Aeolian Dust and Sea Salt in Marine Aerosols over the Arabian Sea during the Southwest Monsoon: Sources and Spatial Variability

Abstract

The nutrient supply to the oceanic surface water is largely regulated by the extrinsic and intrinsic ambient atmospheric aerosol properties, particularly chemistry. In this context, we measured the comprehensive chemical composition (inorganic ions and major elements) of atmospheric aerosols and studied their spatial distribution over the Arabian Sea during the southwest monsoon (SWM) period for 2 consecutive years (2017 and 2018). We found that the water-soluble ionic concentration (WSIC) is dominated by Na+ and Cl-ions followed by Ca 2+ and SO4 2-with a minor contribution of Mg2+ and NO3-. The relative contribution of ionic species to WSIC remained similar for both the monsoon seasons with higher WSIC during 2018 (69.3 ± 63 µ g m-3) compared to 2017 (40.6 ± 33 µ g m-3). Sea salt (estimated from Na+and Cl-ions) and mineral dust (calculated from the Al concentration) are dominant constituents of aerosols, with negligible contribution from anthropogenic species found to be temporally and spatially consistent during both the campaigns. The mineral dust is characterized by a uniform Nd isotopic signature during both campaigns, indicating its source to the Arabian Peninsula during the study period. Source apportionment of mineral dust using the Nd isotope is supported by back-trajectory analyses as well as satellite retrieved (MODIS and CALIPSO) data. The Cl-deficit, which is mainly attributed to the interaction of sea salt with acidic species, is found to be relatively low (10-12%) compared to that during continental outflow over the Arabian Sea, indicating the insignificant contribution from anthropogenic emissions during the SWM months.