Isotopic constraints on non‐photochemical sulfate production in the Arctic winter

The oxygen isotopic composition (Δ 17 O) of non‐seasalt sulfate (NSS) aerosol was measured in samples from Alert, Canada over one year (July 1999–June 2000) and used to quantify the S(IV) oxidants. Measurements of Δ 17 O in NSS are used to evaluate the relative contributions of O 3 , H 2 O 2 , and OH oxidation leading to the formation of SO 4 2− compared to a model of Feichter et al. (1996). The isotopic values suggest that there is a twofold overestimate of ozone oxidation in the model during winter. The isotopic composition is consistent with 10 to 18% contribution from a non‐photochemical oxidation pathway, likely Fe 3+ /Mn 2+ ‐catalyzed O 2 oxidation, during the dark Arctic winter. Isotopic evidence also invokes a 3 to 10% contribution of a mass dependent oxidant during springtime Arctic ozone depletion events.