The UN Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) - An ocean science-policy interface standing the test of time.

Addressing the wide range of marine pollution problems facing the global ocean requires a continual transfer of credible, relevant and timely scientific information to policy and decision makers in coastal and ocean management. The United Nations GESAMP (Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection) is a long-standing scientific advisory group providing such information on a wide range of marine topics and emerging issues of concern to ten UN Sponsoring Organizations. This paper presents an overview of GESAMPs operation and examples of its current work. The group's scientific output is often cited by national governments, inter-governmental groups, and a range of non-governmental groups. Given the growing concerns about ocean health and the impacts of many stressors in an era of climate change, the development of timely and effective ocean policy and decision making would benefit from wider recognition and application of GESAMPs work.

Changing atmospheric acidity as a modulator of nutrient deposition and ocean biogeochemistry.

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.

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.

Observation- and Model-Based Estimates of Particulate Dry Nitrogen Deposition to the Oceans.

Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO3-) and ammonium (NH4+) to the surface waters of the open ocean, with potential impacts on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, ~2900 observations of aerosol NO3- and NH4+ concentrations, acquired from sampling aboard ships in the period 1995 - 2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NOx) on mineral surfaces. Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes, however these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux are therefore very difficult to validate for dry deposition. Here the available observational data were averaged over a 5° × 5° grid and compared to ACCMIP dry deposition fluxes (ModDep) of oxidised N (NOy) and reduced N (NHx) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NOy, NHx and particulate NO3- and NH4+, and surface-level particulate NO3- and NH4+ concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO3- and NH4+) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model - observation ratios, weighted by grid-cell area and numbers of observations, (RA,n) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO3- concentrations (RA,n = 1.4 - 2.9) and under-estimates NH4+ concentrations (RA,n = 0.5 - 0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH4+ in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (RA,n = 0.6 - 2.6 for NO3-, 0.6 - 3.1 for NH4+). Values of RA,n for NHx CalDep - ModDep comparisons were approximately double the corresponding values for NH4+ CalDep - ModDep comparisons due to the significant fraction of gas-phase NH3 deposition incorporated in the TM4 and ACCMIP NHx model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model - observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species concentrations and this cannot be achieved if model products only report dry deposition flux over the ocean.

Persistent sulfate formation from London Fog to Chinese haze.

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Atmospheric transport and deposition of mineral dust to the ocean: implications for research needs.

This paper reviews our knowledge of the measurement and modeling of mineral dust emissions to the atmosphere, its transport and deposition to the ocean, the release of iron from the dust into seawater, and the possible impact of that nutrient on marine biogeochemistry and climate. Of particular concern is our poor understanding of the mechanisms and quantities of dust deposition as well as the extent of iron solubilization from the dust once it enters the ocean. Model estimates of dust deposition in remote oceanic regions vary by more than a factor of 10. The fraction of the iron in dust that is available for use by marine phytoplankton is still highly uncertain. There is an urgent need for a long-term marine atmospheric surface measurement network, spread across all oceans. Because the southern ocean is characterized by large areas with high nitrate but low chlorophyll surface concentrations, that region is particularly sensitive to the input of dust and iron. Data from this region would be valuable, particularly at sites downwind from known dust source areas in South America, Australia, and South Africa. Coordinated field experiments involving both atmospheric and marine measurements are recommended to address the complex and interlinked processes and role of dust/Fe fertilization on marine biogeochemistry and climate.