Isotopic constraints on atmospheric sulfate formation pathways in the Mt. Everest region, southern Tibetan Plateau

Abstract. As an important atmosphere constituent, sulfate aerosols exert profound impacts on climate, ecological environment, and human health. The Tibetan Plateau (TP), identified as the Third Pole, contains the largest land ice masses outside the poles and has attracted wide attention on its environment and climatic change. However, the mechanisms of sulfate formation in this specific region remain poorly characterized. Oxygen-17 anomaly (Δ¹⁷O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the Δ¹⁷O values in atmospheric sulfate collected at a remote site in the Mt. Everest region to decipher the possible formation mechanisms of sulfate in such a pristine environment. Throughout the sampling campaign (April–September 2018), the Δ¹⁷O in non-dust sulfate show an average of 1.7 ± 0.5 ‰ which is higher than most existing data in modern atmospheric sulfate. The seasonality of Δ¹⁷O in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in Δ¹⁷O for both sulfate and nitrate (i.e., minima in warm season and maxima in cold season) observed from diverse geographic sites. This high Δ¹⁷O in non-dust sulfate found in this region clearly indicates the important role of the S(IV) + O₃ pathway in atmospheric sulfate formation promoted by high cloud water pH condition. Overall, our study provides an observational constraint on atmospheric acidity in altering sulfate formation pathways particularly in dust-rich environments, and such identification of key processes provides an important basis for a better understanding of the sulfur cycle in the TP.