Monsoon-driven variability in carbonaceous aerosol sources, chemistry, and optical properties over the equatorial Indian Ocean

Budhavant, Krishnakant Babanrao; Muslim, Ahmed; Romson, Joakim; Satheesh, Sreedharan Krishnakumari; Gustafsson, Örjan

doi:10.5194/egusphere-2026-4276

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https://doi.org/10.5194/egusphere-2026-4276

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20 Aug 2026

| 20 Aug 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Monsoon-driven variability in carbonaceous aerosol sources, chemistry, and optical properties over the equatorial Indian Ocean

Krishnakant Babanrao Budhavant, Ahmed Muslim, Joakim Romson, Sreedharan Krishnakumari Satheesh, and Örjan Gustafsson

Abstract. Carbonaceous aerosols over the equatorial Indian Ocean influence regional radiative forcing and long-range transport, yet integrated observations linking aerosol chemistry, optical properties, transport pathways, and source characteristics remain scarce. Here, we present year-round measurements of PM₂.₅ chemical composition, aerosol optical properties, air-mass transport, and representative dual-carbon isotope signatures from the Maldives Climate Observatory at Gan (MCOG; 0.69° S, 73.15° E) to investigate how seasonal monsoon circulation regulates aerosol transport and atmospheric processing.

During the northeast monsoon (NEM), continental outflow from South and Southeast Asia resulted in significantly higher PM₂.₅ concentrations (5.6 ± 2.7 μg m⁻³), aerosol optical depth (AOD₅₀₀ = 0.17 ± 0.09), and elevated concentrations of black carbon (BC), organic carbon (OC), and non-sea-salt sulfate (nss-SO₄²⁻). In contrast, marine inflow during the southwest monsoon (SWM), together with enhanced precipitation, substantially reduced aerosol loading while increasing the relative contribution of sea-salt aerosol. Diagnostic aerosol ratios revealed significant seasonal shifts in aerosol source characteristics and chemical composition. Although BC mass absorption cross-sections at 658 nm were slightly higher during the SWM, the seasonal difference was not statistically significant. Representative dual-carbon isotope measurements provided independent evidence for mixed fossil and contemporary carbon sources.

These observations demonstrate that monsoon circulation governs aerosol transport, loading, and chemical composition over the equatorial Indian Ocean and provide observational benchmarks for improving the representation of long-range aerosol transport, atmospheric processing, and aerosol–climate interactions in regional chemical transport and Earth system models.

Received: 16 Jul 2026 – Discussion started: 20 Aug 2026

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Krishnakant Babanrao Budhavant, Ahmed Muslim, Joakim Romson, Sreedharan Krishnakumari Satheesh, and Örjan Gustafsson

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RC1:
'Comment on egusphere-2026-4276', Anonymous Referee #1, 01 Sep 2026 reply
This manuscript presents year-round measurements of aerosol chemical composition, optical properties, air-mass transport, and a limited set of dual-carbon isotope measurements at the Maldives Climate Observatory at Gan. The dataset is valuable, and the contrast between continental influence during the NEM and marine influence during the SWM is interesting. However, I think the manuscript currently tends to overstate the strength of some of the seasonal interpretations.
My main comments are:
Seasonal differences: “significant” versus substantial/consistent differences: The manuscript frequently uses “significant” to describe differences between the NEM and SWM. While some variables are statistically different, in other cases the magnitude of the difference does not seem to warrant this language (e.g., page 9, line 239). More generally, I suggest carefully distinguishing statistical significance from the magnitude and physical significance of the difference and toning down the language where appropriate.

Seasonal differences cannot be attributed to air-mass origin alone: The NEM/SWM contrast is interpreted primarily in terms of continental versus marine back trajectories. However, the two periods also differ substantially in meteorological conditions. In particular, Figure S1 shows much more frequent and intense rainfall during the SWM. The role of wet scavenging, as well as differences in RH, wind speed, etc., therefore deserves more explicit consideration. How much of the lower aerosol loading during the SWM is due to cleaner marine air masses, and how much could be due to enhanced precipitation and removal? The discussion should be more careful about attributing the observed seasonal differences simply to transport/source regions.

Interpretation of BC-MAC: I am not convinced that the data (i.e., Fig. 5) support the emphasis on “higher BC-MAC” during the SWM or, more importantly, the interpretation in terms of enhanced atmospheric aging. The mean is indeed higher during the SWM, but the distributions overlap substantially and the difference is not statistically significant (Table S1). Given the strong differences in rainfall and aerosol loading between the seasons, there are also alternative explanations for the observed distribution.

What does the 72-hour averaging imply for the interpretation?: The 72-hour integrated sampling is an important characteristic of the dataset. It is appropriate for characterizing broad seasonal differences, but individual samples cannot resolve variability within the three-day sampling period and may integrate over changing meteorological and transport conditions. The manuscript should be clearer about what is meant by “temporal” or “seasonal variability” and avoid drawing conclusions about individual events or short-term variability that cannot be resolved by the sampling approach.

Implications for the title and overall framing: Taken together, these issues suggest that the title and overall framing may be somewhat stronger than warranted. The study robustly demonstrates differences in aerosol loading and composition between the monsoon regimes, but the evidence for seasonal source variability and changes in BC optical properties is more indirect. I suggest revisiting the title and ensuring that the abstract, hypotheses, graphical abstract, and conclusions consistently reflect the strength of the evidence.

Sections 3.5 and 4 should be combined: Section 3.5 and Section 4 (Conclusions) are repetitive. I suggest combining these sections to make the manuscript more concise and avoid repetition.

Overall, I find the dataset interesting and potentially valuable, but I think the manuscript would benefit considerably from more cautious interpretation of the NEM–SWM differences and a clearer separation of what is directly demonstrated by the observations from what is inferred from the transport and meteorological context.

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Citation: https://doi.org/10.5194/egusphere-2026-4276-RC1
RC2: 'Comment on egusphere-2026-4276', Anonymous Referee #2, 02 Sep 2026 reply

This paper presents a comprehensive analysis of year-round aerosol measurements conducted over the Indian Ocean. While not a particularly novel investigation, the results are definitely important and worthy of publication. The study is scientifically sound, and the paper is clearly written and, most of it, well organized. I have e few issues that should be addressed before recommending accepting the paper for publication.

The used methods are described very well in section 2. The only exception is section 2.1 which has no citations to any earlier work. Does the high-volume aerosol sampling follow a specific protocol defined somewhere?
There is unnecessary repetition here and there. First, the pieces of information given on lines 47-53 and 220-224 are essentially the same, the only difference being that on lines 47-53 this is given as general information with references to earlier work, while on lines 220-224 it is given as a result of this study. There should be a connection between these two, e.g. stating that these observations are consistent with earlier studies. Second, lines 240-241 and 308-309 give exactly the same information. Maybe it would be enough to say on lines 240-241 that the seasonal behaviour of AOD follows that of PM (the text should refer to Fig. 5, not to Fig. 4 as written now).
The results and conclusions of section 3.4 are very qualitative. It is understandable that the limited number of samples precludes more quantitative analysis, but I would have appreciated a bit more concrete conclusions on this issue in the paper.
Finally, there is plenty of repetition between sections 3.5 and 4: lines 389-390 vs. 416-417, lines 395-399 vs. 417-421, and lines 402-404 vs. 428-430. Considering this, and overall contexts of these two sections, I would even recommend combining the sections together into a single final section of the paper.

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Citation: https://doi.org/10.5194/egusphere-2026-4276-RC2
CC1: 'Comment on egusphere-2026-4276', Kavita Bhosle, 04 Sep 2026 reply

The manuscript presents a good year-round observational dataset from MCOG, combining PM₂.₅ chemical composition, aerosol optical properties, air-mass trajectories and carbon isotope measurements. However, I have a major concern about the novelty of the present work.
Many of the important findings reported in this manuscript have already been investigated and reported in earlier studies from Maldives and the Indian Ocean region. For example, transport of polluted continental air during NEM and relatively clean marine air during SWM is already well known. Similarly, higher BC, OC and nss-SO₄²⁻ during continental outflow, higher contribution of sea salt during marine conditions, and the important role of monsoon circulation in controlling aerosol loading and composition have been reported previously. The manuscript itself also refers to several previous studies showing similar results.
More importantly, several earlier studies, including studies from the same or related research group, have already investigated carbonaceous aerosol sources, radiocarbon-based source apportionment, atmospheric ageing and BC optical properties over the Indian Ocean. There are relevant studies published in 2015, 2018, 2020, 2023 and also a recent study in 2024. Therefore, many of the results presented in this manuscript look more like confirmation of the previous findings rather than providing substantially new scientific understanding.
The authors mention that the novelty is the integration of year-round aerosol chemistry, optical properties, transport and isotope measurements at MCOG in one study. This integrated dataset is certainly useful. However, in my view, only combining different measurements in one study may not be sufficient to establish strong scientific novelty unless this combination provides some new process understanding, new source information, or some important result which was not known from the previous studies. At present, this is not coming out clearly from the manuscript.
I also have concern regarding the novelty of the carbon isotope results. The finding that carbonaceous aerosols reaching Maldives have contribution from both fossil and contemporary sources has already been demonstrated in previous radiocarbon studies. In the present study, the number of isotope samples is also limited, and the authors themselves mention that these observations are not sufficient for quantitative seasonal source apportionment. Therefore, it is difficult to consider this as a major new result.
Similarly, BC-MAC is slightly higher during SWM, but this difference is not statistically significant. Therefore, it may not be appropriate to give strong importance to enhanced atmospheric ageing or BC absorption during SWM. Similar changes in BC optical properties during long-range transport towards the Maldives have also been investigated in previous studies, including the recent 2024 work. Therefore, the new scientific information coming from this analysis is also not very clear.
One important aspect which is missing is a direct comparison with the earlier measurements from MCOG/Maldives. Since measurements are available from this region for many years, the present dataset provides a good opportunity to examine whether there has been any real change in the characteristics of South Asian pollution reaching the equatorial Indian Ocean. For example, authors can compare PM₂.₅, BC, OC, sulfate, BC/PM₂.₅, OC/BC, AOD, BC-MAC and fossil/contemporary carbon contribution with earlier observations. If some significant changes are found over the last one or two decades, this can provide a much stronger scientific message.
Overall, the dataset is useful and the measurements are important, but in the present form I find the scientific novelty rather limited. Most of the major conclusions are already known from previous studies and the manuscript mainly reconfirms the established monsoon-driven aerosol behaviour over this region. The authors need to demonstrate clearly what new scientific knowledge is obtained from the present measurements compared to earlier MCOG/Maldives studies. Without such analysis and comparison, it is difficult to identify a sufficiently strong new scientific contribution from this work.

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Citation: https://doi.org/10.5194/egusphere-2026-4276-CC1

Krishnakant Babanrao Budhavant, Ahmed Muslim, Joakim Romson, Sreedharan Krishnakumari Satheesh, and Örjan Gustafsson

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https://doi.org/10.5194/egusphere-2026-4276-supplement

Krishnakant Babanrao Budhavant, Ahmed Muslim, Joakim Romson, Sreedharan Krishnakumari Satheesh, and Örjan Gustafsson

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Short summary

Seasonal monsoon winds transport aerosol particles across the equatorial Indian Ocean. Measurements in the Maldives show that the northeast monsoon carries polluted air from South and Southeast Asia, whereas the southwest monsoon brings cleaner marine air from the southern Indian Ocean. These observations improve our understanding of long-range pollution transport and help improve climate and air-quality models.


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