the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Mar 2026
Aerosol processing during long-range transport governs the chemical composition of haze over the northern Indian Ocean
Abstract. Each year during the winter period, a persistent haze forms over the Indian subcontinent and the northern Indian Ocean. This has been shown to influence regional warming, rainfall patterns and air quality. Previous studies have demonstrated that this haze is largely anthropogenic in origin, with its composition dominated by sulfate, organic compounds and black carbon. Nevertheless, to date, information about its composition has largely been limited to bulk chemical composition and low time-resolution data. Here, we aim to characterise aerosol composition over the Indian Ocean on the molecular level, in order to identify the impact of different sources and processing in this region. High-time-resolution measurements were conducted at the Maldives Climate Observatory, Hanimaadhoo (MCOH) using a Time-of-Flight Aerosol Mass Spectrometer (AMS) and Chemical Ionisation Mass Spectrometer with a Filter Inlet for Gases and Aerosols (FIGAERO-CIMS). Results showed a remarkably uniform composition, despite variability in source regions and total concentration, indicating strong regional mixing of air masses. Sulfate accounted for approximately 52 % of non-refractory sub-micron particulate mass. Eighteen sulfur-containing organic compounds were identified, some for the first time in this location. Tracers of some sources, particularly biomass burning, were identified in the organic mass spectrum. However, the majority of organic mass was dominated by highly-processed compounds such as dicarboxylic acids. Our results underscore the impact of long-range transport and heterogeneous sulfate-driven chemistry on aerosol composition over the Indian Ocean, with important implications for understanding radiative forcing, aerosol-cloud interactions, and regional climate feedbacks in South Asia.
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Status: final response (author comments only)
- CC1: 'Non-volatile PM (Soot / dust / ...)', J. C. Corbin, 16 Mar 2026
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RC1: 'Comment on egusphere-2026-914', Anonymous Referee #1, 10 Apr 2026
This manuscript presents high-time-resolution measurements of aerosol chemical composition at MCOH and provides valuable molecular-level information on sulfate-containing organic compounds and highly oxidized organic aerosol over the northern Indian Ocean. The dataset is interesting and potentially useful for improving our understanding of aerosol characteristics in this region, especially given the limited availability of such measurements at this site. However, in its current form, the manuscript is more convincing in describing the observed aerosol composition than in demonstrating the proposed mechanisms controlling it. In particular, several key interpretations regarding long-range transport aging, aqueous/cloud processing, and the influence of Delhi or Indo-Gangetic Plain outflow are not yet sufficiently supported by the current analysis. I therefore think the a major reversion is needed before it can be considered for publication.
Major comments:
- The manuscript separates the campaign into three periods associated with different air mass trajectories. However, this classification is not fully exploited in the subsequent analysis. Beyond showing bulk compositional similarities and differences in mass loading, the manuscript does not provide a sufficiently detailed analysis of molecular composition, source tracers of each period, or potential processing pathways. As a result, the period classification appears somewhat descriptive rather than mechanistically informative. If the authors aim to argue that aerosol processing during long-range transport governs the observed composition, a deeper comparison among these periods is needed.
- The manuscript argues that photochemical aging, aqueous/cloud processing, and sulfate-driven heterogeneous chemistry during long-range transport govern the aerosol composition at MCOH. However, the current evidence is still largely inferential, based mainly on back trajectories, compositional similarity across periods, high O:C, and the presence of highly oxidized compounds. These observations are consistent with atmospheric processing, but they do not by themselves uniquely demonstrate that these processes occurred during transport.
- The manuscript attributes the high abundance of oxalic acid and other dicarboxylic acids to cloud or aqueous-phase processing, which is plausible. However, the evidence presented here is still indirect. If the authors want to retain this as a central mechanistic conclusion, it would help to include additional analysis linking these species to meteorological or transport indicators relevant to aqueous processing, or at least to more clearly acknowledge that the conclusion is based on molecular interpretation rather than a direct process constraint.
- If the authors intend to argue that the observed aerosol was influenced by aged outflow from Delhi or the Indo-Gangetic Plain, a more explicit period classification would be needed. In particular, the manuscript should identify the periods most strongly affected by air masses passing over Delhi-related source regions, and examine whether these periods exhibit distinguishable chemical signatures, such as source tracers, oxidation characteristics, or specific molecular patterns. Without such stage-resolved analysis, the comparison with Delhi remains suggestive rather than demonstrating that the observed aerosol characteristics at MCOH are specifically linked to aged Delhi outflow.
Specific comments:
- The resolution of figures should be improved.
- Figure 2: The colors used for the chemical species in panel (a) should be consistent with those used in panel (c) for clarity and easier comparison.
- Line 273-274: Please provide more details regarding the anthropogenic sources referred to here.
- Line 275-276: Please clarify what additional information Figure S4 provides and how it supports the discussion in this section.
- Line 304: Since C4H9O4S was identified during this campaign, it would be valuable to further examine its temporal evolution, diurnal pattern, and variations across the different periods. Such analyses could provide useful insight into its potential formation pathways.
- Sections 3.5: More in-depth analysis of the identified organic compounds is needed. For example, what is the mass fraction of each major species, and what are their temporal patterns? In addition, is the high contribution of organic acids and dicarboxylic acids distinct from that observed in other regions? In its current form, this section is largely descriptive and mainly reports several phenomena, but a deeper investigation of the organic molecular composition would substantially strengthen the manuscript.
Citation: https://doi.org/10.5194/egusphere-2026-914-RC1 -
RC2: 'Comment on egusphere-2026-914', Anonymous Referee #2, 24 Apr 2026
This study utilizes advanced HR-ToF-AMS and FIGAERO-CIMS to conduct a detailed characterization of the chemical composition of wintertime, anthropogenic-influenced haze aerosols over the northern Indian Ocean. Obtaining such high-time-resolution and molecular-level joint observational data in this relatively under-sampled region is highly valuable. The dataset contributes positively to our understanding of the evolution of long-range transported aerosols in South Asia. However, despite the exceptional richness and potential of the acquired dataset, the current analysis primarily remains at the level of species identification and bulk composition description. It does not yet fully utilize the quantitative and source apportionment capabilities of both AMS and FIGAERO-CIMS datasets. I suggest that the authors further deepen their data analysis to more robustly substantiate the core conclusion that aerosol aging during long-range transport governs the chemical composition over the northern Indian Ocean.
Major Comments:
- While this study employs top-tier mass spectrometry techniques, the current analysis of OA properties remains largely at the level of bulk classification and qualitative molecular identification. It is highly recommended that the authors perform Positive Matrix Factorization (PMF) on both the AMS organic mass spectra and the FIGAERO-CIMS datasets. For the AMS, PMF would quantitatively deconvolve the contributions of secondary organic aerosols with varying degrees of oxidation (e.g., OOA) and primary emissions (e.g., BBOA, HOA). For the FIGAERO-CIMS, PMF would help categorize hundreds of complex organics into meaningful factors sharing common volatility or source characteristics. Including these standard yet fundamental analyses will provide richer chemical details to support the narrative of aerosol aging during long-range transport.
- The identification of multiple SCO and CHO/CHON compounds is a noteworthy contribution of this study. However, the current analysis relies primarily on species listing and relative signal comparisons. We suggest the authors go further by quantifying the contributions of these compounds — or their compound classes — to total OA (e.g., as relative abundances or semi-quantitative mass fractions), and by examining their temporal co-variation with sulfate, relative humidity, fire counts, and air mass origin. Where secondary formation pathways are proposed for specific SCOs, supporting the discussion with relevant tracer correlations or diagnostic ratios would better constrain the inferred mechanisms. For newly identified species in particular, supplementary evidence such as characteristic thermogram profiles or comparisons with volatility data reported in the literature would strengthen confidence in the proposed assignments.
- The text highlights C₄H₉O₄S⁻ as the most abundant SCO signal observed, proposing it corresponds to diethyl sulfate and claiming this as its first ambient atmospheric detection. Currently, however, the assignment relies primarily on molecular formula matching, lacking further validation such as comparisons with standard thermograms, literature values of known organosulfate volatilities, or a discussion ruling out potential isomers. Given that this is a key highlight of the paper, it is recommended that the authors explicitly acknowledge the uncertainties associated with this identification and provide as much corroborative analysis as possible, allowing readers to objectively assess the robustness of this claim.
Minor Comments:
- The study selects three characteristic sampling periods based on 10-day backward trajectories to compare mass spectra and aerosol composition. It is recommended to supplement this with statistical significance testing (e.g., t-tests on the mass fractions of major components) across these periods. This would provide a more objective evaluation of whether the chemical composition features associated with different source regions are statistically distinct.
- The supplementary material notes that the aerosol was largely neutralized based on an ion balance analysis (indicating sufficient ammonium to neutralize sulfate and nitrate). Given that sulfate chemistry and aqueous-phase processes critically influence the formation pathways of secondary organic aerosols (especially SCOs), it is suggested to briefly discuss in the main text how this neutralized environment might specifically affect the formation mechanisms of the observed sulfur-containing organics (e.g., the efficiency of acid-catalyzed multiphase reactions).
Citation: https://doi.org/10.5194/egusphere-2026-914-RC2
Data sets
Chemical composition of aerosol particles observed at the Maldives Climate Observatory, Hanimaadhoo, January – February 2023 Sophie L. Haslett, Karin Stegelius, Pauline Thompson, Pia Karbiener, Piotr Markuszewski, Krishnakant Budhavant https://doi.org/10.17043/haslett-2026-aerosol-maldives-1
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Although the title and abstract of this manuscript discusses "chemical composition", my cursory reading suggests that the authors did not measure any non-volatile material (soot/dust/tarballs/ash...), although state-of-the-art instrumentation was used to measure volatile PM. Could the title/abstract be clarified during revisions?