Chemical composition of aerosols over the Arabian Sea based on global reanalyses data and on-board ship measurements

Abstract

The knowledge of chemical composition of atmospheric aerosols is key to understand aerosol-cloud-climate interactions and surface water biogeochemistry in the oceanic regions. Despite of strong natural and anthropogenic sources in its upwind regions, the studies on aerosol composition have been very limited over the Arabian Sea. We have comprehensively analyzed the results from global models, ECMWF's CAMS and NASA's MERRA-2 reanalyses, in conjunction with our ship-based measurements. Both models captured the overall spatio-temporal variability in sulphate (SO₄²⁻) and sea salt (r = 0.76–0.86). However, there is large scatter in PM₁₀ and dust variability and the concentrations are typically overestimated by MERRA-2 except during winter, but underestimated by CAMS. Despite of difference in magnitudes, these models successfully reproduced the key seasonal features e.g., winter-time maxima in sulphate (9.0 ± 6.5 μg m⁻³ in MERRA-2, 11.9 ± 6.2 μg m⁻³ in measurements). While sulphate enhancement is most pronounced along India's west coast, the monsoon-time sea salt spike is strongest near east coast of the Middle-East (>200 μg m⁻³) region. Trend analysis results from both models indicate a statistically significant increase in sulphate aerosols over the Arabian Sea during 2003–2022 period (0.4 μgm⁻³y⁻¹ in winter). However, long-term trends in sea salt and dust are not consistent between the two models and underscore a need for further investigations. Insights into aerosol distribution and model performances from this study would aid in planning future expeditions and refining chemistry-climate models, thereby enhancing our understanding of biogeochemical processes in the Indian Ocean.