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.