O3 and PAN in southern Tibetan Plateau determined by distinct physical and chemical processes
Abstract. 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 NOx 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 NOx and VOCs might lead to strongly enhanced O3 and PAN formation in downwind pristine regions, which should be paid more attention in the future.
Wanyun Xu et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2023-182', Anonymous Referee #1, 19 Apr 2023
- AC1: 'Reply on RC1', Gen Zhang, 18 May 2023
RC2: 'Comment on egusphere-2023-182', Anonymous Referee #2, 23 Apr 2023
- AC2: 'Reply on RC2', Gen Zhang, 18 May 2023
Wanyun Xu et al.
Wanyun Xu et al.
Viewed (geographical distribution)
The authors conducted a study to investigate the variations in tropospheric ozone (O3) and peroxyacetyl nitrate (PAN) and their precursors at a high-altitude site in the southern Tibetan Plateau. They found that variations in O3 and PAN were controlled by distinct processes, with O3 being mostly determined by stratosphere-troposphere exchange (STE), while PAN being mostly influenced by tropospheric chemistry and transport. While local surface O3 and PAN both revealed net photochemical production during daytime, the variability of PAN was more controlled by photochemical formation processes than O3. Overall, the manuscript is well organized and fluent in language. The analysis methods and results are scientifically sound. To my knowledge, this is the first study in Tibet presenting simultaneous high time resolution measurements of O3, PAN as well as its precursors, which are highly valuable data worth publishing. Additionally, this is the first evaluation of PAN formation in Tibet, which also fills in a gap of knowledge. Some minor issues need to be addressed before the manuscript can be accepted for publication.