Anthropogenic nitrogen inputs and impacts on oceanic N2O fluxes in the northern Indian Ocean: The need for an integrated observation and modelling approach

Anthropogenically-derived nitrogen input to the northern Indian Ocean has increased significantly in recent decades, based on both observational and model-derived estimates This external nutrient source is supplied by atmospheric deposition and riverine fluxes, and has the potential to affect the vulnerable biogeochemical systems of the Arabian Sea and Bay of Bengal, influencing productivity and oceanic production of the greenhouse-gas nitrous-oxide (N2O). We summarize current estimates of this external nitrogen source to the northern Indian Ocean from observations and models, highlight implications for regional marine N2O emissions using model-based analyses, and make recommendations for measurement and model needs to improve current estimates and future predictions of this impact. Current observationally-derived estimates of deposition and riverine nitrogen inputs are limited by sparse measurements and uncertainties on accurate characterization of nitrogen species composition. Ocean model assessments of the impact of external nitrogen sources on regional marine N2O production in the northern Indian Ocean estimate potentially significant changes but also have large associated uncertainties. We recommend an integrated program of basin-wide measurements combined with high-resolution modeling and more detailed characterization of nitrogen-cycle process to address these uncertainties and improve current estimates and predictions

Pyrogenic iron: The missing link to high iron solubility in aerosols

Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity

Evaluation of labile iron processing in atmospheric models

Conference abstract (August 13-18, 2017. Goldschmidt 2017 in Paris, France

The GESAMP global model intercomparison: Evaluation of labile iron in aerosols

Oral session abstract EGU2018-2243, European Geosciences Union (EGU) General Assembly 2018 (8-13 April, 2018, Vienna, Austria

Evaluation of labile iron formation in the atmosphere

Atmospheric deposition of labile iron (Fe) to the ocean has been suggested to modulate primary ocean productivity and thus indirectly affect the climate. A key process contributing to atmospheric sources of labile Fe is associated with atmospheric acidity, which leads to Fe transformation from insoluble to soluble forms. Significant progress has been made in our understanding of atmospheric inputs of labile Fe from natural and anthropogenic sources to the oceans. However, there are still large uncertainties regarding the relative importance of different sources of Fe and effects of atmospheric processing on the bioavailability of the delivered Fe. Here, we investigate the effects of atmospheric processing on Fe solubility and contribution of different sources of Fe to labile Fe in the atmosphere. We compiled Fe loading and solubility in aerosols from four atmospheric chemistry transport models and a number of field measurements. Fe-containing aerosols from combustion sources are characterized by low loading and high solubility, compared to mineral dust. Therefore, labile Fe loading may be separately attributed to combustion and dust aerosols, assuming their distinct emission sources and atmospheric processes. The results suggest that combustion aerosols substantially contribute to labile Fe loading measured in the atmosphere. Thus, assessments of dust-borne Fe fertilization of the oceans should include Fe-containing mineral aerosols affected by combustion sources.Poster presented at Ocean Sciences Meeting 2018, AGU, ASLO, the Oceanography Society. Portland, Oregon, Feb. 11-16, 201

Brown carbon in atmospheric outflow from the Indo-Gangetic Plain: Mass absorption efficiency and temporal variability

The simultaneous measurements of brown carbon (BrC) and elemental carbon (EC) are made in ambient aerosols (PM2.5), collected from a site in north-east India during November'09-March'10, representing the atmospheric outflow from the Indo-Gangetic Plain (IGP) to the Bay of Bengal (BoB). The absorption coefficient of BrC (b(abs)), assessed from water-soluble organic carbon (WSOC) at 365 nm, varies from 2 to 21 M m(-1) and exhibits significant linear relationship (P < 0.05) with WSOC concentration (3-29 mu g m(-3)). The angstrom exponent (alpha: 8.3 +/- 2.6, where b(abs) approximate to lambda(-alpha)) is consistent with that reported for humic-like substances (HULIS) from biomass burning emissions (BBE). The impact of BBE is also discernible from mass ratios of nss-K+/EC (0.2-1.4) and OC/EC (3.4-11.5). The mass fraction of WSOC (10-23%) in PM2.5 and mass absorption efficiency of BrC (sigma(abs-BrC): 0.5-1.2 m(2) g(-1)) bring to focus the significance of brown carbon in atmospheric radiative forcing due to anthropogenic aerosols over the Indo-Gangetic Plain

Light absorbing organic aerosols (brown carbon) over the tropical Indian Ocean: impact of biomass burning emissions

The first field measurements of light absorbing water-soluble organic carbon (WSOC), referred as brown carbon (BrC), have been made in the marine atmospheric boundary layer (MABL) during the continental outflow to the Bay of Bengal (BoB) and the Arabian Sea (ARS). The absorption signal measured at 365 nm in aqueous extracts of aerosols shows a systematic linear increase with WSOC concentration, suggesting a significant contribution from BrC to the absorption properties of organic aerosols. The mass absorption coefficient (b(abs)) of BrC shows an inverse hyperbolic relation with wavelength (from 300 to 700 nm), providing an estimate of the Angstrom exponent (alpha P, range: 3-19; Av: 9 +/- 3). The mass absorption efficiency of brown carbon (sigma(abs) BrC) in the MABL varies from 0.17 to 0.72 m(2) g(-1) (Av: 0.45 +/- 0.14 m(2) g(-1)). The alpha P and sigma(abs) BrC over the BoB are quite similar to that studied from a sampling site in the Indo-Gangetic Plain (IGP), suggesting the dominant impact of organic aerosols associated with the continental outflow. A comparison of the mass absorption efficiency of BrC and elemental carbon (EC) brings to focus the significant role of light absorbing organic aerosols (from biomass burning emissions) in atmospheric radiative forcing over oceanic regions located downwind of the pollution sources

Latitudinal distributions of atmospheric dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls over the western North Pacific : Sources and formation pathways

The present study aims to assess the molecular distributions of water-soluble dicarboxylic acids (diacids: C-2-C-12), oxocarboxylic acids (C-2-C-9), and -dicarbonyls (glyoxal and methylglyoxal) in aerosols collected over the western North Pacific (WNP) during a summer cruise (August to September 2008). The measured water-soluble organics show pronounced latitudinal distributions with higher concentrations in the region of 30 degrees N-45 degrees N (average 63ngm(-3)) than 10 degrees N-30 degrees N (18ngm(-3)). Mass fraction of oxalic acid (C-2) in total aliphatic diacids (sigma C-2-C-12) showed higher values (7210%) in lower latitude (10 degrees N-30 degrees N) than that (5616%) in higher latitude (30 degrees N-45 degrees N), suggesting a photochemical production of C-2 due to an increased insolation over the tropical WNP. A similar trend was found in other diagnostic ratios such as oxalic to succinic (C-2/C-4) and oxalic to glyoxylic acid (C-2/C-2), which further corroborate an enhanced photochemical aging over the WNP. In addition, relative abundances of oxalic acid in total diacids showed a marked increase as a function of ambient temperature, supporting their photochemical production. Constantly low concentration ratios of adipic and phthalic acids relative to azelaic acid suggest a small contribution of anthropogenic sources and an importance of oceanic sources during the study period. Significant production of C-2 through oxidation of biogenic volatile organic compounds emitted from the sea surface is also noteworthy, as inferred from the strong linear correlations among water-soluble organic carbon, methanesulphonic acid, and oxalic acid. Sea-to-air emission of unsaturated fatty acids also contributes to formation of diacids over the WNP

Stable carbon and nitrogen isotopic composition of fine mode aerosols (PM2.5) over the Bay of Bengal: impact of continental sources

This study reports on stable carbon (delta C-13(TC)) and nitrogen (delta N-15(TN)) isotopic composition of total carbon and nitrogen (TC and TN) in the fine mode aerosols (PM2.5; N = 31) collected over the Bay of Bengal (BoB). The samples represent two distinct wind regimes during the cruise (27 December 2008-28 January 2009); one from the Indo-Gangetic Plain (referred as IGP-outflow) and another from Southeast Asia (SEA-outflow). The PM2.5 samples from the IGP-outflow show higher delta C-13(TC) (-25.0 to -22.8 parts per thousand; -23.8 +/- 0.6 parts per thousand) than those from the SEA-outflow (-27.4 to -24.7 parts per thousand; -25.3 +/- 0.9 parts per thousand). Similarly, delta N-15(TN) varied from +11.8 to +30.6 parts per thousand (+20.4 +/- 5.4 parts per thousand) and +10.4 to +31.7 parts per thousand (+19.4 +/- 6.1 parts per thousand) for IGP-and SEA-outflows, respectively. Based on the literature data, MODIS-derived fire hotspots and back trajectories, we infer that higher delta C-13(TC) in the IGP-outflow is predominantly associated with fossil fuel and biofuel combustion. In contrast, contribution of primary organic aerosols from the combustion of C-3 plants or secondary organic aerosol (SOA) formation from biomass/biofuel-burning emissions (BBEs) can explain the lower delta C-13(TC) values in the SEA-outflow. This inference is based on the significant linear correlations among delta C-13(TC), water-soluble organic carbon and non-sea-salt potassium (nss-K+, a proxy for BBEs) in the SEA-outflow. A significant linear relationship of delta N-15 with NH4+and equivalent mass ratio of NH4+/SO42-is evident in both the continental outflows. Since NH4+abundance dominates the TN over the BoB (>90 %), atmospheric processes affecting its concentration in fine mode aerosols can explain the observed large variability of delta N-15(TN)

High abundances of oxalic, azelaic, and glyoxylic acids and methylglyoxal in the open ocean with high biological activity: Implication for secondary OA formation from isoprene

Atmospheric dicarboxylic acids (DCA) are a ubiquitous water-soluble component of secondary organic aerosols (SOA), which can act as cloud condensation nuclei (CCN), affecting the Earth's climate. Despite the high abundances of oxalic acid and related compounds in the marine aerosols, there is no consensus on what controls their distributions over the open ocean. Marine biological productivity could play a role in the production of DCA, but there is no substantial evidence to support this hypothesis. Here we present latitudinal distributions of DCA, oxoacids and -dicarbonyls in the marine aerosols from the remote Pacific. Their concentrations were found several times higher in more biologically influenced aerosols (MBA) than less biologically influenced aerosols. We propose isoprene and unsaturated fatty acids as sources of DCA as inferred from significantly higher abundances of isoprene-SOA tracers and azelaic acid in MBA. These results have implications toward the reassessment of climate forcing feedbacks of marine-derived SOA