the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2026
Monsoon-driven variability in carbonaceous aerosol sources, chemistry, and optical properties over the equatorial Indian Ocean
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.
- Preprint
(1954 KB) - Metadata XML
-
Supplement
(737 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2026-4276', Anonymous Referee #1, 01 Sep 2026
-
AC1: 'Reply on RC1', Krishnakant Budhavant, 02 Oct 2026
We sincerely thank the reviewer for the careful evaluation of our manuscript and for the constructive comments. We agree that the original manuscript placed overly strong emphasis on some seasonal interpretations, particularly regarding source attribution and BC mass-normalised absorption. In response, we have revised the manuscript to distinguish direct observational evidence from interpretation, explicitly consider meteorological removal processes together with transport, moderate the interpretation of BC-MAC₆₅₈, clarify the temporal resolution of the 72-h filter sampling, and revise the overall framing of the manuscript.
Comment 1: 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.
Response: We thank the reviewer for this important comment and agree that statistical significance should be distinguished from the magnitude and physical relevance of an observed difference. We have therefore revised the manuscript to use “statistically significant” only when referring explicitly to the outcome of a statistical test, and to describe the magnitude of seasonal differences quantitatively or using terms such as “lower”, “higher”, “modest”, or “substantial”, as appropriate.
We have also added a clarification in the statistical methods that statistical significance does not necessarily imply a large or physically important difference and that seasonal contrasts are interpreted by considering the magnitude and variability of the observations together with the statistical tests (Section 2.5, pages 10, lines 258–261).
Accordingly, in Section 3.1 we report that PM₂.₅ and AOD₅₀₀ were approximately 41 % and 35 % lower during the SWM, respectively, while separately reporting that these differences were statistically significant (p < 0.001). We have removed the redundant wording “decreasing significantly/decreased significantly” from these sentences (Section 3.1, page 11, lines 281–285). In contrast, the NEM–SWM difference in BC-MAC₆₅₈ is explicitly described as modest and not statistically significant (Section 3.3.1, page 15, lines 371–374).
Comment 2: 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.
Response: We thank the reviewer and agree that the observed NEM–SWM differences cannot be attributed to air-mass origin alone. We have therefore revised the manuscript to consider changes in meteorological conditions, particularly precipitation and wet scavenging, together with the contrasting transport pathways.
In Section 3.1, we now explicitly note that the SWM is characterised by higher relative humidity and substantially more frequent and intense precipitation and that these conditions are important for aerosol removal, particularly through enhanced wet scavenging (page 10, lines 265–269). We further clarify that cleaner marine inflow during the SWM likely contributes to the lower aerosol burden, while the substantially greater precipitation provides an additional removal mechanism through wet scavenging. Importantly, we explicitly state that the present observations do not permit these two influences to be quantitatively separated (page 10-11, lines 278–296).
Accordingly, throughout the revised manuscript we have avoided attributing the seasonal aerosol differences solely to continental versus marine source regions and instead interpret them as reflecting the combined influence of changing air-mass origin and monsoon meteorology.
Comment 3: 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.
Response: We thank the reviewer and agree that the slightly higher mean BC-MAC₆₅₈ during the SWM should not be interpreted as evidence of enhanced atmospheric ageing. We have therefore substantially moderated this interpretation in the revised manuscript.
We now report that mean BC-MAC₆₅₈ increased only modestly from 8.8 ± 2.9 m² g⁻¹ during the NEM to 10.2 ± 3.2 m² g⁻¹ during the SWM and explicitly state that this difference is not statistically significant (Welch's t-test, p = 0.103; Mann–Whitney U test, p = 0.064). We also emphasise the substantial overlap between the seasonal distributions and treat the slightly higher SWM mean only as a tendency rather than evidence of a systematic seasonal enhancement in BC absorption (Section 3.3.1, page 15, lines 371–374; Table S1).
We have also revised the discussion of atmospheric processing to acknowledge alternative explanations. Specifically, we now state that the chemical indicators do not provide direct evidence of changes in BC coating thickness, morphology, or internal mixing state, and that the BC-MAC₆₅₈ observations do not show a statistically significant seasonal response. Consequently, we do not attribute the slightly higher mean SWM specifically to atmospheric ageing or to enhanced BC coatings. Alternative influences, including differences in source mixtures, wet removal, aerosol composition, and selective persistence of particles during transport, are explicitly acknowledged and cannot be separated with the available measurements (Section 3.3.2, pages 16, lines 398–410).
Accordingly, the revised manuscript now concludes that the seasonal response is much stronger for aerosol abundance and composition than for BC-MAC₆₅₈, and that direct measurements of BC mixing state, coating thickness, and morphology would be required to determine whether atmospheric processing systematically modifies BC absorption during transport to MCOG.
Comment 4: 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 characterising 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.
Response: We thank the reviewer and agree that the 72-hour integrated sampling constrains the temporal scale at which the filter-based measurements can be interpreted. We have clarified this explicitly in the revised manuscript.
In Section 2.1, we now explain that the 72-hour sampling duration was selected to obtain sufficient particulate mass for the chemical, optical, and isotope analyses while retaining resolution of broad seasonal and monsoon-scale variability. We further clarify that each filter represents the average aerosol composition over approximately 3 days and may therefore integrate changes in meteorological conditions and air-mass transport that occur within that interval. Consequently, the filter measurements are interpreted at seasonal and monsoon-regime scales rather than as indicators of individual short-term pollution or meteorological events (page 6, lines 145–151).
This limitation is also explicitly considered in the Results and Discussion, where we state that differences in PM₂.₅ are interpreted at the seasonal and monsoon-regime scale rather than as responses to individual rainfall or transport events (Section 3.1, page 11, lines 292–294). We reiterate this limitation in the Conclusions, where the observed changes are interpreted at the monsoon-regime scale rather than as responses to individual events (page 19, lines 469–471).
Comment 5: 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.
Response: We thank the reviewer and agree that the title and overall framing should distinguish the robust seasonal differences in aerosol loading and composition from the more limited evidence concerning carbon-source variability and BC optical properties. We have therefore revised the title and the framing throughout the manuscript accordingly.
The revised title, “Year-round aerosol variability across monsoon regimes at a near-equatorial Indian Ocean receptor,” now emphasises the principal observational contribution of the study without implying that distinct seasonal source regimes or systematic changes in BC optical properties have been established.
The Abstract has been revised to emphasise the approximately 41 % and 35 % lower PM₂.₅ and AOD₅₀₀ during the SWM and their likely association with the combined effects of changing air-mass origin and monsoon meteorology. In contrast, BC-MAC₆₅₈ is explicitly described as differing only modestly between the NEM and SWM, with no statistically significant seasonal difference, while the isotope measurements are described as showing substantial sample-to-sample variability within both monsoon regimes (page 1, lines 18–29).
The study questions and overall framing have likewise been revised so that BC mass-normalised absorption is evaluated for whether it varies systematically between the monsoon regimes, rather than assuming such a change, and the limited isotope measurements are used as complementary evidence of carbon-source variability rather than quantitative seasonal source apportionment (Introduction, page 4-5, lines 103–125).
The graphical abstract has been revised consistently to state that BC mass-normalised absorption remains broadly comparable between the two monsoon regimes, with no statistically significant seasonal difference, and that the supporting isotope measurements indicate substantial source heterogeneity between samples.
Finally, the Conclusions now emphasise the robust seasonal changes in aerosol abundance and composition while explicitly stating that BC-MAC₆₅₈ differs only modestly and not significantly between the monsoon regimes and that the five isotope measurements provide complementary evidence rather than quantitative seasonal source apportionment (pages 18–19, lines 462–482).
Comment 6: 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.
Response: We thank the reviewer for this suggestion and agree that repetition between Section 3.5 and the Conclusions should be minimised. We have therefore streamlined the Conclusions by removing the repeated discussion of the comparison with earlier observations from Malé, MCOH, and previous short-period measurements at MCOG.
We have retained Section 3.5 as a separate subsection because it provides the detailed quantitative comparison with earlier Maldives and Indian Ocean observations and establishes the regional context of the present dataset. In contrast, Section 4 is now restricted to a concise synthesis of the principal findings and their broader implications. This avoids repeating the historical comparison while preserving the distinct purpose of the two sections (Section 3.5 and section 4, page 18-19 lines 437-482).
Citation: https://doi.org/10.5194/egusphere-2026-4276-AC1
-
AC1: 'Reply on RC1', Krishnakant Budhavant, 02 Oct 2026
-
RC2: 'Comment on egusphere-2026-4276', Anonymous Referee #2, 02 Sep 2026
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.
Citation: https://doi.org/10.5194/egusphere-2026-4276-RC2 -
AC2: 'Reply on RC2', Krishnakant Budhavant, 02 Oct 2026
We thank the reviewer for the positive assessment of the dataset and for the constructive suggestions. We have revised the manuscript accordingly, particularly by strengthening the description of the aerosol-sampling protocol, reducing repetition, clarifying the interpretation and limitations of the isotope measurements, and streamlining the final discussion and conclusions.
Comment 1: 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?
Response: We thank the reviewer for pointing this out. We have revised Section 2.1 to provide additional details on the high-volume aerosol-sampling protocol and to cite earlier studies using the same sampling approach at Maldives receptor observatories. Specifically, we now state that PM₂.₅ was collected using a Digitel DH-77 high-volume aerosol sampler equipped with a PM₂.₅ size-selective inlet and operated at a nominal flow rate of 500 ± 10 L min⁻¹. We further clarify that the sampling and filter-handling procedures followed established approaches previously applied for PM₂.₅ collection at Maldives receptor observatories using the same high-volume sampling system (Budhavant et al., 2018, 2024) (Section 2.1, page 5, lines 137–138).
Comment 2: 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.
Response: We thank the reviewer and agree that the relationship between the established monsoon circulation described in the Introduction and the observations from the present study should be made explicit rather than presented repetitively. We have revised the beginning of Section 3.1 accordingly.
The Introduction now provides the established regional context, namely that the seasonally reversing monsoon circulation brings continental outflow from South and Southeast Asia during the NEM and predominantly marine air masses from the southern Indian Ocean during the SWM. In Section 3.1, we now explicitly connect our observations to this earlier work by stating: “The meteorological observations during 2022 were consistent with the established seasonal reversal of the South Asian monsoon described in previous studies.” We then present the observed NEM–SWM differences in winds, relative humidity, and precipitation (Section 3.1, page 10, lines 265–269).
Comment 2 (continued): 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).
Response: We agree and have revised this part of Section 3.1 to reduce repetition. Rather than repeating the full description of AOD₅₀₀ seasonal variability at this point, we now state that AOD₅₀₀ showed a similar seasonal pattern to PM₂.₅ and provide the corresponding seasonal means and statistical results. We have also corrected the figure citation from Figure 4 to Figure 5 (Section 3.1, page 11, lines 284–285).
Comment 3: 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.
Response: We thank the reviewer and agree that, despite the limited number of isotope samples, the available measurements can support more concrete sample-specific conclusions. We have therefore revised Section 3.4 to provide a more quantitative description of the dual-carbon isotope results while retaining appropriate caution regarding seasonal interpretation.
Specifically, we now report that during the NEM, the February sample was strongly fossil-dominated (ffossil = 0.99), whereas the March sample had a substantially larger contemporary biomass-derived contribution (fbiomass = 0.64), and the January 2023 sample showed an intermediate fossil/biomass-derived composition of 0.61/0.39. During the SWM, the July sample was also strongly fossil-dominated (ffossil = 0.99), whereas the September sample had a substantially larger contemporary biomass-derived contribution (fbiomass = 0.61) (Section 3.4, page 17, lines 417–423).
These results provide a more concrete conclusion from the available isotope measurements: fossil-dominated and mixed fossil–contemporary signatures occur within both monsoon regimes, indicating substantial sample-to-sample source heterogeneity rather than a clear NEM–SWM source separation. At the same time, because only five selectively sampled filters were analysed, we retain the limitation that the isotope dataset cannot support quantitative seasonal source apportionment or statistical comparison between the monsoon periods (Section 3.4, page 18, lines 431–435; Table S2).
Comment 4: 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.
Response: We agree that the original Sections 3.5 and 4 contained substantial repetition, and that this material has been extensively restructured.
To avoid the former repetitive discussion, Section 3.5 has now been replaced by a distinct section entitled “Context of the present observations within earlier Maldives measurements.” This section serves a different purpose: it quantitatively compares the present MCOG observations with previous measurements from Malé, MCOH, and MCOG, including PM₂.₅, BC/EC, OC, nss-SO₄²⁻, BC-MAC, and radiocarbon source information. It also explicitly cautions that differences among these geographically distinct receptor sites should not be interpreted as a simple temporal trend.
Section 4 has consequently been shortened to “Conclusions and broader implications” and now contains only the principal findings of the present study: the combined influence of transport and meteorological removal on aerosol loading, the absence of a statistically significant seasonal BC-MAC₆₅₈ difference, the limited but informative isotope evidence, and the year-round contribution of the MCOG dataset. We therefore did not merge the two sections literally because the revised Section 3.5 now provides a new quantitative historical/regional comparison that is distinct from the Conclusions. However, the repetitive material identified by the reviewer has been removed.
Changes in manuscript: revised Section 3.5, page 18, lines 437–460; revised Section 4, pages 18–19, lines 462–482.
Citation: https://doi.org/10.5194/egusphere-2026-4276-AC2
-
AC2: 'Reply on RC2', Krishnakant Budhavant, 02 Oct 2026
-
CC1: 'Comment on egusphere-2026-4276', Kavita Bhosle, 04 Sep 2026
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.
Citation: https://doi.org/10.5194/egusphere-2026-4276-CC1 -
AC3: 'Reply on CC1', Krishnakant Budhavant, 02 Oct 2026
We thank K. Bhosle for the detailed and constructive comments. We agree that the basic NEM–SWM circulation reversal and many associated aerosol characteristics are already well established from observations in the northern Maldives and the broader northern Indian Ocean. The specific contribution of the present study is to examine and quantify how these features are expressed at the near-equatorial MCOG receptor, approximately 825 km south of Hanimaadhoo, over a complete annual cycle.
Three main findings emerge from this year-round equatorial perspective: (i) aerosol loading decreases substantially from the NEM to the SWM, with PM₂.₅ and AOD₅₀₀ approximately 41 % and 35 % lower, respectively, likely reflecting the combined influence of predominantly marine inflow and enhanced wet removal; (ii) despite these pronounced seasonal changes in aerosol loading and composition, BC-MAC₆₅₈ does not show a statistically significant seasonal difference; and (iii) the isotope subset contains both fossil-dominated and mixed fossil–contemporary signatures within the two monsoon regimes, indicating substantial source heterogeneity among samples rather than a simple seasonal source switch.
The revised manuscript now presents these findings as the principal observational contributions of the study and places them explicitly in the context of earlier measurements from the Maldives.
Comment 1: 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.
Response: We recognise that previous studies have established the broad NEM–SWM circulation reversal and several associated aerosol characteristics over the Maldives and northern Indian Ocean. The present study builds on this foundation by extending the observational perspective to the near-equatorial MCOG receptor, approximately 825 km south of Hanimaadhoo, and by providing continuous coverage across both monsoon regimes and the intervening transition periods over a complete annual cycle.
This year-round near-equatorial perspective allows us to quantify how the established regional aerosol signal is expressed much farther south and to identify features that are not apparent from the monsoon circulation pattern alone. In particular, the present record shows that pronounced seasonal changes in aerosol abundance and composition at MCOG are not accompanied by a statistically significant seasonal difference in BC-MAC₆₅₈. The supporting isotope measurements further reveal substantial source heterogeneity among samples within both monsoon regimes rather than a simple NEM–SWM source separation.
The revised Introduction now explicitly places the study within the context of previous work on South Asian aerosol transport over the Maldives and northern Indian Ocean, including its seasonal transport, chemical composition, carbon sources, and optical properties (Section 1, page 3, lines 55–60). It then identifies the specific observational gap addressed here: substantially fewer year-round observations are available from the near-equatorial southern Maldives, leaving uncertainty regarding how strongly the northern Indian Ocean aerosol signal persists toward the equator and how transport and monsoon-related removal jointly shape the aerosol population reaching this remote receptor.
We have made this distinction explicit in the revised study-objective paragraph:
“Rather than treating the established NEM–SWM contrast itself as a new finding, we use these observations to quantify how the combined effects of monsoon circulation, air-mass origin, precipitation and atmospheric removal regulate aerosol loading and composition at this near-equatorial receptor.”
The revised manuscript also clarifies how MCOG complements previous observations from MCOH. MCOH is closer to the Indian subcontinent and is particularly well positioned to characterise South Asian continental outflow, whereas MCOG lies near the equator and experiences a stronger contrast between Northern Hemisphere continental influence during the NEM and predominantly Southern Hemisphere marine influence during the SWM. The present observations therefore provide a complementary constraint on the persistence and seasonal expression of the South Asian aerosol signal farther south and show that pronounced seasonal changes in aerosol loading and composition do not necessarily translate into a corresponding change in BC mass-normalised absorption.
Changes in manuscript: Section 1, page 3, lines 55–60 and 78–87; page 4-5, lines 88-92, 103-125.
Comment 2: 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.
Response: The value of the integrated year-round dataset is that it allows us to identify two specific results that are not evident from any one measurement alone. First, PM₂.₅, AOD₅₀₀, BC, OC, and nss-SO₄²⁻ all decrease markedly from NEM to SWM, while BC-MAC₆₅₈ remains statistically indistinguishable between the two regimes; thus, the strong seasonal change in aerosol abundance and composition is decoupled from a comparable change in BC mass-normalised absorption. Second, the isotope subset shows that source composition does not follow a simple monsoon binary: both fossil-dominated and mixed fossil–contemporary signatures occur within NEM and SWM. These findings, together with the full-year transport and meteorological context, are now emphasised as the specific contribution of the MCOG dataset rather than the measurement integration itself. The revised study is framed around three observational questions: (i) how transport pathways and meteorological removal jointly regulate aerosol loading and composition, (ii) whether BC mass-normalised absorption differs systematically between contrasting monsoon regimes despite large seasonal changes in aerosol abundance and composition, and (iii) what complementary information the limited isotope measurements provide when considered together with earlier Maldives and Indian Ocean observations (Section 1, pages 4–5, lines 102–125).
The five isotope measurements are presented as supporting sample-specific source constraints rather than as evidence for a new seasonal source regime or for quantitative seasonal source apportionment. Section 2.5 now explains that the fossil and contemporary biomass-derived fractions were estimated using a two-end-member Δ¹⁴C mass-balance approach and explicitly states that these fractions should not be interpreted as seasonal-mean source contributions (page 10, lines 252–257).
Section 3.4 has also been revised to provide a more concrete but appropriately cautious interpretation. The February and July samples were strongly fossil-dominated (ffossil = 0.99), whereas the March and September samples showed substantially larger contemporary biomass-derived contributions (fbiomass = 0.64 and 0.61, respectively); the January 2023 sample showed an intermediate source composition. The resulting conclusion is that substantial sample-to-sample source heterogeneity occurs within both monsoon regimes, rather than a clear NEM–SWM separation. We therefore explicitly state that the isotope results do not establish a distinct seasonal source regime (Section 3.4, page 17, lines 417–423).
This more cautious interpretation is also reflected in the Abstract and Conclusions, where the isotope observations are described as supporting evidence of source heterogeneity rather than as a major new seasonal source-apportionment result.
Comment 3: 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.
Response: The revised manuscript no longer presents the slightly higher SWM mean as evidence of enhanced atmospheric ageing or enhanced BC absorption.
The revised analysis now reports that at MCOG mean BC-MAC₆₅₈ increased only modestly from 8.8 ± 2.9 m² g⁻¹ during the NEM to 10.2 ± 3.2 m² g⁻¹ during the SWM, and that this difference is not statistically significant (Welch's t-test, p = 0.103; Mann–Whitney U test, p = 0.064). We therefore describe the higher SWM mean only as a tendency and conclude that BC mass-normalised absorption remained broadly comparable between the monsoon regimes (Section 3.3.1, page 15, lines 371–377).
We have also revised Section 3.3.2 to clarify that BC-MAC₆₅₈ is influenced by several factors and cannot serve as a direct or definitive measure of atmospheric ageing. The revised discussion explicitly states that the present dataset cannot attribute the slightly higher SWM BC-MAC₆₅₈ specifically to atmospheric ageing or enhanced BC coatings. Alternative influences, including differences in source mixtures, wet removal, aerosol composition, and selective persistence of particles during transport, are now acknowledged. We further state that direct measurements of BC mixing state, coating thickness, and particle morphology would be required to establish such a mechanism (Section 3.3.2, page 16, lines 395–410).
We agree that changes in BC optical properties during long-range transport toward the Maldives have already been investigated, including in our 2024 study. The revised manuscript therefore places the present BC-MAC₆₅₈ observations explicitly in the context of those earlier measurements rather than presenting them as a new ageing mechanism. Section 3.5 now compares the present year-round MCOG observations with previous Maldives measurements, including the winter SAPOEX observations at MCOG reported by Budhavant et al. (2024) (page 18, lines 454–460).
Consistent with this revised interpretation, the Abstract, graphical abstract, and Conclusions have also been modified. The graphical abstract now states that BC mass-normalised absorption remains broadly comparable across the monsoon regimes, and the Conclusions explicitly state that the observations do not demonstrate enhanced BC absorption due to atmospheric ageing.
Comment 4: 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.
Response: We thank the commenter for this particularly useful suggestion. We have added an entirely new section, Section 3.5, “Context of the present observations within earlier Maldives measurements,” which quantitatively compares the present observations with earlier datasets from Malé, MCOH, and MCOG.
Where directly comparable observations were available, the revised section compares PM₂.₅, EC/BC, OC, nss-SO₄²⁻, BC-MAC, and radiocarbon-derived source information. Specifically, we discuss the year-round 2013 Malé and MCOH measurements, the winter 2014–2015 MCOH observations, five summer-monsoon seasons at MCOH, and the recent winter SAPOEX observations at MCOG. These comparisons place the present measurements within the longer observational record available for the Maldives and northern Indian Ocean (Section 3.5, pages 18, lines 437–460).
However, after making this comparison, we found that the available datasets do not provide a sufficiently consistent basis for establishing a robust long-term temporal trend. Malé, MCOH, and MCOG represent different receptor environments, and the previous measurements also differ in season, sampling duration, and observational design. We have therefore avoided interpreting differences among these datasets as evidence of a simple increase or decrease in South Asian pollution over the last one or two decades.
This limitation is now stated explicitly in the manuscript (page 18, lines 458-460):
“Rather than indicating a simple long-term increase or decrease, the comparison shows that aerosol characteristics across the Maldives depend strongly on receptor location, season, transport history, and wet removal.”
The comparison nevertheless clarifies the specific contribution of the present dataset: it extends previous short-duration observations at MCOG to year-round coverage of both monsoon regimes and the intervening transition periods, thereby providing a contemporary near-equatorial observational benchmark.
Overall response to the novelty concern
In response to these comments, we have repositioned the manuscript so that its contribution lies in what the year-round near-equatorial observations demonstrate rather than in the already established monsoon reversal itself. The MCOG record quantifies a pronounced NEM–SWM change in aerosol burden and composition under the combined influence of transport and wet removal, while showing no statistically significant seasonal difference in BC-MAC₆₅₈. The supporting isotope measurements additionally indicate substantial source heterogeneity among samples within both monsoon regimes rather than a simple seasonal source switch.
Together, these observations show that at this far-south, near-equatorial receptor, monsoon-related changes in aerosol abundance, composition, source influence, and removal are substantially more pronounced than the seasonal response of BC mass-normalised absorption. The new comparison with earlier Maldives measurements places these findings within the broader regional observational record while avoiding an unsupported interpretation in terms of a long-term temporal trend.
Citation: https://doi.org/10.5194/egusphere-2026-4276-AC3
-
AC3: 'Reply on CC1', Krishnakant Budhavant, 02 Oct 2026
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 131 | 77 | 74 | 282 | 38 | 80 | 75 |
- HTML: 131
- PDF: 77
- XML: 74
- Total: 282
- Supplement: 38
- BibTeX: 80
- EndNote: 75
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
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:
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.