As an important atmosphere constituent, sulfate aerosols exert profound impacts on climate, the 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 widespread attention for its environment and climatic change. However, the mechanisms of sulfate formation in this specific region still remain poorly characterized. An oxygen-17 anomaly (Î.sup.17 O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the Î.sup.17 O 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 Î.sup.17 O in non-dust sulfate show an average of 1.7 0/00Â±0.5 0/00, which is higher than most existing data on modern atmospheric sulfate. The seasonality of Î.sup.17 O in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in Î.sup.17 O for both sulfate and nitrate (i.e., minima in the warm season and maxima in the cold season) observed from diverse geographic sites. This high Î.sup.17 O in non-dust sulfate found in this region clearly indicates the important role of the S(IV)+O.sub.3 pathway in atmospheric sulfate formation promoted by conditions of high cloud water pH. 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.