the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2025
Relation between total-column and near-surface NO2 based on in-situ and PANDORA ground-based remote sensing observations
Abstract. Nitrogen dioxide (NO2) is a major pollutant which at high concentrations may affect human health. It is also a photochemically reactive gas which is important for the oxidation potential of the atmosphere and acts as a precursor for the formation of aerosol particles and ozone. However, monitoring of near-surface (NS) NO2 faces the challenge of spatial discontinuity due to large distances between ground-based monitoring stations, whereas satellite remote sensing provides column-integrated concentrations (total column, TC) which are related to NS concentrations in a complicated manner. In this study, the relation between TC and near-surface (NS) NO2 concentrations is studied using TC NO2 data from remote sensing observations using a Pandora and NS NO2 concentrations from in-situ observations, which were located at the Beijing-RADI site (Beijing, China) during January 2022. The ratio between TC and NS NO2 concentrations varies throughout the day with substantially different relations in the morning and afternoon. During the night and morning the atmosphere was vertically stratified, with disconnected layers which prevented vertical mixing of atmospheric constituents. In the afternoon, these layers connected allowing for vertical mixing and transport between the surface and the top of the boundary layer. Thus the prohibition of vertical transport in the morning and the mixing in the afternoon resulted in different relations between the NS and TC NO2 concentrations. These different relationships have consequences for the use of satellite remote sensing to estimate NS NO2 concentrations.
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Status: open (until 26 Mar 2025)
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RC1: 'Comment on egusphere-2025-360', Anonymous Referee #1, 02 Mar 2025
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This manuscript evaluated the relationship between total-column and near-surface NO2 in winter in Beijing in using in-situ and PANDORA ground-based remote sensing observations. Detailed vertical distribution was obtained from Lidar observations. Then the possible influence of the atmospheric boundary layer evolution on the near-surface pollutants was discussed. Also, the coupling mechanism between the transport of pollutants and the complex vertical distribution of the atmospheric boundary layer was analyzed using back trajectories and weather maps. The manuscript is well-written and fits the scope of the journal. It provides an interesting case study of the vertical distribution changes of pollutants in the boundary layer based on remote sensing technology. However, there are a few aspects that need to be addressed to further improve the quality of the manuscript.
1. The manuscript highlighted the use of PANDORA for stratification inversion. This is an interesting point, but a detailed explanation of its credibility as well as its role in practical applications are lacking. Please further elaborate on the advantages of PANDORA technology to state its importance and accuracy to readers.
2. The authors used total concentrations when analyzing NO2 obtained from ground-based remote sensing, but compared the tropospheric concentrations of NO2 with that from satellite sensor TROPOMI. This needs to be clarified.
3. The observation was divided into three periods, in which PM2.5 continued to rise during Period II and suddenly dropped dramatically on the 25th. In the case study, the authors only chose the two cases of Period I. The cases during Period II were not analyzed. Please explain this in detail.
4. In the case studies of the 14th and 18th, the PANDORA remote sensing only had observations during the daytime. This makes sense. However, there was a difference in the vertical distribution of NO2 from PANDORA versus Lidar. I can understand that the Lidar signal comes more from the scattering of aerosol particulate matter, but the vertical decoupling of NO2 does not appear in the PANDORA observations. Although the authors have some explanation for this, I think it needs further clarification of these differences.
5. Based on the multi-temporal backward trajectory and weather maps and in conjunction with the remote sensing imagery from TROPOMI, both cases were in good agreement with the regional transport pattern. In the vertical distribution evolution, it was also consistent with the expectation of winter boundary layer development. Although the authors have explained the boundary layer development and the conformity of the backward trajectories at different heights, the differences between the two cases were not yet clearly summarized. Despite the presence of multiple layers in the boundary layer in both mornings, there is a correlation between the distribution of pollution concentrations in the layers and the regional pollution intensity and transport direction, which is presented in both cases. It would be great if the authors could verify whether this is the case and summarize the differences, which I believe would be interesting.
6. I have some confusion about Figure 7. Although it showed regular data, the ratios were not the correlation coefficients or slopes that were used in the case studies. The slopes were fixed in the cases where TC correlates well with NS NO2 concentrations, whereas the ratios were not slopes, which were highly variable. I think the authors need to give a more detailed explanation of how they are related and different.
7. There are also some other minor issues that need further improvement, such as the format of the citation.
Line 167: “… north, east, and west (Figure 1). ”, for figures, the manuscript used fig more often, please make sure that they are consistent.
Line 205: “… precision of 0.01 DU and a nominal accuracy of 0.1 DU Herman et al. (2009)”, the citation should be in parentheses as a whole.
Line 260: “PM2.5 is measured using beta rays generated”, “2.5” should be subscripted.
Line 304: “e.g., Atkinson, 2000; Boersma et al., 2009; Y. Zhang et al., 2016”, “Y. Zhang” should be “Zhang”. Please double check all the citations so that the format is consistent.
Line 442: “… with a wind speed of 2 m/s”, for units, superscripts are used more often than slashes throughout the manuscript. Please revise it to be consistent.Citation: https://doi.org/10.5194/egusphere-2025-360-RC1 -
RC2: 'Comment on egusphere-2025-360', Anonymous Referee #2, 10 Mar 2025
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The manuscript "Relation between total-column and near-surface NO2 based on in-situ and PANDORA ground-based remote sensing observations" by Zhang et al. investigates the relationship between total column (TC) and near-surface (NS) NO2 concentrations using field experiments, supported by ancillary data and model analysis. The study highlights the value of Pandora observations in capturing and understanding the dynamic vertical distribution of NO2. Additionally, the use of a backward trajectory model in two case studies effectively illustrates air mass motions at different altitudes, providing meaningful insights into the evolution of TC-NS relationships.
Overall, the study is well-structured and employs a clear methodology. However, concerns regarding its novelty, broader implications, and generalizability may limit its impact. Therefore, major revisions are required to justify its publication in Atmospheric Chemistry and Physics (ACP).
Major comments:
- The study is based on a single station (Beijing-RADI) and a short observation period (January 10-29, 2022), which limits the applicability of the results to other regions and seasons. Since the authors emphasize the complexity of the TC/NS NO2 relationship, it raises concerns that this limited dataset may not fully capture its variability. It is suggested that the authors provide a detailed justification for the data selection, explaining why the short-term observations are still appropriate for investigating this relationship. Additionally, the representativeness of the case study should be explicitly discussed, particularly whether the findings are expected to hold across different meteorological conditions and locations. If possible, a comparison of the TC-NS NO2 relationship between the Beijing-RADI site and other sites is recommended to strengthen the generalizability of the conclusions.
- The study emphasizes the importance of investigating the TC-NS NO2 relationship for satellite-based monitoring of NS NO2 concentration and discusses how the TC/NS ratio changes with time and meteorology. However, the manuscript does not directly analyze the relationship between satellite TC NO2 and NS NO2, instead focusing solely on the Pandora TC NO2 and NS NO2 Given that the authors acknowledge biases between Pandora and satellite TC NO2 in the introduction, it is recommended that additional analyses be included to assess how TC/NS variations impact actual satellite NO2 applications on NO2 monitoring.
- The manuscript does not clearly establish the novelty of the study compared to prior works. The motivation expressed in lines 145-148 is not sufficiently developed to justify the study’s significance. While the paper states that accurate TC/NS NO2 information was not previously available for China, it does not explicitly explain why this makes the study novel or how it differs from previous research on TC/NS relationships. To strengthen the motivation, the authors should clarify what specific gaps in the literature they are addressing and explicitly compare their approach to existing studies.
- The manuscript presents an analysis of the TC/NS NO2 ratio, but its physical significance and relevance to satellite-based NO2 applications remain unclear. Besides, the sensitivity of the ratio to meteorological factors is not quantified. To strengthen the analysis, the authors should clarify the rationale for using this ratio, ensure a consistent interpretation, assess meteorological influences, and explicitly connect their findings to the use of satellite NO2 data for estimating near-surface NO2
Specific comments:
- Lines 58-60: The phrase 'providing vertical total column and tropospheric densities' is unclear. It would be more precise to explicitly distinguish total column density, stratospheric column density, and tropospheric column density. In this manuscript, 'TC' should consistently and explicitly refer to the 'tropospheric vertical column density' to avoid ambiguity.
- Lines 85-88: Thompson’s work focuses on the complexity of the TC-NS NO2 relationship, which does not align well with the paragraph’s main discussion on Pandora vs. satellite TC NO2 This reference would be more appropriate in a section specifically addressing TC-NS variability rather than in a discussion of measurement consistency between Pandora and satellite data.
- Lines 109-114: This sentence is excessively long, making it difficult to follow. It is recommended to split it into two sentences to improve readability and ensure clarity of the message.
- Figure 2: It is suggested to add dashed lines in Figure 2 to clearly indicate the boundaries between the three periods, improving readability.
- Lines 315-322: The manuscript presents PM5 observations and results but does not explicitly explain their relevance to NO2 analysis. To improve clarity, the authors should justify the inclusion of PM2.5 data and clarify how it supports the study’s objectives, ensuring a stronger connection between NO2 assessment and aerosol pollution.
- Line 484: The TC/NS ratio mentioned in Line 484 is not found in Figure 3b. Please clarify whether it is missing or referenced incorrectly.
- Figure 3: There appear to be unexpected content between Figures 3c and 3d, likely due to figure cropping. Please check and correct this issue.
- Figure 4: To enhance clarity, it is suggested to add necessary subtitles for the three subfigures in Figure 4c, such as local time. Additionally, the resolution of Figure 4 appears low and should be improved. (Same for Figure 6)
- Figure 6: The middle panel of Figure 6c contains unexpected content. Please verify and correct this issue.
- Lines 691-693: The outlook on potential large-scale implementation is vague. The authors should specify which shortcomings to be resolved.
Citation: https://doi.org/10.5194/egusphere-2025-360-RC2
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