Atmospheric deposition of mineral dust and associated nutrients over the Equatorial Indian Ocean.

Aerosols are potential supplier of nutrients to the surface water of oceans and can impact biogeochemical processes particularly in the remote locations. The nutrient data from atmospheric supply is poorly reported from the Indian Ocean region. In this study, we present atmospheric nutrients such as reactive nitrogen species (Nitrate, Ammonium, Organic nitrogen), micro-nutrients (e.g. Fe, Mn and Cu) concentration along with mineral dust in the aerosol samples collected over meridional transect during summer (April-May 2018) and monsoon (June-July 2019) months. A significant spatial variation of dust was observed during summer (0.6-22.8 μg m ^-3 ) and monsoon (2.8-25.1 μg m ^-3 ) months with a decreasing trend from north to south. Dust as well as other nutrient species shows a general north to south decreasing trend, however, no such trend was seen in the soluble trace elements (TEs) concentration. Anthropogenic species like NH ₄ ⁺ and nss-K ⁺ were found below detection limit during monsoon campaign. The fractional solubility (in percentage) of Fe, Mn and Cu were estimated by measuring their concentration in ultrapure water leach which averaged around 0.99 ± 1.12, 31.0 ± 14.9 and 31.1 ± 25.4, respectively during summer and 0.09 ± 0.08, 6.0 ± 8.9, 16.7 ± 9.6, respectively, during monsoon period. Correlation of soluble Fe with total Fe and total acidic species suggest varying dust sources is an important controlling factor for the fraction solubility of Fe with negligible contribution from the chemical processing. However, a significant correlation was observed between total acid and fractional solubility of Mn and Cu suggest role of chemical processing in enhancement of their solubility. Dry deposition flux of aeolian dust was estimated for both campaign using Al concentration and relatively higher fluxes were observed for summer (12.6 ± 8.4 mg·m ^-2 ·d ^-1 ) and monsoon (8.7 ± 8.4 mg·m ^-2 ·d ^-1 ) months as compared to model based estimates reported in the literature. Contrastingly, estimated deposition flux of soluble Fe from both campaign displays relatively lower values as compared to model based results which underscores a need for re-evaluation of biogeochemical models with real-time data.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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