulfur. Elucidating the specific low-pH reaction mechanisms governing this Fe-HMS link is critical to advance fundamental understanding of coastal sulfur transformations and aerosol impacts.