Abstract. 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.
Seasonality in the Δ<sup>33</sup>S measured in urban aerosols highlights an additional oxidation pathway for atmospheric SO<sub>2</sub>
