Wildfires Drive Sulfur Isotope Anomalies on the Himalayan-Tibetan Plateau

Mass-independent fractionation (MIF) of sulfur (S) isotopes has been reported for tropospheric sulfate aerosols; however, the mechanisms responsible for these anomalies remain poorly constrained. Here, we present δ34S and Δ33S isotope compositions of sulfate aerosols (PM2.5) collected from two sites on the Himalayan-Tibetan Plateau (HTP) region, encompassing both the forest wildfire period and the nonwildfire period on the southern slopes of the Himalayas. Elevated Δ33S values were observed during the wildfire season at both QOMS and SETS, suggesting that forest wildfire emissions are an important contributor to sulfur MIF signals over the HTP. We hypothesize that at least two heterogeneous formation pathways contribute to the generation of S-MIF during the wildfire season: (i) a black carbon (BC)-catalyzed sulfate formation pathway associated with negative Δ33S anomalies, and (ii) a pathway linked to (NH4)2SO4 formation under NH3-rich conditions, marked by positive Δ33S anomalies. In addition, stratospheric intrusions contribute minimally to the observed Δ33S variability, with their signatures largely overprinted by wildfire-related processes, particularly in spring. Our results provide new observational constraints on sulfur isotope fractionation mechanisms in the modern troposphere and highlight the important role of wildfire-driven heterogeneous chemistry in modulating the atmospheric sulfur cycle over the HTP.