Triplet-Excited Dissolved Organic Matter Efficiently Promoted Atmospheric Sulfate Production: Kinetics and Mechanisms

Photochemical generation is an important pathway for atmospheric sulfate formation. However, the roles of atmospheric co-existing photosensitive dissolved organic matter (DOM) in sulfate formation are still unclear. The experimental results in this work provide evidence that atmospheric photosensitizers produce active intermediates to oxidize S(IV) into S(VI) under illumination. Quenching experiments of eight photosensitive model compounds (PS) demonstrate that their triplet-excited states (3PS*) dominate sulfate formation for the photosensitizing pathway with a contribution of over 90%, and 1O2 plays an important role in sulfate formation. The results using humic acid (HA) and water-soluble organic carbon in vehicle exhaust particles (WSOC) as representatives of atmospheric photosensitizers further verify that triplet-excited DOM (3DOM*) is the main reactive species for sulfate formation, which is consistent with the results of PS. Our findings provide new insights into the photochemical formation pathways of atmospheric sulfate.