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.