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 Delta S-33 signatures (from -0.6 parts per thousand to 0.5 parts per thousand) 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 delta S-34-values (n = 155) vary from 2.0 parts per thousand to 11.3 parts per thousand (+/- 0.2 parts per thousand, 2 sigma), the Delta S-33-values from 0.080 parts per thousand to 0.341 parts per thousand (+/- 0.01 parts per thousand, 2 sigma) and the Delta S-36-values from 1.082 parts per thousand to 1.751 parts per thousand (+/- 0.2 parts per thousand, 2 sigma). Our study evidences a seasonality for both the delta S-34 and Delta S-33, 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 delta S-34 coupled to the highest mean Delta S-33-values. The Delta S-33-values are higher during both summer and winter, and are < 0.1 parts per thousand during both spring and autumn. As these higher Delta S-33-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 Delta S-33-values of urban aerosols (i.e. -0.6 parts per thousand to 0.5 parts per thousand) 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.