Tracing the origin of elevated springtime atmospheric sulfate on the southern Himalayan-Tibetan plateau

The Himalayan Tibetan Plateau (HTP) is one of the world's most climate-sensitive regions outside the polar regions. Here, the climate–air quality–hydrological cycle affecting sulfate aerosols remains sparsely investigated, with their source origin(s) requiring further investigation. We tracked the evolution of sulfur mass-independent isotope fractionation [S-MIF i.e., Δ33S ≠ 0]—as a potential source tracer—in springtime aerosol sulfate over the southern HTP. In a first, at the southern HTP high-altitude receptor site Qomolangma-Mt. Everest station, QOMS, ∼4300 m a.s.l., elevated sulfate concentrations and S-MIF were both found to be associated with biomass burning aerosols (SO42− and Δ33S vs. K+: R2 = 0.92 [P < 0.001] and 0.61[P < 0.005], respectively). This is in stark contrast to Δ33S aerosol records from the central HTP and a downwind mountainous site wherein anomalous sulfur has been linked to stratospheric intrusions in the past, and geological lake records from the region which link the origin of modern sulfate on the HTP to the influx of mineral dust, respectively. The findings suggest that there are yet unknown biomass combustion-related processes (e.g., crop-residue and waste burning, wildfires) plausibly generating positive MIF in sulfur, which could have implications for historical S-isotope records. Comparing the triple-S-isotope imprint in aerosol sulfate on the HTP and its surrounding regions reveals the existence of spatial heterogeneity in the dominance of competing sulfate transport and formation processes with implications for the regional tropospheric chemical and radiation budgets.