the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 May 2025
Impact of particulate matter reductions on aerosol HO2 uptake and rising surface ozone pollution in India
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 NOx 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 NOx emissions by 25–50 %. Thus, we recommend integrated strategies addressing aerosols, precursor emissions and regional meteorology to combat ozone pollution.
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.- Preprint
(6842 KB) - Metadata XML
-
Supplement
(4388 KB) - BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1056', Anonymous Referee #1, 04 Jun 2025
-
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 NOx reduction scenarios, 80 % of the increase in ozone due to HO2 uptake on aerosols can be resolved through NOx 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 (NOx- 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 NOx-limited and VOC-limited seem reversed. In a NOx-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, NOx “concentrations” are not directly compared to VOC “concentrations” so I would revise “concentration of NOx 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 NOx 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
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 479 | 122 | 13 | 614 | 28 | 13 | 23 |
- HTML: 479
- PDF: 122
- XML: 13
- Total: 614
- Supplement: 28
- BibTeX: 13
- EndNote: 23
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Please see the attached review.