| Summary: | <p>Sulfur isotope analysis provides a unique probe for source-specific information and certain atmospheric reactions. Globally, atmospheric aerosols in urban locations exhibit significant sulfur mass-independent fractionation (i.e., S-MIF, Δ<sup>33</sup>S ≠ 0). The origin(s) of these S-MIF anomalies remains unclear, thereby limiting the interpretation and/or application of such signals. Here, we conducted dual-isotope (Δ<sup>33</sup>S and δ<sup>34</sup>S) fingerprinting of sulfate aerosols from summertime megacity Delhi in south Asia. A shift toward concomitantly high Δ<sup>33</sup>S (from 0.2‰ to 0.5‰) and low δ<sup>34</sup>S (from 5‰ to 1‰) values was observed with the influx of mineral dust. The Fe:Al ratio showed significant correlations with both sulfate loadings (<em>R</em><sup>2</sup> = 0.84) and Δ<sup>33</sup>S signatures (<em>R</em><sup>2</sup> = 0.77). Contrary to the prevailing paradigm, this observational evidence suggests that mineral-dust-associated sulfate exhibits S-MIF anomalies. Atmospheric processing of mineral dust plausibly leads to the production of these anomalies. Our evaluation suggests that an inherent mechanism(s) remains elusive. Although hindered by end-member uncertainties, we show that S-MIF signals can be source apportioned to quantitatively constrain the fraction of mineral-dust-associated sulfate in urban locations. The influx of mineral-dust-associated sulfate can influence urban air pollution affecting air quality and/or human health and as such requires monitoring. Urban Δ<sup>33</sup>S signals can therefore be used to trace this sulfate fraction, thereby improving our understanding of sulfate aerosol dynamics.</p>
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