Seasonality in the Δ33S measured in urban aerosols highlights an additional oxidation pathway for atmospheric SO2
Sulfates present in urban aerosols collected worldwide usually exhibit significant non-zero Δ33S 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 PM10 aerosols collected over the year 2013 at five stations within the Montreal Island (Canada), each characterized by distinct types and levels of pollution. The δ34S-values (n= 155) vary from 2.0 ‰ to 11.3 ‰ (±0.2 ‰, 2σ), the Δ33S-values from −0.080 ‰ to 0.341 ‰ (±0.01 ‰, 2σ) and the Δ36S-values from −1.082 ‰ to 1.751 ‰ (±0.2 ‰, 2σ). Our study evidences a seasonality for both the δ34S and Δ33S, 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 δ34S coupled to the highest mean Δ33S-values. The Δ33S-values are higher during both summer and winter, and are < 0.1 ‰ during both spring and autumn. As these higher Δ33S-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 Δ33S-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 SO2 in the presence of mineral dust may play a role in generating such large ranges of S-MIF.