Comment on egusphere-2023-182

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Tropospheric ozone (O₃) and peroxyacetyl nitrate (PAN) are both photochemical pollutants harmful to the ecological environment and human health. In this study, measurements of O₃ 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 O₃ and PAN were in fact controlled by distinct physiochemical processes. During the dry spring season, airmasses rich in O₃ were associated with high altitude westerly airmasses that entered the TP from the west or the south, which frequently carried high loadings of stratospheric O₃ 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 O₃ entrainment into the troposphere of the northern TP, which travelled southwards to NMC within low altitudes via northerly winds in front of ridges or closed high pressures over the TP. Westerly and southerly airmasses, however, revealed low O₃ levels due to the overall less stratospheric O₃ within the troposphere of low latitude regions. PAN, however, was only rich in westerly or southerly airmasses 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 O₃ level at NMC was mostly determined by stratosphere-troposphere exchange (STE), which explained 77 % and 88 % of the observed O₃ concentration in spring and summer, respectively. However, only 0.1 % of the springtime day-to-day O₃ variability could by STE processes, while 22 % was explained during summertime. Positive net photochemical formation was estimated for both O₃ and PAN based on observation-constrained box modelling. Near surface photochemical formation could not explain the high O₃ level observed at NMC and was also not the factor determining the day-to-day variability of O₃, however, it captured events with elevated PAN concentrations and was able to explain its diurnal variations. Both O₃ and PAN formation were highly sensitive to NO_x levels, with PAN being also quite sensitive to VOCs concentrations. Under the rapid development of transportation network and the urbanization inside the TP, increased emissions and loadings in NO_x and VOCs might lead to strongly enhanced O₃ and PAN formation in downwind pristine regions, which should be paid more attention in the future.