R. Rengarajan

Geochemical cycling in the Hooghly estuary, India

U-Th decay series isotopes, δ 18 O and Si measurements in the river estuarine waters and sediments of the polluted Hooghly estuary as well as the surface waters of the Bay of Bengal, its high salinity end member, are reported. Dissolved Si indicates that there are probably two mixing regimes, dissolved U behaviour is nonconservative and δ 18 O behaves conservatively in the overall estuarine region. Isotopes of reactive elements, viz. 234 Th and 210 Po, are removed from the estuarine waters in < 2 days and < 1 month, respectively, which is due to high suspended matter (30-301 mg 1 -1 ), 228 Ra and 226 Ra are profusely released into the estuarine waters in the low to mid-salinity regions. As expected, the opposite trend is observed in the case of estuarine sediments and suspended matter. Reactive isotopes of Th, 210 Pb and 210 Po are enriched, whereas Ra isotopes are depleted with respect to their parent nuclides in the estuarine sediments and suspended matter. 232 Th/Al ratio appears well suited to study the distribution and mixing of the bed load sediments of the Ganga-Brahmaputra (G-B) and the Hooghly rivers with those from other rivers on the Bay of Bengal floor.

Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of particle scavenging processes

Vertical and temporal variations in the activities of234Th,210Po and210Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties.A common feature of the234Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of234Th is deficient in the surface 200 m with respect to its parent,238U. This gross deficiency is attributed to the preferential removal of234Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved234Th are comparable in these two basins. The temporal variations in the234Th-238U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m), the scavenging residence time of234Th ranges from 30 to 100 days.The surface and deep waters of both the seas show an enhanced deficiency of dissolved210Po relative to210Pb and that of210Pb relative to226Ra. The deficiencies of both210Po and210Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of210Po and210Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved210Po/210Pb and210Pb/226Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.

Dual‐carbon‐isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India

Large-scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source-diagnostic stable an ...

Secondary organic aerosol over an urban environment in a semi-arid region of western India

Carbonaceous species in PM10 and PM2.5 samples, collected from an urban location at Ahmedabad in India during summer, were analyzed to study variability in water-soluble organic carbon (WSOC) and secondary organic carbon (SOC) along with atmospheric water vapor content. A significant correlation between WSOC and SOC was found indicating major contribution of soluble organic compounds by secondary organic aerosol formation. A strong inverse dependence of WSOC and SOC on atmospheric water vapor content is observed in both PM10 and PM2.5 (at <45% relative humidity, RH) during daytime; whereas data collected during monsoon season at higher RH conditions do not exhibit such relation. Aerosol liquid water content (LWC) calculated from thermodynamic equilibrium model suggests that the decrease in secondary organic aerosol (SOA) with increase in RH occurs when LWC is absent or insignificant amount. The inverse correlation in summertime indicates possible decrease in the extent of heterogeneous photochemical oxidation of precursor volatile organic compounds on mineral aerosol surface with increase in ambient water vapor. These results have implications for SOA estimations on regional scales especially in arid and semi-arid regions where significant amount of fine mineral dust is present.

Atmospheric transport of mineral dust from the Indo‐Gangetic Plain: Temporal variability, acid processing, and iron solubility

Atmospheric transport of chemical constituents from the Indo-Gangetic Plain (IGP) to the Bay of Bengal is a conspicuous seasonal feature that occurs during the late NE-monsoon (December–March). With this perspective, aerosol composition and abundance of mineral dust have been studied during November 2009 to March 2010 from a sampling site (Kharagpur: 22.3°N, 87.3°E) in the IGP, representing the atmospheric outflow to the Bay of Bengal. The chemical composition of PM2.5 suggests the dominance of nss- SO42− (6.9–24.3 µg m−3); whereas the abundance of mineral dust varied from 3 to 18 µg m−3. The concentration of aerosol iron (FeTot) and its fractional solubility (Fews % = Fews/FeTot *100, where Fews is the water-soluble fraction of FeTot) varied from 60 to 1144 ng m−3 and from 6.7 to 26.5%, respectively. A striking similarity in the temporal variability of total inorganic acidity (TIA =  NO3− + nss- SO42−) and Fews (%) provides evidence for acid processing of mineral dust (alluvium) during atmospheric transport from the IGP. The contribution of TIA to water-soluble inorganic species [(nss- SO42− +  NO3−)/ΣWSIS], mass ratios of Ca/Al and Fe/Al, and abundance of dust (%) and Fews (%) in the IGP-outflow are similar to the aerosol composition over the Bay of Bengal. With the rapid increase in anthropogenic activities over south and south-east Asia, the enhanced fractional solubility of aerosol iron (attributed to acid processing of mineral dust) has implications to further increase in the air-sea deposition of Fe to the Ocean surface.

Spatial variability of upper ocean POC export in the Bay of Bengal and the Indian Ocean determined using particle‐reactive 234Th

The northern Indian Ocean is globally significant for its seasonally reversing winds, upwelled nutrients, high biological production and expanding oxygen minimum zones. The region acts as sink and source for atmospheric CO2. However, the efficiency of the biological carbon pump to sequester atmospheric CO2 and export particulate organic carbon from the surface is not well known. To quantify the upper ocean carbon export flux and to estimate the efficiency of biological carbon pump in the Bay of Bengal and the Indian Ocean, seawater profiles of total 234Th were measured from surface to 300 m depth at 13 stations from 19.9°N to 25.3°S in a transect along 87°E, during spring intermonsoon period (March – April 2014). Results showed enhanced in situ primary production in the equatorial Indian Ocean and the central Bay of Bengal, and varied from 13.2 mmol C m−2 d−1 to 173.8 mmol C m−2 d−1. POC export flux in this region varied from 0 to 7.7 mmol C m−2 d−1. Though high carbon export flux was found in the equatorial region, remineralization of organic carbon in the surface and sub-surface waters considerably reduced organic carbon export in the Bay of Bengal. Annually recurring anticyclonic eddies enhanced organic carbon utilization and heterotrophy. Oxygen minimum zone developed due to stratification and poor ventilation was intensified by sub-surface remineralization. 234Th-based carbon export fluxes were not comparable with empirical statistical model estimates based on primary production and temperature. Region-specific refinement of model parameters is required to accurately predict POC export fluxes.

234Th based carbon export flux along the Indian GEOTRACES GI02 section in the Arabian Sea and the Indian Ocean

234Th (t1/2 = 24.1 d), present in seawater, is a naturally occurring particle-reactive radionuclide formed through the radioactive decay of its parent, 238U (t1/2 = 4.47 x 109 y). The 234Th:238U disequilibrium is exploited to quantify fluxes of elements moving out of the euphotic zone by attaching on to sinking particles. Under the Indian GEOTRACES programme, high-resolution sampling in the upper 300 m depth was carried out at 11 stations in the Arabian Sea and the Indian Ocean during April – May 2014 from 17°N to 16°S along 65°E transect to estimate the 234Th based particulate organic carbon (POC) export flux from the upper ocean. Average 234Th fluxes integrated to 100 m depth were 2612 and 1968 dpm m-2 d-1 for the Arabian Sea and the Indian Ocean, respectively. The estimated POC export fluxes ranged from negligible to 9.0 mmol m-2 d-1 and the 234Th based POC export efficiencies were <2 to 5%. For the same season, the Arabian Sea and the Bay of Bengal showed highly contrasting carbon export trends (mean: 4.0 and 0.8 mmol C m-2 d-1, respectively). The modeled POC export fluxes from in situ and satellite derived primary production are higher than the 234Th based values for the Laws and Dunne models and are comparable for the Henson model. The modeled POC fluxes which depend on surface temperature and primary production could be further refined for the seasonal cycle in biological productivity and associated differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity and associated DOC export.