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

Abstract

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.

Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ<sup>33</sup>S reservoir of sulfate aerosols?

“…More locally, a recent study reported a I. Genot et al: Oxygen and sulfur mass-independent isotopic signatures narrow range from −0.57 ‰ to 11.33 ‰ for sulfur emitted by transport and industries in Paris (Au Yang et al, 2020). Lee et al (2002) carried out vegetation and diesel combustion experiments, resulting in δ 34 S and δ 18 O values between 9.55 ‰ and 16.42 ‰ and between 5.5 and 10.5 ‰, respectively, with O of ∼ 0 ‰, forming primary sulfates without mass-independent signatures.…”

Carbonaceous Fractions Contents and Carbon Stable Isotope Compositions of Aerosols Collected in the Atmosphere of Montreal (Canada): Seasonality, Sources, and Implications

“…Although most of the TC mass is concentrated in the finest PM fractions (up to 90%; Sudheer et al, 2016), our results showed that the carbonaceous fractions studied here were significantly contributing in our TSP aerosols samples (from 5 to 26%). For all carbon fractions, there was a tendency to observe the lowest concentrations at station 99 (Kruskal-Wallis ANOVA test at 0.05 level, Table 1), located at the western end of the island, that is representing the local background (e.g., Au Yang et al, 2019). The rest of the stations showed similar results for all carbon fractions, suggesting a homogeneous influence of carbonaceous aerosols for most of the urban and industrial parts of island covered by these stations.…”

“…Four monitoring stations (number 03, 06, 13, and 99; Figure 1) were selected as they represent the major environmental conditions encountered on the island in terms of atmospheric contamination (Au Yang et al, 2019): (i) station 03 ("Saint-Jean-Baptiste") is located at the north-eastern end of the island and is typically influenced by local petro-chemical industries, (ii) station 06 ("Anjou") is located close to the intersection of two of the main high-traffic highways (highways 40 and 25) on the island, (iii) station 13 ("Drummond") is located downtown and represents the urban background, whereas iv) station 99 ("Sainte-Anne-de-Bellevue") is located at the western end of the island in a semi-rural environment about 35 km WSW of Montreal downtown and is more likely to sample regional aerosols generated upstream the city and transported to Montreal (Au Yang et al, 2019).…”

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

“… Mass independent sulfur isotope effects observed in coal, tropospheric aerosols and SO 2 in the modern atmosphere. Data compilation is obtained from Lin et al., Shaheen et al., and Au Yang et al …”

“…The major challenge to explain the observed sulfur isotopic anomalies in combustion‐related primary sulfates is that a proper and quantitative description of the isotopic partitioning and exit channels such as gas‐to‐solid formation processes is required. A most recent study measured positive Δ 36 S values in some aerosol samples collected from Canada, in contrast to the early combustion chamber experiment and most ambient sulfate aerosols display negative Δ 36 S values (Figure ). On a view of isotopic mass balance, this result is not surprising, though a quantitative explanation remains lacking.…”