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The mass absorption efficiency (MAE) of light absorbing water-soluble organics, representing a significant fraction of brown carbon (BrC), has been studied in fine mode aerosols (PM2.5) from a source region (Patiala: 30.2 °N, 76.3 °E) of biomass burning emissions (BBEs) in the Indo-Gangetic Plain (IGP). The mass absorption coefficient of BrC at 365 nm (babs-365), assessed from absorption spectra of aqueous extracts, exhibits significant linear relationship with water-soluble organic carbon (WSOC) for day (R2 = 0.37) and night time (R2 = 0.77) samples; and slope of regression lines provides a measure of MAE of BrC (daytime: ∼0.75 m2 g−1 and night time: 1.13 m2 g−1). A close similarity in the temporal variability of babs-365 (for BrC) and K+ in all samples suggests their common source from BBEs. The babs-365 of BrC follows a power law (babs-λ ≈ λ−α; where α = angstrom exponent) and averages around 5.2 ± 2.0 M m−1 (where M = 10−6). A significant decrease in the MAE of BrC from the source region (this study) to the downwind oceanic region (over Bay of Bengal, Srinivas and Sarin, 2013 ) could be attributed to relative increase in the contribution of non-absorbing WSOC and/or photo-bleaching of BrC during long-range atmospheric transport. The atmospheric radiative forcing due to BrC over the study site accounts for ∼40% of that from elemental carbon (EC).

The air-sea deposition of nutrients (N, P and Fe) to the oceanic regions located downwind of pollution sources in south and south-east Asia is gaining considerable attention in the present-day scenario of climate change. We report here a case study on the atmospheric outflow of nutrients from the Indo-Gangetic Plain (IGP) to the Bay of Bengal (BoB). Air mass back trajectories suggest conspicuous downwind transport of chemical constituents from the IGP to the BoB during the late NE-monsoon (January-April); thus, representing BoB as one of the unique oceanic regions influenced by anthropogenic sources over a short span of 3 - 4 months. During the course of this study (November’09 – March’10), nutrient (NO3 - , NH4 + , NOrg, PO4 3- and Fews) concentrations in the atmospheric outflow show pronounced temporal variability. The inorganic nitrogen (NH4 + -N: ~ 90 % of NInorg) dominates the total soluble nitrogen (NTot). Although the contribution of organic nitrogen is not significant, the mass ratio of NOrg/NTot in the outflow varied from 0.07 to 0.40. The abundances of PInorg and Fews varied from 0.4 to 4.8 nmol m -3 and 0.2 to 0.6 nmol m -3 , respectively. The high abundance of K + and significant (P-value < 0.05) correlation with PInorg and Fews, suggests their significant contribution from biomass burning emissions (BBEs). The characteristic mass ratios of nutrients (NInorg/NTot: 0.92 ± 0.13, NOrg/NTot: 0.21 ± 0.11, and PInorg/nss-Ca 2+ : 0.35 ± 0.23) in the IGPoutflow show striking similarity with those reported over the BoB. These results have implications to further increase in the atmospheric deposition of nutrients and their impact on biogeochemistry of surface Bay of Bengal.

Temporal variability (November'09–March'10) in the mass concentrations of PM 2.5 , mineral dust, organic carbon and elemental carbon (OC and EC), water-soluble organic carbon (WSOC) and inorganic species (WSIS) has been studied in the atmospheric outflow to the Bay of Bengal from a sampling site [Kharagpur: 22.02°N, 87.11°E] in the Indo-Gangetic Plain (IGP). Based on diagnostic ratios of carbonaceous species [OC/EC ≈ 7.0 ± 2.2, WSOC/OC ≈ 0.52 ± 0.16, and K + /EC ≈ 0.48 ± 0.17], we document dominant impact from biomass burning emissions (wood-fuel and post-harvest agricultural-waste burning) in the IGP-outflow. Relatively high concentration of sulphate (SO 4 2 − ≈ 6.9–25.3 μg m − 3 ; SO 4 2 − /ΣWSIS = 45–77%) and characteristic ratios of nss-SO 4 2 − /EC (3.9 ± 2.1) and nss-SO 4 2 − /OC (0.61 ± 0.46) provide information on absorption/scattering properties of aerosols. Based on quantitative assessment of individual components of PM 2.5 , we document aerosol organic carbon-to-organic mass (OC to OM) conversion factor centring at 1.5 ± 0.2 (range: 1.3–2.7) in the atmospheric outflow from IGP. The aerosol composition over the Bay of Bengal shows striking similarity with the diagnostic ratios documented for the IGP-outflow. Relatively high conversion factor for assessing the mass of organic aerosols over the Bay of Bengal (1.1–3.7) provides evidence for their oxidation during long-range atmospheric transport.

This manuscript presents a comprehensive data set on concentrations of crustal elements (Al, Ca, Fe and Mg) and trace metals (Pb, Cd, Cu, Mn, Cr, Co and Ni) in ambient aerosols studied from the marine atmospheric boundary layer (MABL) of the Bay of Bengal during the period of continental outflow (January–April). Based on cruises conducted during January'09 ( winter ) and March–April'06 ( spring inter-monsoon ), mass concentrations (ng m − 3 ) of Pb (9–57), Cd (0.01–2.8), Cr (1–17) and Mn (4–103) in the outflow from the Indo-Gangetic Plain were higher during the winter compared to those in the spring inter-monsoon (Pb: 3.2–13.5; Cd: 0.1–1.6; Cr: 0.2–7.2 and Mn: 4.6–42.7). In contrast, Cu concentration was relatively high during the spring inter-monsoon (3.2–27.9 ng m − 3 ); whereas concentrations of Co and Ni were not significantly different (p-value > 0.05) during the two seasons. In general, enrichment factors of Pb and Cd are higher (EF Pb : ~ 225; EF Cd : ~ 1165) over the southern Bay (south of 15°N) associated with south-east Asian outflow. A significant (p-value 4 2 − reaffirms their anthropogenic origin. Another notable feature of temporal data relates to significantly higher ratios of crustal elements (Ca/Al and Fe/Al) associated with the long-range transport of mineral dust from the Thar Desert during the spring inter-monsoon. The deposition of mineral dust (0.3–6 g m − 2 yr − 1 ) to the Bay of Bengal is consistent with model results reported in the literature. However, the dominance of anthropogenic trace metals and their air–sea deposition must be considered in any assessment of factors that influence the marine ecosystem in the Bay of Bengal.

This study presents the first data set on atmospheric input of N, P and Fe to the Northern Indian Ocean. Based on the chemical analysis of ambient aerosols, collected from the marine atmospheric boundary layer (MABL) during the continental outflow (January-April), we document that dry-deposition fluxes (μmolm(-2) d(-1)) of N (2-167), P (0.5-4.8) and Fe (0.02-1.2) to the Bay of Bengal are significantly higher compared to those over the Arabian Sea [N: 0.2-18.6; P: 0.3-0.9; Fe: 0.001-0.015]. Using atmospherically derived P and Fe, C-fixation (1.1 Pg yr(-1)) in the Bay of Bengal is dominated by anthropogenic sources. In contrast, C-fixation (0.03 Pg yr(-1)) in the Arabian Sea is limited by P and Fe. This is attributed to the poor fractional solubility of atmospheric mineral dust transported to the Arabian Sea. However, N-fixation by diazotrophs in the two oceanic regions is somewhat similar (0.5 Tg yr(-1)). Our estimate of N-deposition (0.2 Tg yr(-1)) to the Northern Indian Ocean is significantly lower compared to model results (~800-1200 mg-Nm(-2)yr(-1)≈5.7-8.6 Tg yr(-1) by Duce et al. (2008); ~4.1 Tg yr(-1) by Okin et al. (2011); ~0.8 Tg yr(-1) by Kanakidou et al. (2012)). An overestimate of N-deposition by models could arise due to inappropriate parameterization of temporal variability associated with the continental outflow spread over only four months.

The continental outflow from south and south-east Asia, persisting during January to March, dominates the widespread dispersal of pollutants over tropical Bay of Bengal. With a view to assess the impact of anthropogenic sources on surface ocean biogeochemistry, concentrations of water-soluble inorganic and organic nitrogen (N Inorg , N Org ), their spatial variability and dry-deposition fluxes have been studied in two size fractions (PM 2.5 and PM 10 ) collected during Jan-2009 from the MABL of Bay of Bengal. The mass concentration of N Inorg (NH 4 + + NO 3 − , range: 18 to 565 nmol m − 3 ) dominates the total soluble nitrogen (N Tot = N Inorg + N Org, ) in the fine mode (PM 2.5 ), and occurs mainly as NH 4 + (range: 16 to 561 nmol m − 3 ). The mass-ratio of N Inorg in PM 2.5 to PM 10 centers around 0.85, suggesting that contribution of coarse mode NO 3 − is relatively insignificant. The dominant contribution of N Inorg (as NH 4 + ) to N Tot is also evident based on the data from earlier cruises (Feb–Mar-2001, Feb-2003 and Mar–Apr-2006) conducted in the Bay of Bengal. Water-soluble N Org also dominates the fine fraction and accounts for no more than 40% of N Tot , with relatively high concentrations along coastal regions. A significant linear relationship among N Org , NH 4 + , nss-K + and EC (p-value Inorg + N Org ) to the Bay of Bengal ranges from 2 to 167 μmol m − 2 d − 1 . The upper estimate of N-deposition is somewhat comparable with the model based fluxes, and can support up to 13% of the Primary Production in the Bay of Bengal.

The chemical composition (involving water-soluble inorganic constituents, crustal elements and carbonaceous species) of size-segregated aerosols (PM 10 and PM 2.5 ), collected from the marine atmospheric boundary layer (MABL) of the Bay of Bengal (BoB) during 27th Dec' 08–28th Jan' 09, has been studied to ascertain the factors controlling the spatio-temporal variability in the fractional solubility of aerosol iron. Based on the air-mass back-trajectory (AMBT) analyses and chemical proxies, continental outflow from the two major source regions has been identified, viz: (1) outflow from the Indo-Gangetic Plain (IGP) sampled over north-BoB (N-BoB); and (2) south-east Asian (SEA) outflow over south-BoB (S-BoB). A significant linear relationship among fractional Fe solubility [WS-Fe (%)] and nss-SO 4 2− over N-BoB (characterized by higher abundance of aerosol iron (Fe A ) and SO 4 2− ) provides evidence for the acid processing of mineral dust during atmospheric transport from IGP. The enhancement in the solubility of aerosol constituents is also evident from a linear increase in nss-Ca 2+ with nss-SO 4 2− . In contrast, a temporal shift in the winds, representing the outflow from south-east Asia and aerosol composition over south-BoB, exhibit enhanced fractional solubility of aerosol Fe (range: 11.4 to 49.7%) associated with the lower abundance of dust ( − 3 of Fe A ) and nss-SO 4 2− ( − 3 ). These observations suggest the dominance of combustion sources (biomass burning and fossil-fuel) in dictating the aerosol iron solubility over south Bay of Bengal. The impact of the anthropogenic sources is also ascertained based on the covariance of WS-Fe with K + and OC (organic carbon); as well as enrichment factor of heavy metals (Pb and Cd) associated with the outflow from south-east Asia.