Baker Alexander R.

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Baker
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Alexander R.
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  • Article
    Methods for the sampling and analysis of marine aerosols : results from the 2008 GEOTRACES aerosol intercalibration experiment
    (Association for the Sciences of Limnology and Oceanography, 2013-02) Morton, Peter L. ; Landing, William M. ; Hsu, Shih-Chieh ; Milne, Angela ; Aguilar-Islas, Ana M. ; Baker, Alexander R. ; Bowie, Andrew R. ; Buck, Clifton S. ; Gao, Yuan ; Gichuki, Susan ; Hastings, Meredith G. ; Hatta, Mariko ; Johansen, Anne M. ; Losno, Remi ; Mead, Chris ; Patey, Matthew D. ; Swarr, Gretchen J. ; Vandermark, Amanda ; Zamora, Lauren M.
    Atmospheric deposition of trace elements and isotopes (TEI) is an important source of trace metals to the open ocean, impacting TEI budgets and distributions, stimulating oceanic primary productivity, and influencing biological community structure and function. Thus, accurate sampling of aerosol TEIs is a vital component of ongoing GEOTRACES cruises, and standardized aerosol TEI sampling and analysis procedures allow the comparison of data from different sites and investigators. Here, we report the results of an aerosol analysis intercalibration study by seventeen laboratories for select GEOTRACES-relevant aerosol species (Al, Fe, Ti, V, Zn, Pb, Hg, NO3 , and SO42 ) for samples collected in September 2008. The collection equipment and filter substrates are appropriate for the GEOTRACES program, as evidenced by low blanks and detection limits relative to analyte concentrations. Analysis of bulk aerosol sample replicates were in better agreement when the processing protocol was constrained (± 9% RSD or better on replicate analyses by a single lab, n = 7) than when it was not (generally 20% RSD or worse among laboratories using different methodologies), suggesting that the observed variability was mainly due to methodological differences rather than sample heterogeneity. Much greater variability was observed for fractional solubility of aerosol trace elements and major anions, due to differing extraction methods. Accuracy is difficult to establish without an SRM representative of aerosols, and we are developing an SRM for this purpose. Based on these findings, we provide recommendations for the GEOTRACES program to establish consistent and reliable procedures for the collection and analysis of aerosol samples.
  • Preprint
    Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean
    ( 2007-04-26) Planquette, Helene ; Statham, Peter J. ; Fones, Gary R. ; Charette, Matthew A. ; Moore, C. Mark ; Salter, Ian ; Nedelec, Florence H. ; Taylor, Sarah L. ; French, M. ; Baker, Alexander R. ; Mahowald, Natalie M. ; Jickells, T. D.
    The annual phytoplankton bloom occurring north of the Crozet Plateau provides a rare opportunity to examine the hypothesis that natural iron fertilisation can alleviate HNLC conditions normally associated with the Southern Ocean. Therefore, during CROZEX, a large multidisciplinary study performed between November 2004 and January 2005, measurements of total dissolved iron (DFe, ≤ 0.2 μm) were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples. DFe concentrations were determined by flow injection analysis with N,N-dimethyl- pphenylenediamine dihydrochloride (DPD) catalytic spectrophotometric detection. DFe concentrations varied between 0.086 nM and 2.48 nM, with low values in surface waters. Enrichment of dissolved iron (>1 nM) at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of ~0.07 nM.km-1 at the time of sampling. Using lateral and vertical diffusion coefficients derived from Ra isotope profiles and also estimates of atmospheric inputs, it was then possible to estimate a DFe concentration of ~0.55 nM to the north of the islands prior to the bloom event, which is sufficient to initiate the bloom, the lateral island source being the largest component. A similar situation is observed for other Sub-Antarctic Islands such as Kerguelen, South Georgia, that supply dissolved iron to their surrounding waters, thus, enhancing chlorophyll concentrations.
  • Article
    Global oceanic emission of ammonia : constraints from seawater and atmospheric observations
    (John Wiley & Sons, 2015-08-13) Paulot, Fabien ; Jacob, Daniel J. ; Johnson, Martin T. ; Bell, Tom G. ; Baker, Alexander R. ; Keene, William C. ; Lima, Ivan D. ; Doney, Scott C. ; Stock, Charles A.
    Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia (NHx(sw)), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here we compare [NHx(sw)] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [NHx(sw)] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NHx within observed ranges. The resulting global ocean emissions is 2.5 TgN a−1, much lower than current literature values (7–23 TgN a−1), including the widely used Global Emissions InitiAtive (GEIA) inventory (8 TgN a−1). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NHx over most of the ocean in the Northern Hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a−1, comparable in magnitude to other natural sources from open fires and soils.
  • Preprint
    Fractional solubility of aerosol iron : synthesis of a global-scale data set
    ( 2012-04-06) Sholkovitz, Edward R. ; Sedwick, Peter N. ; Church, Thomas M. ; Baker, Alexander R. ; Powell, Claire F.
    Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to biological availability is the proportion of aerosol iron that enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and estimated %FeS values for ~1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including a new data set from the Atlantic Ocean. Despite the wide variety of methods that have been used to define 'soluble' aerosol iron, our global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. The hyperbolic trends that we observe for both global- and regional-scale data are adequately described by a simple two-component mixing model, whereby the fractional solubility of iron in the bulk aerosol reflects the conservative mixing of 'lithogenic' mineral dust (high FeT and low %FeS) and non-lithogenic 'combustion' aerosols (low FeT and high %FeS). An increasing body of empirical and model-based evidence points to anthropogenic fuel combustion as the major source of these non-lithogenic 'combustion' aerosols, implying that human emissions are a major determinant of the fractional solubility of iron in marine aerosols. The robust global-scale relationship between %FeS and FeT provides a simple heuristic method for estimating aerosol iron solubility at the regional to global scale.
  • Dataset
    Fractional solubility of aerosol iron : synthesis of a global-scale data set [revised]
    ( 2012-03-26) Sholkovitz, Edward R. ; Sedwick, Peter N. ; Church, Thomas M. ; Baker, Alexander R. ; Powell, Claire F.
    Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to biological availability is the proportion of aerosol iron that enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and estimated %FeS values for ~1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including a new data set from the Atlantic Ocean. Despite the wide variety of methods that have been used to define 'soluble' aerosol iron, our global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. The hyperbolic trends that we observe for both global- and regional-scale data are adequately described by a simple two-component mixing model, whereby the fractional solubility of iron in the bulk aerosol reflects the conservative mixing of 'lithogenic' mineral dust (high FeT and low %FeS) and non-lithogenic 'combustion' aerosols (low FeT and high %FeS). An increasing body of empirical and model-based evidence points to anthropogenic fuel combustion as the major source of these non-lithogenic 'combustion' aerosols, implying that human emissions are a major determinant of the fractional solubility of iron in marine aerosols. The robust global-scale relationship between %FeS and FeT provides a simple heuristic method for estimating aerosol iron solubility at the regional to global scale.
  • Dataset
    Fractional solubility of aerosol iron : synthesis of a global-scale data set
    ( 2011-10-05) Sholkovitz, Edward R. ; Sedwick, Peter N. ; Church, Thomas M. ; Baker, Alexander R. ; Powell, Claire F.
    Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to bioavailability is the proportion of aerosol iron that enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and %FeS values for ~1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including new data from the Atlantic Ocean. The global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data falling along an inverse hyperbolic trend. The large dynamic range in %FeS (0-95%) varies with FeT in a manner similar to that identified for aerosols collected in the Sargasso Sea by Sedwick et al. (2007), who posit that the trend reflects near-conservative mixing between air masses that carry lithogenic mineral dust (with high FeT and low %FeS) and non-soil-dust aerosols such as anthropogenic combustion emissions (with low FeT and high %FeS), respectively. An increasing body of empirical evidence points to the importance of aerosol source and composition in determining the fractional solubility of aerosol iron, such that anthropogenic combustion emissions appear to play a critical role in determining this parameter in the bulk marine aerosol. The robust global-scale relationship between %FeS and FeT may provide a simple heuristic method for estimating aerosol iron solubility at the regional to global scale.