Single-Particle Measurements reveal previously overlooked formation of Hydroxymethanesulfonate particle in Non-Fog Conditions: The Crucial Role of Iron

Hydroxymethanesulfonate (HMS) is formed through liquid-phase reactions of formaldehyde and SO₂and is a key tracer for cloud processes. This study analyzes eight months of hourly single- particle data from Hong Kong, identifying 76,515 HMS particles and observing a seasonal decline from winter to summer. Interestingly, higher hourly HMS particle number was occasionally observed during non-fog conditions compared to fog, particularly with elevated iron (Fe) levels. Statistical and machine learning analyses of 5,000 hours of data revealed that Fe's effect on HMS depends on pH and precursors concentrations. Under non-fog conditions, the Fe-sensitive zone spans a broader pH range, and precursors have a greater influence on Fe's effect on HMS. Overall, this study underscores the complex mechanisms of HMS formation in coastal regions, revealing the significant impact of environmental factors, particularly the favorable role of Fe in non-fog conditions.