Organic aerosols contribute significant uncertainty to climate-forcing feedbacks due to poorly constrained secondary organic aerosol (SOA) formation pathways. This study investigates biogenic SOA (BSOA) in the wintertime marine atmospheric boundary layer over the Bay of Bengal-a critical receptor region at the intersection of distinct outflows from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA). Through measurements of isoprene, alpha-/beta-pinene, and beta-caryophyllene oxidation products in PM2.5, we find beta-caryophyllene SOA to be the most abundant BSOA class. BSOA concentrations correlate with levoglucosan in both outflows, suggesting a potential link to the oxidation of precursors coemitted with biomass-burning products. While aerosol liquid water content (ALWC) is comparable across outflows, BSOA exhibit significant linear relationships with anthropogenic non-sea-salt-SO42- and ALWC specifically within the SEA outflow. These associations, coupled with observed NH4+/SO42- equivalent ratios (0.77 +/- 0.12), suggest that acid-catalyzed aqueous-phase processing may contribute to BSOA formation in the SEA outflow over the open ocean. This covariability highlight the importance of distinct continental outflow signatures to understand BSOA evolution in marine environments, while acknowledging the limitations of indirect acidity proxies and idealized ALWC assumptions.

The temporal variation for a three-year period (2000–2002) in the atmospheric abundances of principal nitrogen species (NH4+ and NO3−) has been studied in rain and aerosols from an urban city (Ahmedabad, 23.0°N, 72.6°E) located in a semi-arid region of western India. Their concentrations in ambient aerosols over the annual seasonal cycle exhibit large variation [NH4+: < 0.001 to 1.3 (GM = 0.25 μg/m3); NO3−: 0.09 to 4.4 (GM = 1.3 μg/m3)]; with systematically higher concentrations during Nov-Feb (drier period) and relatively low during Jun-Aug (wet season). In comparison, abundances of NH4+ and NO3− in individual precipitation events (n = 91) collected during the southwest monsoon (Jun-Aug) for three years varied as NH4+: <1.0 to 220 (VWM: 30 μeq/L) and NO3−: 1.3 to 115 (VWM: 13 μeq/L). Using corresponding rainfall data, the wet-deposition fluxes of NH4+ during 2000, 2001 and 2002 have been derived: 364, 327 and 297 mg/m2/y, respectively; which are considerably higher than the dry-deposition fluxes (16, 11 and 16 mg/m2/y). The wet deposition of NO3− over the three years varied as 392, 569 and 487 mg/m2/y in comparison to dry-deposition fluxes averaging as 201, 134 and 137 mg /m2/y. These results also indicate that both dry and wet deposition of NO3− accounts for as much as 65

Human activities and climate change substantially threaten coastal areas, impacting ecosystem functions, services, and human-wellbeing. Trace elements, from both natural and anthropogenic sources, can contaminate coastal regions, and at high concentrations may become toxic to marine biota. Climate change is likely to affect the sources, sinks and cycling of trace elements in coastal systems: for example, riverine runoff is set to increase as precipitation in the Arctic intensifies, and more frequent extreme floods are expected to activate previously deeply buried trace elements. Furthermore, changes in human activity under a warming climate, such as increased Arctic shipping and potential geoengineering projects such as ocean alkalinity enhancement, will likely introduce more trace elements to coastal ecosystems. Advancing our understanding of trace element cycling is at present limited by factors including lack of data coverage in the Global South, challenges in studying multi-stressor effects and ecosystem responses, lack of long-term data, and the difficulty in parametrizing robust models in coastal environments. Climate change is likely to impact coastal trace element contamination through natural processes, such as river runoff and human activities such as shipping. A focus on increased data coverage in the Global South, long-term and multiple stressors studies and improved ecosystem models are promising avenues to improve our understanding.

The chemical composition of total suspended particulate (TSP) matter and secondary aerosol formation have been studied during wintertime fog and haze events from urban sites (Allahabad and Hisar) in the Indo-Gangetic Plain. The atmospheric abundances of elemental carbon (EC), organic carbon (OC), water-soluble OC (WSOC) suggest that organic matter is a major component of TSP, followed by concentrations of sulphate and nitrate under varying meteorological conditions. The concentrations of EC, OC, and WSOC show a nearly 30

Climate change interacts with the sources and cycling of contaminants, such as radionuclides, in the environment. In this review, we discuss the implications of climate change impacts on existing and potential future sources of radionuclides associated with human activities to the marine environment. The overall effect on operational releases of radionuclides from the nuclear and non-nuclear sectors will likely be increased interference or prevention of normal operations due to weather-related events. For certain radioactive waste dumped at sea and sunken nuclear submarines, the impact of climate change and ocean acidification on the release of radionuclides and their subsequent fate in the marine environment should be considered further. Fluxes from secondary sources of radionuclides in the marine and terrestrial environment and cryosphere will change in response to climate change impacts such as sea level rise, warming and changes in precipitation patterns. In addition, climate change impacts may increase the risk of releases of radionuclides from operational and legacy wastes on land to the marine environment. Overall, our synthesis highlights that there is a need to understand and assess climate change impacts on sources of radionuclides to the marine environment to meet environmental and management challenges under future climate scenarios.

The effects of climate change (CC) on contaminants and their potential consequences to marine ecosystem services and human wellbeing are of paramount importance, as they pose overlapping risks. Here, we discuss how the interaction between CC and contaminants leads to poorly constrained impacts that affects the sensitivity of organisms to contamination leading to impaired ecosystem function, services and risk assessment evaluations. Climate drivers, such as ocean warming, ocean deoxygenation, changes in circulation, ocean acidification, and extreme events interact with trace metals, organic pollutants, excess nutrients, and radionuclides in a complex manner. Overall, the holistic consideration of the pollutants-climate change nexus has significant knowledge gaps, but will be important in understanding the fate, transport, speciation, bioavailability, toxicity, and inventories of contaminants. Greater focus on these uncertainties would facilitate improved predictions of future changes in the global biogeochemical cycling of contaminants and both human health and marine ecosystems.

Anthropogenic emissions to the atmosphere have increased the flux of nutrients, especially nitrogen, to the ocean, but they have also altered the acidity of aerosol, cloud water, and precipitation over much of the marine atmosphere. For nitrogen, acidity-driven changes in chemical speciation result in altered partitioning between the gas and particulate phases that subsequently affect long-range transport. Other important nutrients, notably iron and phosphorus, are affected, because their soluble fractions increase upon exposure to acidic environments during atmospheric transport. These changes affect the magnitude, distribution, and deposition mode of individual nutrients supplied to the ocean, the extent to which nutrient deposition interacts with the sea surface microlayer during its passage into bulk seawater, and the relative abundances of soluble nutrients in atmospheric deposition. Atmospheric acidity change therefore affects ecosystem composition, in addition to overall marine productivity, and these effects will continue to evolve with changing anthropogenic emissions in the future.

Atmospheric transport and the subsequent air-to-sea deposition of water-soluble iron (Fews), an essential micronutrient for the phytoplankton growth, have a profound influence on the biogeochemical cycles of carbon and nitrogen. Sources of Fews include contributions from poorly soluble natural mineral dust and highly soluble anthropogenic aerosols from biomass burning emissions and fossil-fuel combustion in the continental outflows. Apart from the source/emission contributions, atmospheric processing of aerosol iron (FeTot) by inorganic acidic species (e.g., non-sea-salt or nss-SO42- and NO3-) and/or organic acids also affect the supply of Fews to the surface waters that are downwind of pollution sources. Among these, the least understood process is the oxalic acid-mediated photochemical cycling of Fews. Laboratory studies have clearly demonstrated an enhancement in the fractional solubility of aerosol iron (i.e., Fews (%) = Fews/FeTot ×100) via the oxalic acid complexation with FeTot and subsequent photochemical reduction process. However, lacking support from the field measurements limits our ability to incorporate the proposed mechanism in the current biogeochemistry models. This study is designed with the overarching goal of investigating the role of oxalic acid on the Fews (%) over a coastal ocean (i.e., the Bay of Bengal: BoB) influenced by the atmospheric outflow from the Indo-Gangetic Plain (IGP) and South-east Asia (SEA) during the winter season. We analysed 31 PM2.5 samples for the mass concentrations of FeTot, Fews and other chemical composition including nss-SO42-, NO3-, oxalic acid and related polar compounds as well as stable carbon isotopic composition of oxalic acid (δ13Coxalic). Strong positive linear relationship of oxalic acid with FeTot and significant inverse linear relationship between δ13Coxalic and Fews over the BoB clearly emphasize the role of oxalic acid on the Fews (%). These findings comply with the notion that oxalic acid formed from the precursor water-soluble organic acids in the deliquescent aerosols, is complexed with aerosol-Fe and undergoes through successive photochemical reactions, contributing to an overall increase in the Fews (%).

Anthropogenic emissions of nitrogen and sulphur oxides and ammonia have altered the pH of aerosol, cloud water and precipitation, with significant decreases over much of the marine atmosphere. Some of these emissions have led to an increased atmospheric burden of reactive nitrogen and its deposition to ocean ecosystems. Changes in acidity in the atmosphere also have indirect effects on the supply of labile nutrients to the ocean. For nitrogen, these changes are caused by shifts in the chemical speciation of both oxidized (NO3- and HNO3) and reduced (NH3 and NH4+) forms that result in altered partitioning between the gas and particulate phases that affect transport. Other important nutrients, notably iron and phosphorus, are impacted because their soluble fractions increase due to exposure to low pH environments during atmospheric transport. These changes affect not only the magnitude and distribution of individual nutrient supply to the ocean but also the ratios of nitrogen, phosphorus, iron and other trace metals in atmospheric deposition. Since marine microbial populations are sensitive to nutrient supply ratio, the consequences of atmospheric acidity change include shifts in ecosystem composition in addition to overall changes in marine productivity. Nitrogen and sulphur oxide emissions are decreasing in many regions, but ammonia emissions are much harder to control. The acidity of the atmosphere is therefore expected to decrease in the future, with further implications for nutrient supply to the ocean. This presentation will explore the impact of increased atmospheric acidity since the Industrial Revolution, and the projected acidity decreases, on atmospheric nutrient supply and its consequences for the biogeochemistry of the ocean.

The atmospheric abundance of oxalic acid significantly influences the supply of soluble iron (Fe-ws), an essential micronutrient for the phytoplankton growth, to the ocean surface. We used the linear relationship between stable carbon isotopic composition of oxalic acid (delta C-13(oxalic)) and Fe-ws in marine aerosols collected from a coastal ocean, the Bay of Bengal (BoB), to probe the likely photochemical cycling of total aerosol iron (Fe-Tot). In winter, the BoB is influenced by continental pollution from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA). Despite higher levels of oxalic acid, anthropogenic SO42-, and Fe-T(ot), in IGP outflow, we observe higher fractional solubility of aerosol iron [Fe-ws (%) = Fe-ws/Fe-T(ot) x 100] in the SEA outflow (11.4-49.7%). Coincidently, we observe an inverse linear relationship between delta C-13(oxalic) (from -18.4 to -8.8 parts per thousand) and Fe-ws (3.5-38.0 ng m(-3)) in the SEA outflow (slope = -0.18; R-2 = 0.42; p < 0.05) but not in the IGP outflow. This relationship indicates the catalytic behavior of Fe-Tot, in the oxidation of precursor water-soluble organics to oxalic acid, yielding Fe-ws Unlike the IGP outflow, SEA outflow aerosols (i.e., having excess free inorganic acidity and low aerosol pH) are more acidic and experience higher solar influx over the remote BoB. These findings emphasize the concept that oxalic acid formation from the precursor organics in deliquescent aerosols is catalyzed by Fe-Tot, contributing to an overall increase of Fe-ws (%).

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.

In this paper, we synthesize the size distribution and optical properties of the atmospheric water-soluble fraction of light-absorbing organic carbon (brown carbon; BrC) in the continental outflow from the Indo-Gangetic Plain (IGP) in South Asia to the North Indian Ocean. A comparison of the mass absorption coefficient of water-soluble BrC (babs-WSBrC-365nm) in PM2.5 with that in PM10 sampled over the Bay of Bengal reveals the dominance of BrC in fine mode. Furthermore, the babs-BrC-365nm shows a significant linear relationship with mass concentrations of airborne particulate matter, water-soluble organic carbon and non-sea-salt-K+ in the continental outflow from the IGP. This observation emphasizes the ubiquitous nature and significant contribution of water-soluble BrC from biomass burning emissions (BBEs). Comparing the absorption properties from this study with global datasets, it is discernible that BBEs dominate BrC absorption. Furthermore, the imaginary refractive index of water-soluble BrC (kWSBrC-365nm) in marine aerosols sampled over the North Indian Ocean during November is significantly higher than during December to January. Thus, significant temporal variability is associated with crop-residue burning emissions in the IGP on the composition of BrC over the North Indian Ocean. Our estimates show that the babs-WSBrC-365nm and kWSBrC-365nm from post-harvest crop-residue burning emissions in the IGP are much higher than the BBEs from the southeastern United States and Amazonian forest fires. Another major finding of this study is the lack of significant relationship between kWSBrC-365nm and the mass ratio of elemental carbon to particulate organic matter, as previously suggested by chamber experiments to model varying BrC absorption properties in ambient aerosols. Therefore, considerable spatio-temporal variability prevails among emission sources (wood burning vs. crop-residue burning), which needs to be considered when assessing the regional radiative forcing of BrC relative to major absorbing elemental carbon.

Biomass burning (BB), a pivotal source of both primary and secondary organic aerosols (POA and SOA, respectively), affects the regional and global climate. We have used stable carbon isotopic composition (delta C-13(TC)) of total carbon (TC) and BB tracers (anhydrosugars, sugars, and sugar alcohols) to elucidate the relative significance of POA and SOA over the Bay of Bengal, influenced by the long-range transport from the Indo-Gangetic Plain (IGP-outflow) and Southeast Asia (SEA-outflow) during a winter cruise. The molecular distributions of anhydrosugars (levoglucosan, Lev; galactosan, Gal; mannosan, Man) are different between IGP- (Lev > Gal > Man) and SEA-outflows (Lev > Man > Gal). The positive linear/nonlinear relationships of delta C-13(TC) with total sugar-C, K+, water-soluble organic carbon (WSOC), and TC in BBOA from the SEA-outflow are in sharp contrast to those from Mt. Tai, China and Rondonia, Brazil in summer; mainly because of the prevailing differences in ambient photochemical processing. The Keeling plots (delta C-13(TC) versus 1/TC, 1/WSOC, and 1/Lev) in the SEA-outflow revealed a mixing of "C-13-enriched POA" and C-13-depleted "fresh-SOA" of BB origin. Because the sugar compounds are mostly water-soluble and become bioavailable in the surface waters, we estimate the air-to-sea depositions of sugar-C and WSOC over the Bay of Bengal to contribute to no more than 0.1% and 13%, respectively, of their supply via peninsular rivers.

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.

This work reports on the current status of the global modeling of iron (Fe) deposition fluxes and atmospheric concentrations and the analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry transport (CTMs) and general circulation (GCMs) models participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, The Atmospheric Input of Chemicals to the Ocean. The global total Fe (TFe) emission strength in the models is equal to ∼ 72 Tg Fe yr−1 (38–134 Tg Fe yr−1) from mineral dust sources and around 2.1 Tg Fe yr−1 (1.8–2.7 Tg Fe yr−1) from combustion processes (the sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr−1, accounting for both mineral dust and combustion aerosols. The mean global deposition fluxes into the global ocean are estimated to be in the range of 10–30 and 0.2–0.4 Tg Fe yr−1 for TFe and LFe, respectively, which roughly corresponds to a respective 15 and 0.3 Tg Fe yr−1 for the multi-model ensemble model mean.The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parameterizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicates that most models overestimate surface level TFe mass concentrations near dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe concentrations near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere ( ∼ 14), larger than that of the Southern Hemisphere ( ∼ 2.4) for the ensemble model mean. This model intercomparison and model–observation comparison study reveals two critical issues in LFe simulations that require further exploration: (1) the Fe-containing aerosol size distribution and (2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions.

(1) JAMSTEC, Yokohama, Japan (akinorii@jamstec.go.jp), (2) Utrecht Univ., Utrecht, The Netherlands, (3) Univ. of Crete, Heraklion, Greece, (4) Cornell Univ., Ithaca, USA, (5) Univ. of East Anglia, Norwich, UK, (6) Physical Research Laboratory, Ahmedabad, India, (7) Stockholm Univ., Stockholm, Sweden, (8) Rutgers Univ., Newark, USA, (9) Florida State Univ., Tallahassee, USA, (10) Univ. of Georgia, Savannah, USA, (11) Univ. of Tasmania, Tasmania, Australia, (12) North Carolina State Univ., Raleigh, USA, (13) NASA Ames Research Center, Mountain View, USA, (14) Argonne National Laboratory, Argonne, USA„ (15) Texas AM Univ., College Station, USA

Abstract. This work reports on the current status of global modelling of iron (Fe) deposition fluxes and atmospheric concentrations and analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry-transport (CTMs) and general circulation (GCMs) models have participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, The Atmospheric Input of Chemicals to the Ocean. The global total Fe (TFe) emissions strength in the models is equal to ~ 72 Tg-Fe yr−1 (38–134 Tg-Fe yr−1) from mineral dust sources and around 2.1 Tg-Fe yr−1 (1.8–2.7 Tg-Fe yr−1) from combustion processes (sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg-Fe yr−1, accounting for mineral dust and combustion aerosols together. The multi model ensemble global TFe and LFe deposition fluxes into the global ocean are calculated to be ~ 15 Tg-Fe yr−1 and ~ 0.3 Tg-Fe yr−1, respectively. The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parametrizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicate that most models overestimate surface level TFe mass concentrations near the dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe loading near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere (~ 14), larger than the Southern Hemisphere (~ 2.4) for the ensemble model mean. This model intercomparison and model–observation comparison study reveals two critical issues in LFe simulations that require further exploration: 1) the Fe-containing aerosol size distribution and 2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions.