Quantifying Contributions of Sulfate, Hydroxymethanesulfonate, and Additional S(IV) Compounds to Wintertime Aerosol Particles

Reactions of dissolved sulfur dioxide (SO2) with aldehydes form particulate hydroxyalkylsulfonates, including hydroxymethanesulfonate (HMS), hydroxyethanesulfonate (HES), dihydroxyethanesulfonate (DHES), and hydroxyacetylmethanesulfonate (HAMS). Recent work has shown enhanced production of HMS within aqueous particles at low temperatures; however, little is known about the presence of HES, DHES, and HAMS. During Jan.–Feb. 2022 in Fairbanks, Alaska, these hydroxyalkylsulfonates were identified and quantified within 340,877 individual aerosol particles using single-particle mass spectrometry. HMS was identified within 27 ± 3% of the particle number concentration in the size range of 0.1–1.0 μm. The primary source of the majority (∼94%, by number) of individual HMS-containing particles was identified as residential heating (wood and oil combustion). The average HMS mass fraction within these HMS-containing particles was 7.0%. The number fraction of HMS-containing particles increased with particle diameter and plateaued at ∼500 nm, consistent with aqueous-phase processing. Therefore, locally emitted residential heating combustion particles accumulated water, promoting aqueous-phase reactions and secondary organosulfur formation. Notably, HES, DHES, and HAMS accounted for 48 ± 14% of the average 0.1–1.0 μm S(IV) mass. Together, the identified hydroxyalkylsulfonates, including HMS, comprised ∼90% of the measured S(IV) mass, with inorganic S(IV) estimated at ∼10%. Therefore, focusing measurements and modeling solely on sulfate and/or HMS may underestimate organic S(IV) and misrepresent the sulfur budget.