Impact of particulate matter reductions on aerosol HO<sub>2</sub> uptake and rising surface ozone pollution in India

Gopikrishnan, Gopalakrishna Pillai; Westervelt, Daniel M.; Kuttippurath, Jayanarayanan

doi:10.5194/egusphere-2025-1056

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https://doi.org/10.5194/egusphere-2025-1056

Preprints

12 May 2025

| 12 May 2025

Impact of particulate matter reductions on aerosol HO₂ uptake and rising surface ozone pollution in India

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

Abstract. Atmospheric aerosols significantly contribute to air pollution and influence atmospheric chemistry, impacting air quality and public health. Decrease in aerosols can hinder the radical uptake sink of HO₂, and thus increase NO_x and OH, and subsequently increase ozone levels. This study investigates the seasonal variations of PM₁₀ and aerosol surface area and their effect on surface ozone levels in India, using the GEOS-Chem Chemical Transport Model for the years 2018 and 2022, two years with high and low simulated PM₁₀ concentrations, respectively. The results reveal substantial seasonal variations in PM₁₀ and aerosol surface area. In winter (DJF), higher PM₁₀ and aerosol surface area in the Indo-Gangetic Plain (IGP) and western Central India (CI) result from biomass burning and industrial activity, while coastal regions show lower aerosol surface area. A decrease in aerosol surface area is seen during the pre-monsoon (MAM) and monsoon (JJAS), followed by an increase in the post-monsoon (ON) season. As a result, aerosol-induced HO₂ uptake during winter and post-monsoon lowers ozone concentrations by approximately 30 μg/m³ in 2022 when compared to that of 2018. In contrast, during monsoon in 2022, the decrease in aerosol surface area caused an ozone increase of 10–20 μg/m³ when compared to that of 2018. On average, eighty percent of this increase in surface ozone due to reduction in PM can be mitigated by reducing anthropogenic NO_x emissions by 25–50 %. Thus, we recommend integrated strategies addressing aerosols, precursor emissions and regional meteorology to combat ozone pollution.

Received: 06 Mar 2025 – Discussion started: 12 May 2025

Competing interests: One author (JK) is an editor of ACP. The authors declare there is no other competing interest

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Journal article(s) based on this preprint

06 Feb 2026

Sensitivity of photochemical surface ozone formation regimes to emissions and meteorology in India

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

Atmos. Chem. Phys., 26, 1907–1929, https://doi.org/10.5194/acp-26-1907-2026,https://doi.org/10.5194/acp-26-1907-2026, 2026

Short summary

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-1056', Anonymous Referee #1, 04 Jun 2025

Please see the attached review.

Citation: https://doi.org/10.5194/egusphere-2025-1056-RC1
RC2: 'Comment on egusphere-2025-1056', Anonymous Referee #3, 15 Aug 2025

Review of “Impact of particulate matter reductions on aerosol HO₂ uptake and rising surface ozone pollution in India”
The study by Gopikrishnan et al. reports the effect of HO₂ uptake on aerosols in the years 2018 (high aerosol year) and 2022 (low aerosol year) in the entire India region using the GEOS-Chem model. In 2022, during winter and post-monsoon seasons, HO₂ uptake on aerosols reduced ozone while this is reversed during the monsoon season due to lower aerosols. Based on NO_x reduction scenarios, 80 % of the increase in ozone due to HO₂ uptake on aerosols can be resolved through NO_x mitigation. Overall, the study reports important results on the impact of this new chemical regime and the following revisions are suggested for improvement.
Major comments:
The modeled to observed O₃ discrepancies seem significant in Figure S1. The authors provide several factors (lines 176 to 183) that could cause model to obs discrepancies such as uncertainties in the precursor emissions, ozone sinks, and the coarse resolution of the model. However, the modeled O₃ is significantly higher than observations in all the ground station reported values shown in Figure S1 throughout the entire year. The model overestimates the obs by up to ~75 ppb (assuming that the y-axis is in µg/m³) and in many periods ~ 50 ppb which seems way too high compared to previous literature on GEOS-Chem model studies in Asia. Since the paper mainly discusses about different chemical regimes (NO_x- vs VOC- vs aerosol-limited) it seems worthwhile to confirm that the emissions have been set up properly in the model. More details on the model setup should be added in section 2.2. (e.g., meteorology and emission inventory used).
Similar to Ivatt et al. (2022), it would be useful to add sensitivity tests by adjusting the HO₂ uptake coefficient as this number has a wide reported range (0.08-0.4) and could change the relative importance of each chemical regime.
There are details on the entire region of India that doesn’t seem highly relevant to the main point of the paper which makes the paper difficult to read. Throughout sections 3.1.1, 3.1.2, and 3.1.3, the authors describe trends in PM10 or ozone for each region/year/season and provide previous literature studies to derive causations of the modeled trend. In section 4.1 this continues describing a figure in the supporting information. This level of detail can be useful for the interested readers but not necessarily needed for the discussion. I suggest leaving only relevant information in the main text and moving the rest to the supporting information.
Minor comments:
(lines 37 – 41) The statements about NO_x-limited and VOC-limited seem reversed. In a NO_x-limited regime, radical-radical termination reactions dominate, and higher levels of VOCs won’t lead to significant O₃ increase. In a VOC-limited environment, loss through NO₂ + OH dominates and reducing VOCs will be more efficient in lowering O₃. Also, NO_x“concentrations” are not directly compared to VOC “concentrations” so I would revise “concentration of NO_x is low compared to VOCs”.
Line 49 Add definition for “MDA-8”
Line 51 How many days of ozone exceedances were reported for the 2018 base year?
Line 51 Should 100 ppb be 100 µg/m³?
Figure 1 Colors for NW and Central are too similar. Suggest changing the color of one of them. Northeast India is shown as “NE” in the figure legend but “NEI” in the caption.
Figure S1 Suggest adding units on the y-axis.
Line 300 The relationship between PM₁₀ and ozone is not always inverse. Should clarify.
Line 354 western -> Western
Figure 6 The color scale is a little difficult to see especially the blue and red gradations. Suggest adjusting.
Lines 406-410 If the peroxy-radical self-reactions dominate, it is the VOCs that suppress ozone through the self-reactions.
Line 512 “Furthermore, scaling down the NO_x emissions in the model simulations by 25 and 50 % shows a reduction of about 5-10 and 10-15 µg/m³ of surface ozone in India” Are these numbers referring to the bottom graph in Figure 8? It seems more like 1-4 µg/m³ for a 25 % and 4-8 µg/m³ for a 50 % reduction.

Citation: https://doi.org/10.5194/egusphere-2025-1056-RC2
AC1: 'Comment on egusphere-2025-1056', Daniel Westervelt, 22 Sep 2025

Please see the attached document for responses to both reviewers.

Citation: https://doi.org/10.5194/egusphere-2025-1056-AC1

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-1056', Anonymous Referee #1, 04 Jun 2025

Please see the attached review.

Citation: https://doi.org/10.5194/egusphere-2025-1056-RC1
RC2: 'Comment on egusphere-2025-1056', Anonymous Referee #3, 15 Aug 2025

Review of “Impact of particulate matter reductions on aerosol HO₂ uptake and rising surface ozone pollution in India”
The study by Gopikrishnan et al. reports the effect of HO₂ uptake on aerosols in the years 2018 (high aerosol year) and 2022 (low aerosol year) in the entire India region using the GEOS-Chem model. In 2022, during winter and post-monsoon seasons, HO₂ uptake on aerosols reduced ozone while this is reversed during the monsoon season due to lower aerosols. Based on NO_x reduction scenarios, 80 % of the increase in ozone due to HO₂ uptake on aerosols can be resolved through NO_x mitigation. Overall, the study reports important results on the impact of this new chemical regime and the following revisions are suggested for improvement.
Major comments:
The modeled to observed O₃ discrepancies seem significant in Figure S1. The authors provide several factors (lines 176 to 183) that could cause model to obs discrepancies such as uncertainties in the precursor emissions, ozone sinks, and the coarse resolution of the model. However, the modeled O₃ is significantly higher than observations in all the ground station reported values shown in Figure S1 throughout the entire year. The model overestimates the obs by up to ~75 ppb (assuming that the y-axis is in µg/m³) and in many periods ~ 50 ppb which seems way too high compared to previous literature on GEOS-Chem model studies in Asia. Since the paper mainly discusses about different chemical regimes (NO_x- vs VOC- vs aerosol-limited) it seems worthwhile to confirm that the emissions have been set up properly in the model. More details on the model setup should be added in section 2.2. (e.g., meteorology and emission inventory used).
Similar to Ivatt et al. (2022), it would be useful to add sensitivity tests by adjusting the HO₂ uptake coefficient as this number has a wide reported range (0.08-0.4) and could change the relative importance of each chemical regime.
There are details on the entire region of India that doesn’t seem highly relevant to the main point of the paper which makes the paper difficult to read. Throughout sections 3.1.1, 3.1.2, and 3.1.3, the authors describe trends in PM10 or ozone for each region/year/season and provide previous literature studies to derive causations of the modeled trend. In section 4.1 this continues describing a figure in the supporting information. This level of detail can be useful for the interested readers but not necessarily needed for the discussion. I suggest leaving only relevant information in the main text and moving the rest to the supporting information.
Minor comments:
(lines 37 – 41) The statements about NO_x-limited and VOC-limited seem reversed. In a NO_x-limited regime, radical-radical termination reactions dominate, and higher levels of VOCs won’t lead to significant O₃ increase. In a VOC-limited environment, loss through NO₂ + OH dominates and reducing VOCs will be more efficient in lowering O₃. Also, NO_x“concentrations” are not directly compared to VOC “concentrations” so I would revise “concentration of NO_x is low compared to VOCs”.
Line 49 Add definition for “MDA-8”
Line 51 How many days of ozone exceedances were reported for the 2018 base year?
Line 51 Should 100 ppb be 100 µg/m³?
Figure 1 Colors for NW and Central are too similar. Suggest changing the color of one of them. Northeast India is shown as “NE” in the figure legend but “NEI” in the caption.
Figure S1 Suggest adding units on the y-axis.
Line 300 The relationship between PM₁₀ and ozone is not always inverse. Should clarify.
Line 354 western -> Western
Figure 6 The color scale is a little difficult to see especially the blue and red gradations. Suggest adjusting.
Lines 406-410 If the peroxy-radical self-reactions dominate, it is the VOCs that suppress ozone through the self-reactions.
Line 512 “Furthermore, scaling down the NO_x emissions in the model simulations by 25 and 50 % shows a reduction of about 5-10 and 10-15 µg/m³ of surface ozone in India” Are these numbers referring to the bottom graph in Figure 8? It seems more like 1-4 µg/m³ for a 25 % and 4-8 µg/m³ for a 50 % reduction.

Citation: https://doi.org/10.5194/egusphere-2025-1056-RC2
AC1: 'Comment on egusphere-2025-1056', Daniel Westervelt, 22 Sep 2025

Please see the attached document for responses to both reviewers.

Citation: https://doi.org/10.5194/egusphere-2025-1056-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Daniel Westervelt on behalf of the Authors (22 Sep 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Oct 2025) by Benjamin A Nault

RR by Anonymous Referee #4 (14 Nov 2025)

RR by Anonymous Referee #5 (08 Dec 2025)

Suggestions for revision or reasons for rejection

Gopikrishnan et al. use GEOS-Chem simulations to investigate photochemical surface ozone formation in India for 2018 and 2022. They found that substantial variations in PM10 and aerosol surface area, as well as changing meteorology, influence the surface ozone formation. While the topic is relevant to ACP, the manuscript is poorly presented because the data discussion is not well linked to the included literature findings, the process-level discussion is lacking, and the abbreviations are misused. The manuscript will require major revisions before it can be considered for publication. I highly recommend that the authors consider what information GEOS-Chem can provide, what underlies the variations in simulations across regions, and which processes are included in GEOS-Chem. The current presentation in the manuscript does not demonstrate a good understanding of GEOS-Chem from the authors.

Major
1. Line 88: Why PM10 but not PM2.5? Reasonings should be provided to illustrate why the impact of PM10 is important on surface O3 pollution.
2. Unit for O3: It is more common to express the unit for O3 with ppb rather than ug/m3.
3. Abbreviations of regions: I would suggest avoiding the use of the abbreviations for regions. I have to go back to section 2.1 many times to check what they are when reviewing the manuscript.
4. Figure 2: It is not easy to spot the difference between observations and model estimates, regarding the claim in Lines 270-271. I would suggest calculating the differences between observations and model estimates for sites and summarizing them in a table or figure. Please also check that the colour scheme is colour-blind friendly.
5. Section 3.1.1: The current discussion on the differences between observation and model estimates is problematic. For each region, the authors cite only papers in the discussion. There is no real discussion about whether there is a systematic reason causing such differences. It would be better if the authors could look at how GEOS-Chem performed in other regions.
6. Lines 309-327: Could you elaborate on how the BC scavenges ozone and how the accumulation of PM suppresses the planetary boundary layer and enhances pollutant trapping? Were these processes included in your model run?
7. Lines 330-351: More discussions are needed on how the change in PM affects the solar radiation. What are the contributions via aerosol direct and indirect effects?
8. Lines 420-433: When mentioning different aerosol species like biomass burning and dust aerosols, it would be good to include the spatial distributions of their aerosol surface areas like Figures S8 and S9.
9. Lines 489-491: There is no process-level analysis on how meteorology affects the precursor lifetimes, photolysis rates, and vertical mixing?

Minor
1. Lines 271-272: How can you come up with the claim “IGP also shows higher PM₁₀ (120–140 μg/m³) from industrial, vehicular and agricultural emissions including biomass and crop-residue burning”?
2. Lines 283-284: What is the scientific evidence for the stratospheric-to-troposphere transport? Was this process included in the GEOS-Chem model?
3. Lines 285-287: How were the stagnant conditions defined here?
4. Line 335: Any plot to support the increased solar radiation penetration?
5. Lines 341-344: Where is the evidence for stagnant meteorological conditions?
6. Lines 437-438: What is the supporting evidence for fire events and dust storms? The same applies to other places.
7. Lines 488-489: Which plot or table shows the combined influence of regional circulation and boundary layer height?
8. Lines 495-496: How did you conclude that “the HO₂ uptake effect is an order of magnitude smaller than the direct meteorological impact (20–40 μg/m³)”?
9. Line 499: Did you define “persistently polluted environments” at somewhere?

Technical
1. Line 49: Please include the full name for the PM when it is used for the first time.
2. Lines 50 – 53: The sentence is long to read. Please cut it into two sentences.
3. Lines 65: Why not use the abbreviation for ozone and particulate matter? O3 and PM have been introduced before.
4. Line 96: Is it the right place to cite Figures S1 and S2?
5. Lines 108-126: Could you please divide the whole paragraph into several paragraphs based on regions? It is too long to read as a whole paragraph. If you could manage to present the information in a table, that would be best.
6. BLO3: I do not think the use of BLO3 is necessary. It occurs only three times.
7. Figure S9: Could you provide the full names for different aerosol species in the caption?
8. Line 350: What is HxOy?
9. Lines578-579: It is unclear that “changing meteorological conditions can be reduced by additional efforts in reducing anthropogenic NOx emissions”

Hide

ED: Reconsider after major revisions (18 Dec 2025) by Benjamin A Nault

AR by Daniel Westervelt on behalf of the Authors (30 Dec 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (31 Dec 2025) by Benjamin A Nault

AR by Daniel Westervelt on behalf of the Authors (05 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (12 Jan 2026) by Benjamin A Nault

AR by Daniel Westervelt on behalf of the Authors (22 Jan 2026)

Journal article(s) based on this preprint

06 Feb 2026

Sensitivity of photochemical surface ozone formation regimes to emissions and meteorology in India

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

Atmos. Chem. Phys., 26, 1907–1929, https://doi.org/10.5194/acp-26-1907-2026,https://doi.org/10.5194/acp-26-1907-2026, 2026

Short summary

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

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https://doi.org/10.5194/egusphere-2025-1056-supplement

Gopalakrishna Pillai Gopikrishnan, Daniel M. Westervelt, and Jayanarayanan Kuttippurath

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This study examines the inverse effect of aerosol surface area and particulate matter (PM) reduction on air quality and atmospheric chemistry, particularly on surface ozone levels. Aerosols act as surfaces for the uptake of hydroxyl radicals (HO₂), which are essential for controlling ozone formation. Reducing aerosols and PM may enhance surface ozone formation, thus worsening air quality. However, further efforts to decrease NO_x emissions could mitigate this rise in surface ozone levels.


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Impact of particulate matter reductions on aerosol HO2 uptake and rising surface ozone pollution in India

Journal article(s) based on this preprint

Interactive discussion

Interactive discussion

Peer review completion

Suggestions for revision or reasons for rejection

Suggestions for revision or reasons for rejection

Journal article(s) based on this preprint

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Impact of particulate matter reductions on aerosol HO₂ uptake and rising surface ozone pollution in India