Sulfate aerosols exert a net cooling effect on the earth-atmosphere system, yet their radiative forcing remains associated with largest of uncertainties in the assessment of climate change. One of the contributing factors is the poor understanding of the sulfate formation pathways, which are thought to be following mostly the mass-dependent fractionation model (i.e., Δ 33 S ~ 0). However, globally, urban sulfate aerosols exhibit significant non-zero Δ 33 S compositions (from -0.6‰ to +0.6‰), resulting in sulfur mass-independent fractionation (S-MIF) processes. The origin(s) of these S-MIF anomalies remain(s) unclear. Here, we conducted dual-isotope (Δ 33 S, δ 34 S) probing of sulfate aerosols from summertime megacity Delhi in South Asia. A shift towards concomitantly high Δ 33 S (from +0.2‰ to +0.5‰) and low δ 34 S (from +5‰ to +1‰) values was observed with the influx of mineral dust. The Fe-to-Al tracer showed significant correlations with sulfate loadings (R 2 =0.84) and Δ 33 S signatures (R 2 =0.77). As such, we postulate that the SO2 photo-oxidation on mineral dust generates S-MIF anomaly ~ +0.35±0.10‰, thereby also explaining the previously observed Δ 33 S values worldwide. Together, the findings help deconvolute S-isotope dynamics in urban regions wherein, contrary to prevailing paradigm, non-anthropogenic factor (i.e., mineral dust) is found to influence the aerosol sulfate-induced pollution affecting air quality/human health. SYNOPSISSulfate-related pollution in urban regions-affecting air quality/human health-could be linked to non-anthropogenic factor i.e., mineral dust.

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