Seasonality in the Δ 33 S measured in urban aerosols highlights an additional oxidation pathway for atmospheric SO 2

Sulfates present in urban aerosols collected worldwide usually exhibit significant non-zero Δ 33 S signatures (from −0.6 ‰ to 0.5 ‰) whose origin still remains unclear. To better address this issue, we recorded the seasonal variations of the multiple sulfur isotope compositions of PM 10 aerosols collected over the year 2013 at five stations within the Montreal Island (Canada), each characterized by distinct types and levels of pollution. The δ 34 S-values ( n = 155) vary from 2.0 ‰ to 11.3 ‰ (±0.2 ‰, 2 σ ), the Δ 33 S-values from −0.080 ‰ to 0.341 ‰ (±0.01 ‰, 2 σ ) and the Δ 36 S-values from −1.082 ‰ to 1.751 ‰ (±0.2 ‰, 2 σ ). Our study evidences a seasonality for both the δ 34 S and Δ 33 S, which can be observed either when considering all monitoring stations or, to a lesser degree, when considering them individually. Among them, the monitoring station located at the most western end of the island, upstream of local emissions, yields the lowest mean δ 34 S coupled to the highest mean Δ 33 S-values. The Δ 33 S-values are higher during both summer and winter, and are < 0.1 ‰ during both spring and autumn. As these higher Δ 33 S-values are measured in “upstream” aerosols, we conclude that the mechanism responsible for these highly positive S-MIF also occurs outside and not within the city, at odds with common assumptions. While the origin of such variability in the Δ 33 S-values of urban aerosols (i.e. −0.6 ‰ to 0.5 ‰) is still subject to debate, we suggest that oxidation by Criegee radicals and/or photooxidation of atmospheric SO 2 in the presence of mineral dust may play a role in generating such large ranges of S-MIF.