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

Abstract

<p><strong>Abstract.</strong> Sulfates present in urban aerosols collected worldwide usually exhibit significant non-zero Δ<sup>33</sup>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<sub>10</sub> 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<sup>34</sup>S-values (n = 155) vary from 2.0 to 11.3 ‰ (± 0.2 ‰, 2σ), the Δ<sup>33</sup>S-values from −0.080 to 0.341 ‰ (± 0.01 ‰, 2σ) and the Δ<sup>36</sup>S-values from −1.082 to 1.751 ‰ (± 0.2 ‰, 2σ). Our study evidences a seasonality for both the δ<sup>34</sup>S and Δ<sup>33</sup>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<sup>34</sup>S coupled to the highest mean Δ<sup>33</sup>S-values. The Δ<sup>33</sup>S-values are higher during both summer and winter, and are < 0.1 ‰ during both spring and autumn. As these higher Δ<sup>33</sup>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<sup>33</sup>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<sub>2</sub> in presence of mineral dust may play a role in generating such large ranges of S-MIF.</p>

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

“…Although oxidation with O 2 TMI at T = 50 • C could produce negative 33 S down to −0.37 ‰, which would account for the lowest 33 S observed in black crusts, this oxidation pathway would also produce larger 36 S down to −1.50 ‰ at odds with the 36 S reported in the black crust. Their potential combination cannot account for sulfate aerosol data from the literature (Au Yang et al, 2019;Guo et al, 2010;Lin et al, 2018b;Romero and Thiemens, 2003;Shaheen et al, 2014) or for the black crust as it would result in slightly negative 33 S-36 S values that could not explain the 33 S as low as −0.34 ‰ (yellow frames in Fig. 6).…”

“…This suggestion primarily relies on the similarities between 33 S-36 S values of sulfate aerosols and laboratory experiments of SO 2 photolysis conducted at different wavelengths (Romero and Thiemens, 2003) and on the correlation between 35 S-specific activity and 33 S values (Lin et al, 2018b). However, these studies never addressed the absence of the complementary negative 33 S reservoir, which is required to balance the positive 33 S reservoir (see Au Yang et al, 2019). In this respect, it is worth mentioning that volcanic and stratospheric aerosols trapped in Antarctic ice cores (see Gautier et al, 2018, and references therein) show both positive 33 S (up to ∼ 2 ‰) and complementary negative 33 S values (down to −1 ‰) and weighed average 33 S = 0 ‰ explained by prior partial deposition.…”

“…In this respect, it is worth mentioning that volcanic and stratospheric aerosols trapped in Antarctic ice cores (see Gautier et al, 2018, and references therein) show both positive 33 S (up to ∼ 2 ‰) and complementary negative 33 S values (down to −1 ‰) and weighed average 33 S = 0 ‰ explained by prior partial deposition. Stratospheric fluxes are actually too low to account for 33 S > 0.1 ‰ (Lin et al, 2016;Au Yang et al, 2019). Accordingly, some other authors rather tried to explain the positive anomalies of most aerosols with "tropospheric" chemical reactions, which are SO 2 oxidation by the main oxidant including NO 2 , H 2 O 2 , OH, O 3 , and O 2 TMI, but experimental data result in a maximum 33 S values of ∼ 0.2 ‰ for all studied reactions (Au Yang et al, 2018;Harris et al, 2013b).…”

“…At present, however, it is difficult to reach a consistent budget for tropospheric SO 2 oxidation (chemically and isotopically). Indeed, most of rural and urban sulfate aerosols have positive 33 S values (Au Yang et al, 2019;Guo et al, 2010;Han et al, 2017;Lin et al, 2018b;Romero and Thiemens, 2003;Shaheen et al, 2014), implying either a source of SO 2 with 33 S > 0 ‰ (which has not been identified yet as all known sources have 33 S ∼ 0 ‰; Lin et al, 2018b) or more likely processes such as SO 2 photolysis in the stratosphere (e.g., Farquhar et al, 2001), contributing to the 33 S-enriched tropospheric sulfate reservoir from initial SO 2 with 33 S = 0 ‰, which should be balanced by a 33 S-depleted reservoir but which remains scarce (see Shaheen et al, 2014;Han et al, 2017;Lin et al, 2018a). Negative 33 S values were suggested to result specifically from combustion (Lee et al, 2002) and/or OCS (carbonyl sulfide) photolysis (Lin et al, 2011).…”

“…As none of the most significant tropospheric SO 2 oxidation reactions can account for 33 S anomalies in sulfate aerosols (Au Yang et al, 2018;Guo et al, 2010;Han et al, 2017;Harris et al, 2013b;Lee et al, 2002;Lin et al, 2018a, b;Romero and Thiemens, 2003;Shaheen et al, 2014), this leads to the suggestion that either some reactions or SO 2 sources have been overlooked. Finally, a recent study highlights the possibility of SO 2 oxidation on mineral dust surfaces resulting in 33 S-depleted sulfate deposition in rural environments and subsequent 33 S enrichment of residual SO 2 transported to cities (Au Yang et al, 2019), but the negative 33 S values are still missing. In this respect, black crusts potentially represent new ways to sample the atmosphere in urban regions at a relatively global scale.…”