Decadal-Scale Observations of the Impact of South Asian Pollution Outflow on the Radiation over the Northern Indian Ocean

This study examines long-term trends in aerosol loading, chemical composition, and radiative effects over the northern Indian Ocean using the Maldives Climate Observatory at Hanimaadhoo (MCOH) as a receptor site for South Asian outflow. Nearly two decades (2004–2025) of in situ measurements, satellite observations, and reanalysis products are combined to assess changes in aerosol optical depth (AOD), surface solar radiation, sulfate aerosol concentrations, and associated climate-relevant feedback. AERONET observations at MCOH show a mean AOD of 0.30 ± 0.09 with a near-zero long-term trend (0.0017 ± 0.01 decade⁻¹), consistent with MODIS satellite estimates. Seasonal AOD exhibits modest increases during winter, pre-monsoon, and post-monsoon periods, and a slight decline during the monsoon. Clear-sky pyranometer measurements indicate a weak but persistent decline in surface-reaching global shortwave radiation (−3.0 ± 2.3 W m⁻²; −1.4%), consistent with regional dimming trends from MERRA-2 reanalysis (−1.6 ± 0.7 W m⁻² decade⁻¹), with the strongest dimming occurring during the pre-monsoon season. Concurrently, column-integrated water vapour increases significantly (+0.19 ± 0.07 cm decade⁻¹), suggesting potential feedback that may enhance atmospheric warming. Filter-based chemical analyses from 2006 to 2025 reveal a persistent long-term increase in sulfate aerosols concentrations (0.25 ± 0.05 µg m⁻³ yr⁻¹).  Sulfate aerosols a major secondary pollutant derived from SO₂ emissions play an important role in climate-cooling agent through the scattering of solar radiation. Despite rapid socio-economic development across South Asia, emission control measures have been effective over the long term in reducing the magnitude of the increasing trend to about half of that observed in the first decade. Together, these results highlight the complexity of aerosol–radiation–water vapour interactions and emphasize the need for both sustained long-term observations and improved modelling to better constrain climate impacts of air pollution in the South Asian region.