the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Nov 2024
Regional transport of aerosols from Northern India and its impact on boundary layer dynamics and air quality over Chennai, a coastal megacity in Southern India
Abstract. Westerly driven regional transport of aerosols from the heavily polluted North India towards south-eastern India is a prevalent phenomenon during the winter season. Here, the regional aerosol transport events on the boundary layer dynamics and air quality over Chennai, a tropical South Asian megacity, are investigated. The long-term satellite data enables us to depict such regional transport events prolonged for a few days, accounting for ~10–13 per cent of the winter season. The occurrence of these regional transport events is increasing over time in southeastern India which are associated with relatively calmer conditions under anticyclonic wind circulation over north India extending to south India. The transported aerosol layer is generally located around ~1–3 km across the entire southeastern India, capped by the strong atmospheric temperature inversion. The regional aerosol/ haze transport significantly reduces the boundary layer height (ABL-H) by ~38 % compared to clear sky conditions ( ~2–2.5 km). Consequently, an increase in PM2.5 is observed to be ~30–35 % in association with the strong heating aloft ABL (~1.2–2.5 K), suppression of ABL-H and anticyclonic circulation over north India. This study provides robust observational evidence on the importance of regional transport of aerosols on air quality of downwind megacities and warrants more observational and modelling studies to constrain the inherent aerosol-induced effects on boundary layer dynamics.
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CC1: 'Comment on egusphere-2024-3093', Mukunda M Gogoi, 21 Dec 2024
Given the current state of the manuscript, I recommend substantial revisions and resubmission, addressing the following comments to align with the journal's standards for scientific quality, clarity, and rigor.
- Mention the specific version of the CALIPSO data used in the current study. This is critical for understanding the availability and applicability of the data, especially since CALIPSO data is not available beyond 2020/2023, depending on the version.
- Line 105: It was mentioned that that CALIPSO data was used “during December–March between 2015 and 2024.” However, the CALIPSO dataset does not extend beyond 2020/2023 (depending on the version). This needs clarification.
- Line 130 (U, V, and Speed): Mention “Speed,” but it is unclear what this refers to in the context of winds (U, V components). Clarify whether this represents the resultant wind speed or another parameter.
- On what basis it is identified that aerosols being transported from the Indo-Gangetic Plain (IGP) to Chennai? Was any pathway analysis (e.g., HYSPLIT trajectories, wind back-trajectory models, etc.) carried out to confirm the transport of aerosols?
- Elaborate more on how clear days and Regional Transport Events (RTE) are categorized, including specific criteria and thresholds.
- The methodology section is weak and lacks details. I recommend to rewrite this section and provide clear and elaborate explanations for all methods, including RTE categorization, endurance estimation, and aerosol transport identification.
- On what basis is the endurance of RTE days estimated? Specify the data source used to determine the endurance period. What criteria or threshold are used to distinguish clear days from RTE days on a diurnal scale? Are there any previous studies reporting RTE endurance over the Indian domain? Citing prior works could strengthen your findings.
- The region referred to as the “south-eastern coast box” is not shown in any figure. Include this region clearly in a figure for better understanding.
- The RH on clear days is relatively higher than on RTE days. How does hygroscopic growth contribute to the increase in endurance during RTE periods, given these RH differences?
- Differences in wind fields between RTE and clear days alone may not be sufficient evidence for aerosol optical depth (AOD) enhancement over the south-eastern coast and peninsular region. Provide additional supporting evidence or analysis to strengthen this.
- Line 205: Define TAL (Transported Aerosol Layer) when it first appears in the text.
- Lines 241–257 Why were total mean diurnal changes studied instead of individual day differences? Analyzing day-specific differences may offer greater insights into variability.
- Care should be taken to reduce the several typo errors in the manuscript.
Figure 1: Panels (a, b): Why is there a data gap in the bottom-left corner (65°E–70°E and 5°N–10°N)? Panels (c, d): At what altitude are the wind vectors shown? Why are wind vectors missing over the Tibetan Plateau during RTE days, while they are visible on clear days? Panel (e): The description is unclear. How are the weighted counts estimated? Are they based on monthly data or for the entire study period? Panel (f): Why are wind speed differences over high-altitude regions masked? Justify this masking and its impact on interpretation. Suggest adding Chennai and Karaikal locations on Figure 1 for geographic reference.
Figure 2: Is it possible to highlight the region of interest on the map where the CALIPSO profiles are considered? This would improve clarity. Were there any close overpasses of CALIPSO to the MPL station? If yes, how consistent are the CALIPSO profiles when compared to the MPL observations? Clarify how the Atmospheric Boundary Layer Height (ABL-H) and Transported Aerosol Layer (TAL) are identified from MPL profiles.
Figure 4: Panel (a): Is this plot representative of Chennai or Karaikal? Provide clarification. Panel (c): Is the exponential fit used in the figure the most appropriate fit for the data? If possible, justify the choice of the exponential model or test alternative fits for robustness. Provide clarity regarding climatology values of ABL-H and PM2.5. Specify the data sources for these values. Indicate which data points in the figure correspond to RTE and clear days to improve interpretability.
Provide additional figures and clarifications, such as: Highlighting CALIPSO regions of interest in Figure 2. Marking the south-eastern coast box explicitly. Comparing CALIPSO overpasses to MPL observations.
Citation: https://doi.org/10.5194/egusphere-2024-3093-CC1 -
AC1: 'Reply on CC1', Saleem Ali, 07 Apr 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3093/egusphere-2024-3093-AC1-supplement.pdf
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RC1: 'Comment on egusphere-2024-3093', Anonymous Referee #2, 02 Jan 2025
Overview/General Comment
The manuscript entitled "Regional transport of aerosols from Northern India and its impact on boundary layer dynamics and air quality over Chennai, a coastal megacity in Southern India." by Ali et al. attempted to study the critical influence of aerosols on boundary layer dynamics. The research problem is highly significant and holds considerable importance within the scientific community. However, the results are very loosely presented with inadequate justification. The present analysis ranging form ABL identification to TAL transport, relevant interpretation requires greater depth and clarity to attempt the present problem. The estimation of ABL-H, ABL-AOD need to be revised in all the sections before presenting the analysis. Given these shortcomings, I believe substantial revisions are necessary. Therefore, I recommend resubmission, provided the authors address the following comments,
Specific Comments
Line no 105: The statement "during December – March between 2015 and 2024" is inaccurate as the CALIPSO mission ended on August 1, 2023. needs clarification on this statement.
Line no 153: The authors refer to "The climatological mean spatial distribution of columnar AOD." However, defining 2015 to 2024 as a climatological period is inappropriate. Please revise accordingly.
Line no 158-160: The statement, "The RTE and clear days are observed to be 119 and 70 days... RTE shows an enhancement of more than 0.7 over the eastern coast of India compared to clear days," requires clarification: (1) What latitude/longitude range was averaged to derive the value of 0.7? (2) Is the RTE enhancement consistently above 0.7 for all 119 days?
Line no 195-197: “decreasing gradient in aerosol extinction values between surface and 2 km” between RTE and Clear days. This observation is very important. However, it is also important the uncertainty associated with it, (i) author mentioned Fig 2a and 2b is a mean extinction between Dec and March 2015 to 2023 and the (ii) passes considered for the present study is ± 5 deg longitudes which include land and ocean. Considering that the following uncertainty arises,
December features strong ABL inversion (~1 km), while March inversions can extend to ~2.5 km. Similarly, oceanic passes may yield aerosol peaks near 1 km, whereas land passes could show concentrations up to 2 km. I recommend presenting RTE and clear-day analyses for individual months and separating land and ocean data to address these uncertainties.
Line no 210: “Can be seen above the ABL during the RTE periods” I recommend to show the 3 or 5 days backward trajectory analysis from surface upto 4 km during RTE and Clear sky days.
Line no 213 – 214: “temporal variation ….AOD” I have noticed several instance the ABL AOD towards Zero. This reflects the ABL estimation in figure 2c has been associated with uncertainty. For instance, on 27-01-2024 after 18 LT the ABL height and the first altitude bin are at the same altitude. In real atmosphere it never exists. This needs to be taken care in all the section of ABL height identification otherwise the claim of ABL height reduction, boundary layer dynamics may not be valid . It is also to be noted the entire manuscript depends on ABL identification to discuss the TAL and BL dynamics.
Line no 217: “..ABL (integrated extinction within surface and ABL-H) decreases from ~0.4 to less than 0.2..” re-estimation of ABL altitude, ABL-AOD needed.
Line no 220-221: “….the altitude, where the TAL presents, is observed to be warming….” “…contrary to the earlier findings, the surface temperature is also observed to be warming …” it is confusing. 1) What analysis supports the warming conclusion? Temperature profiles typically decrease with height in Fig 2c. 2) Which earlier findings author mentioning? As similar how authors concluding the surface temperature warming? Is author mentioning the diurnal variation ? with respect to what it is warming ?
Line no 224: “..ABL-H also decreased from ~1.4 km to ~0.3 km..” I can see the ABL temperature inversion of at ~ 1.0 km during 25-01-2018 from the radiosonde observations.
Line 225: “The climatological ABL-H”.. can be replaced to “The mean ABL-H..” use of climatological may not valid.
Line no 226-227: “RTE events in altering the boundary layer dynamics” it is loosely stated, I recommended authors should propose the mechanism, appropriate explanation with proper evidence.
Line no 237: “strong inversion at the altitude of ABL and the top of the TAL during the hazy days, attributed to the aerosol heating” the strong inversion layer is related to the large-scale seasonal variation Sinha et al (2013), Ganguly et al (2006). Interpretation needs to be revisited.
Line no 242-243: “RTE (10 days) and clear (6 days) day cases within 2018 and 2023 obtained from MPL”. I recommend the author to present within a month or within the season of particular year to avoid the uncertainty related to inter and intra variability.
Line 249-255. “….significantly reduce the short-wave radiation of incoming solar radiation, hence reducing the surface heat flux and development of ABL….” The author’s claim is not relevant from the present observation. I recommend perform flux direct observations or at least from reanalysis data to support this statement.
Line 259 – 298: Section 3.2. I did not understand why the author decided to present the meteorological observation between Chennai and Karaikal. When the TAL is from the IGP?. Spatial analysis, or data from stations along the transport pathway from IGP to Chennai, would be more appropriate.
Section 3.3. The analysis presented is insufficient to substantiate discussions on BL dynamics. Consider including spatial analyses from CALIPSO or other reanalysis datasets to strengthen this section.
Minor Comments:
Line 100: The statement "AOD over land at 1 km" mention including Ocean as well.
Line 146: The phrase "eastern coast within ±5° longitudes" is ambiguous. Clarify whether this refers to a specific latitude/longitude or the entire eastern coast. Define the latitude/longitude range considered in this study.
Line 155: The authors state "during these months is evident," but do not specify which months are being referenced.
Line 205: The acronym TAL should be introduced here, rather than in Line 209, for better readability and context.
Lines 156–157: Specify the latitude/longitude range averaged to derive the AOD values for RTE days (0.42 ± 0.08) and clear-sky days (0.23 ± 0.06).
Line 239: Replace "during the dry season" with "during the winter season" for consistency and clarity.
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AC2: 'Reply on RC1', Saleem Ali, 07 Apr 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3093/egusphere-2024-3093-AC2-supplement.pdf
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AC2: 'Reply on RC1', Saleem Ali, 07 Apr 2025
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