the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Mar 2023
O3 and PAN in southern Tibetan Plateau determined by distinct physical and chemical processes
Wanyun Xu
Yuxuan Bian
Weili Lin
Yingjie Zhang
Yaru Wang
Gen Zhang
Xiaobin Xu
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.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
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- Final revised paper
Journal article(s) based on this preprint
Wanyun Xu et al.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-182', Anonymous Referee #1, 19 Apr 2023
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.
Minor comments:
- Lines 136-156: A slight concern on the PSCF analysis is that this is typically more suitable for the source analysis of atmospheric components with longer lifetimes, while PAN can have a long-lifetime in the free troposphere, O3 seems to be more short-lived.
- Lines 198-205: The authors only provided Propy-Equivalent VOCs concentrations, however, to put observations into context with those in other studies, it might also be necessary to provide direct VOCs concentrations in ppb or ppbC. How do VOCs levels compare with previous observations in Tibet or similar high-altitude sites around the world? Since the authors discuss O3 and PAN photochemistry afterwards, this might be important for understanding the difference or similarities in their results with those in literature.
- Lines 212-215: Again, how do NO2 concentrations compare with previous observations in Tibet (if there are any)? The authors mention that there are natural and anthropogenic emissions in NOx, can you name the detailed sources influencing NOx variations at Nam CO? How do these sources impact the diurnal variation of NOx?
- Lines 252-260: Here the authors conclude that O3 and PAN decreases after sunset are caused by dry deposition as well as the cutoff of free-tropospheric input, indicating that local formations O3 and PAN are weak. However, in the later sections, PAN formation at least was revealed to be strong. Is there a contradiction?
- Lines 258-264: PAN often revealed high concentrations at moderate PBLH, which the authors explained as “Whereas PAN in the free troposphere might have had higher variability, which resulted in largely different enhancements of PAN upon down mixing.”. Might it be that local PAN formation was controlling the large variability of PAN?
Technical comments:
- Line 18 and later in text: “physiochemical” should be replaced with be “physicochemical”.
- Line 28: “could by” should be “could be explained by”
- Line 45: Change “nearly proportional to its OVOCs precursors” to “nearly proportionally to its OVOC precursors”.
- Line 440: “reflects” should be “reflect”
- Line 446: The “of” should be “or” in “from the west of from the south”
Citation: https://doi.org/10.5194/egusphere-2023-182-RC1 - AC1: 'Reply on RC1', Gen Zhang, 18 May 2023
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RC2: 'Comment on egusphere-2023-182', Anonymous Referee #2, 23 Apr 2023
This paper reports spring and summer season O3 and PAN levels and their variations in Tibet and presents an analysis of what has been controlling the changes of those two most important photochemical air pollutants at such a high-altitude site under very clean conditions. Different impacts of transport and photochemistry on O3 and PAN have been detected, revealing distinct transport pathways and different sensitivity to photochemical formation processes. Overall, the topic of the paper fits well into the scope of Atmospheric Chemistry and Physics, the observation and analysis methods used are scientifically sound and the results revealed sufficient novelty. The manuscript can be accepted for publishing after the following questions have been well addressed.
- Although the authors reported the contributions of transport or STE and local chemistry to the observed O3 and PAN concentrations, most of the results are descriptive, particularly before section 3.3. It’s better to discuss the results more quantitatively and compare them with relevant studies.
- The writing style needs to be improved. It is difficult to understand some conclusions because some key information in the figures is omitted.
- Almost all the modeled PAN concentrations are higher than the observed values, while it is completely opposite for O3. My question is whether it is suitable to evaluate the importance of local chemistry for O3 and PAN chemistry using a box model under an extreme environment such as Tibet, where is greatly affected by transport or STE？
- Did you warm up the model before simulating? How did you evaluate your model performance?
- Figure 2b, it is difficult to differentiate the different components of VOCs. Figure 3b, what are the dotted lines
- Please define the w wind, u wind, v wind.
- Line 28: “However, only 0.1% of the springtime day-to-day O3 variability could by STE processes, …”. Something is missing in this sentence.
- Line 32: Make sure you really mean “diurnal” here and not “day-to-day”.
- Line 115: There should be a comma before “which”
- Line 288-291: South Asian countries obviously strong pollution sources close to Tibet, revealing PAN transport, especially in summer, however, O3 transport seemed weaker in comparison (Fig. 8). Is that because of enhanced dry deposition in southerly low altitude airmasses?
- Line 339: make sure of the consistency between “air mass” and “airmass”.
- Lines 411-412: Since local photochemistry highly overestimates observed concentrations, does that mean that free tropospheric input and PBL growth results in diluted PAN concentrations?
- Lines 420-421: OVOCs contributed overall largely to the total VOCs concentrations and the authors suggest that PAN revealed certain sensitivity towards OVOCs. Which OVOCs contributed most to the formation of PAN?
- Line 445-446: “…, airmasses rich in O3 were mainly associated with high altitude westerly airmasses that either entered the TP from the west of from the south, …” Replace “of” with “or”?
Citation: https://doi.org/10.5194/egusphere-2023-182-RC2 - AC2: 'Reply on RC2', Gen Zhang, 18 May 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-182', Anonymous Referee #1, 19 Apr 2023
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.
Minor comments:
- Lines 136-156: A slight concern on the PSCF analysis is that this is typically more suitable for the source analysis of atmospheric components with longer lifetimes, while PAN can have a long-lifetime in the free troposphere, O3 seems to be more short-lived.
- Lines 198-205: The authors only provided Propy-Equivalent VOCs concentrations, however, to put observations into context with those in other studies, it might also be necessary to provide direct VOCs concentrations in ppb or ppbC. How do VOCs levels compare with previous observations in Tibet or similar high-altitude sites around the world? Since the authors discuss O3 and PAN photochemistry afterwards, this might be important for understanding the difference or similarities in their results with those in literature.
- Lines 212-215: Again, how do NO2 concentrations compare with previous observations in Tibet (if there are any)? The authors mention that there are natural and anthropogenic emissions in NOx, can you name the detailed sources influencing NOx variations at Nam CO? How do these sources impact the diurnal variation of NOx?
- Lines 252-260: Here the authors conclude that O3 and PAN decreases after sunset are caused by dry deposition as well as the cutoff of free-tropospheric input, indicating that local formations O3 and PAN are weak. However, in the later sections, PAN formation at least was revealed to be strong. Is there a contradiction?
- Lines 258-264: PAN often revealed high concentrations at moderate PBLH, which the authors explained as “Whereas PAN in the free troposphere might have had higher variability, which resulted in largely different enhancements of PAN upon down mixing.”. Might it be that local PAN formation was controlling the large variability of PAN?
Technical comments:
- Line 18 and later in text: “physiochemical” should be replaced with be “physicochemical”.
- Line 28: “could by” should be “could be explained by”
- Line 45: Change “nearly proportional to its OVOCs precursors” to “nearly proportionally to its OVOC precursors”.
- Line 440: “reflects” should be “reflect”
- Line 446: The “of” should be “or” in “from the west of from the south”
Citation: https://doi.org/10.5194/egusphere-2023-182-RC1 - AC1: 'Reply on RC1', Gen Zhang, 18 May 2023
-
RC2: 'Comment on egusphere-2023-182', Anonymous Referee #2, 23 Apr 2023
This paper reports spring and summer season O3 and PAN levels and their variations in Tibet and presents an analysis of what has been controlling the changes of those two most important photochemical air pollutants at such a high-altitude site under very clean conditions. Different impacts of transport and photochemistry on O3 and PAN have been detected, revealing distinct transport pathways and different sensitivity to photochemical formation processes. Overall, the topic of the paper fits well into the scope of Atmospheric Chemistry and Physics, the observation and analysis methods used are scientifically sound and the results revealed sufficient novelty. The manuscript can be accepted for publishing after the following questions have been well addressed.
- Although the authors reported the contributions of transport or STE and local chemistry to the observed O3 and PAN concentrations, most of the results are descriptive, particularly before section 3.3. It’s better to discuss the results more quantitatively and compare them with relevant studies.
- The writing style needs to be improved. It is difficult to understand some conclusions because some key information in the figures is omitted.
- Almost all the modeled PAN concentrations are higher than the observed values, while it is completely opposite for O3. My question is whether it is suitable to evaluate the importance of local chemistry for O3 and PAN chemistry using a box model under an extreme environment such as Tibet, where is greatly affected by transport or STE？
- Did you warm up the model before simulating? How did you evaluate your model performance?
- Figure 2b, it is difficult to differentiate the different components of VOCs. Figure 3b, what are the dotted lines
- Please define the w wind, u wind, v wind.
- Line 28: “However, only 0.1% of the springtime day-to-day O3 variability could by STE processes, …”. Something is missing in this sentence.
- Line 32: Make sure you really mean “diurnal” here and not “day-to-day”.
- Line 115: There should be a comma before “which”
- Line 288-291: South Asian countries obviously strong pollution sources close to Tibet, revealing PAN transport, especially in summer, however, O3 transport seemed weaker in comparison (Fig. 8). Is that because of enhanced dry deposition in southerly low altitude airmasses?
- Line 339: make sure of the consistency between “air mass” and “airmass”.
- Lines 411-412: Since local photochemistry highly overestimates observed concentrations, does that mean that free tropospheric input and PBL growth results in diluted PAN concentrations?
- Lines 420-421: OVOCs contributed overall largely to the total VOCs concentrations and the authors suggest that PAN revealed certain sensitivity towards OVOCs. Which OVOCs contributed most to the formation of PAN?
- Line 445-446: “…, airmasses rich in O3 were mainly associated with high altitude westerly airmasses that either entered the TP from the west of from the south, …” Replace “of” with “or”?
Citation: https://doi.org/10.5194/egusphere-2023-182-RC2 - AC2: 'Reply on RC2', Gen Zhang, 18 May 2023
Peer review completion
Journal article(s) based on this preprint
Wanyun Xu et al.
Wanyun Xu et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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