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: open (until 01 May 2026)
- CC1: 'Non-volatile PM (Soot / dust / ...)', J. C. Corbin, 16 Mar 2026 reply
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RC1: 'Comment on egusphere-2026-914', Anonymous Referee #1, 10 Apr 2026
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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
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?