Isotopic evidence for importance of atmospheric acidity on sulfate formation in the Mt. Everest region

<p>Oxygen-17 anomaly (Δ<sup>17</sup>O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the Δ<sup>17</sup>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. The Δ<sup>17</sup>O in non-dust sulfate show higher values than most existing data in modern atmospheric sulfate. The seasonality of Δ<sup>17</sup>O in non-dust sulfate exhibits high values in the pre-monsoon and low values in the monsoon, opposite to the seasonality in Δ<sup>17</sup>O for both sulfate and nitrate (i.e., minima in warm season and maxima in cold season) observed from diverse geographic sites. This high Δ<sup>17</sup>O in non-dust sulfate found in this region clearly indicates the important role of the S(IV) + O<sub>3</sub> pathway in atmospheric sulfate formation promoted by high cloud water pH conditions. In turn, this study highlights observational evidence that atmospheric acidity plays an important role in controlling sulfate formation pathways particularly for dust-rich environments.</p>