O₃ and PAN in southern Tibetan Plateau determined by distinct physical and chemical processes

Tropospheric ozone (O-3) and peroxyacetyl nitrate (PAN) are both photochemical pollutants harmful to the ecological environment and human health. In this study, measurements of O-3 and PAN as well as their precursors were conducted from May to July 2019 at Nam Co station (NMC), a highly pristine high-altitude site in the southern Tibetan Plateau (TP), to investigate how distinct transport processes and photochemistry contributed to their variations. Results revealed that, despite highly similar diurnal variations with steep morning rises and flat daytime plateaus that were caused by boundary layer development and downmixing of free-tropospheric air, day-to-day variations in O-3 and PAN were in fact controlled by distinct physicochemical processes. During the dry spring season, air masses rich in O-3 were associated with high-altitude westerly air masses that entered the TP from the west or the south, which frequently carried high loadings of stratospheric O-3 to NMC. During the summer monsoon season, a northward shift of the subtropical jet stream shifted the stratospheric downward entrainment pathway also to the north, leading to direct stratospheric O-3 entrainment into the troposphere of the northern TP, which traveled southwards to NMC within low altitudes via northerly winds in front of ridges or closed high pressures over the TP. Westerly and southerly air masses, however, revealed low O-3 levels due to the overall less stratospheric O-3 within the troposphere of low-latitude regions. PAN, however, was only rich in westerly or southerly air masses that crossed over polluted regions such as northern India, Nepal or Bangladesh before entering the TP and arriving at NMC from the south during both spring and summer. Overall, the O-3 level at NMC was mostly determined by stratosphere-troposphere exchange (STE), which explained 77 % and 88 % of the observed O-3 concentration in spring and summer, respectively. However, only 0.1 % of the springtime day-to-day O-3 variability could be explained by STE processes, while 22 % was explained during summertime. Positive net photochemical formation was estimated for both O-3 and PAN based on observation-constrained box modeling. Near-surface photochemical formation was unable to account for the high O-3 level observed at NMC, nor was it the determining factor for the day-to-day variability of O-3. However, it was able to capture events with elevated PAN concentrations and explain its day-to-day variations. O-3 and PAN formation were both highly sensitive to NOx levels, with PAN being also quite sensitive to volatile organic compound (VOC) concentrations. The rapid development of transportation networks and urbanization within the TP may lead to increased emissions and loadings in NOx and VOCs, resulting in strongly enhanced O-3 and PAN formation in downwind pristine regions, which should be given greater attention in future studies.