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<p>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 (<span class="inline-formula">Δ¹⁷O</span>) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the <span class="inline-formula">Δ¹⁷O</span> 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 <span class="inline-formula">Δ¹⁷O</span> in non-dust sulfate show an average of <span class="inline-formula">1.7 ‰±0.5</span> ‰, which is higher than most existing data on modern atmospheric sulfate. The seasonality of <span class="inline-formula">Δ¹⁷O</span> in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in <span class="inline-formula">Δ¹⁷O</span> 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 <span class="inline-formula">Δ¹⁷O</span> in non-dust sulfate found in this region clearly indicates the important role of the <span class="inline-formula">S(IV)+O₃</span> 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.</p>