No abstract

Water-soluble species constitute a significant fraction (up to 60–70%) of the total aerosol loading in the marine atmospheric boundary layer (MABL). The “indirect” effects, that is, climate forcing due to modification of cloud properties depend on the water-soluble composition of aerosols. Thus, the characterization of aerosols over the MABL is of greater relevance. Here, we present 1-year long aerosol chemical composition data of PM10 and PM2.5 at a costal location in the northeastern Arabian Sea (Goa; 15.45°N, 73.20°E, 56 m above the sea level). Average water-soluble ionic concentration (sum of anion and cation) is highest (25.5 ± 6.9 and 19.6 ± 5.8 μg·m−3 for PM10 and PM2.5, respectively) during winter season and lowest during post-monsoon (17.3 ± 9.1 and 14.4 ± 8.1 μg·m−3 for PM10 and PM2.5, respectively). Among water-soluble ionic spices, SO42- ion was found to be dominant species in anions and NH4+ is dominant in cations, for both PM10 and PM2.5 during all the seasons. These observations clearly hint to the contribution from anthropogenic emission and significant secondary inorganic species formation. Sea-salt (calculated based on Na+ and Cl−) concentration shows significant temporal variability with highest contribution during summer seasons in both fractions. Sea-salt corrected Ca2+, an indicator of mineral dust is found mostly during summer months, particularly in PM10 samples, indicates contribution from mineral dust emissions from arid/semiarid regions located in the north/northwestern India and southwest Asia. These observations are corroborated with back-trajectory analyses, wherein air parcels were found to derive from the desert area in summer and Indo-Gangetic Plains (a hot spot for anthropogenic emissions) during winter. In addition, we also observe the presence of nss-K+ (sea-salt corrected), for PM2.5, particularly during winter months, indicating influence of biomass burning emissions. The impact on aerosol chemistry is further assessed based on chloride depletion. Chloride depletion is observed very significant during post-monsoon months (October and November), wherein more than 80 up to 100% depletion is found, mediated by excess sulfates highlighting the role of secondary species in atmospheric chemistry. Regional scale characterization of atmospheric aerosols is important for their better parameterization in chemical transport model and estimation of radiative forcing.

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 Ca2+ 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 μg m–3) compared to 2017 (40.6 ± 33 μg 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.

Atmospheric dust deposition supplies a significant amount of macro-and micronutrients to surface ocean water, thus playing a vital role in modulating phytoplankton growth. In this context, we measured soluble and total bioactive trace elements (TEs; Fe, Mn, and Cu) along with eolian dust concentration in aerosol samples collected over a wide area of the Arabian Sea during the southwest monsoon (SWM) month of two consecutive years (2017 and 2018). Our objective in this study is to assess the spatial variability of TEs and quantify their fluxes over the Arabian Sea. Relatively higher mineral dust concentrations were observed during the campaign period compared to earlier studies over the Arabian Sea. A large spatial variability in total Fe (91−2830 ng m −3 ) and soluble Fe (0.3−32.7 ng m −3 ) concentrations was observed in contrast to Mn and Cu. However, the operational fractional solubility (defined as the fraction of soluble metal leached using ultrapure deionized water out of the total metal) is relatively low (less than 3.6%) for Fe as compared to Mn (range: 6−87%) and Cu (4−79%). Higher enrichment factors were observed for Mn (range: 1−37) and Cu (range: 6−96), particularly in the northern sector, suggesting the partial contribution of TEs from the anthropogenic emissions that is further corroborated by air-mass back trajectory analyses. An inverse hyperbolic relationship between fractional solubility and total metal concentration is observed for Fe (similar to previous observations); however, no such association is found in case of Mn and Cu. The dry deposition fluxes of Fe, Mn, and Cu to surface water of the Arabian Sea were estimated as 757, 38.8, and 1.4 μg m −2 day −1 , respectively. These fluxes are on the higher side compared to those reported from other oceanic regions of the globe.