Atmospheric Transport and Deposition of Mineral Dust to the Ocean

Critical Review pubs.acs.org/est

Atmospheric Transport and Deposition of Mineral Dust to the Ocean: Implications for Research Needs Michael Schulz,†,* Joseph M. Prospero,‡ Alex R. Baker,§ Frank Dentener,∥ Luisa Ickes,⊥ Peter S. Liss,§ Natalie M. Mahowald,# Slobodan Nickovic,⊥ Carlos Pérez García-Pando,▽ Sergio Rodríguez,○ Manmohan Sarin,◆ Ina Tegen,¶ and Robert A. Duce$ †

Norwegian Meteorological Institute, Oslo, Norway Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, United States § School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom ∥ European Commission, JRC, Institute for Environment and Sustainability, Ispra, Italy ⊥ World Meteorological Organization, Geneva, Switzerland # Cornell University, Ithaca, United States ▽ NASA Goddard Institute for Space Studies, New York, United States ○ Izaña Atmospheric Research Centre, Santa Cruz de Tenerife, Spain ◆ Department of Planetary & Geosciences, Physical Research Laboratory, Ahmedabad, India ¶ Leibniz Institute for Tropospheric Research, Leipzig, Germany $ Departments of Oceanography and Atmospheric Sciences, Texas A&M University, College Station, United States ‡

ABSTRACT: 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.

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INTRODUCTION

and other potential micronutrients. Changes in productivity could conceivably play a major role in climate by modulating the ocean−atmosphere carbon cycle and altering atmospheric CO2 levels.8 Evidence for the importance of such a dust−ocean−climate feedback process is still limited. A significant correlation of dust with climate indicators is found in paleo-records9 such as ice cores. It has been suggested that mineral dust input in the

Aerosols play an important role in climate by altering the radiative balance of the atmosphere both directly by scattering and absorbing solar and terrestrial radiation and indirectly by affecting cloud microphysical properties.1 There is specific interest in mineral dust, which is a prominent, and often dominant, component in aerosols over large regions of the Earth.2 Furthermore, the emission rates of dust from land surfaces are themselves strongly affected by changes in climate so that there is the possibility of a negative (or positive) climate feedback via dust-climate forcing.3−5 Of particular interest in this paper is the idea that increasing dust levels can also increase oceanic primary productivity, which in large areas of the ocean (∼25%) is limited by the availability of iron.6,7 Mineral dust typically contains substantial amounts of Fe, P, © 2012 American Chemical Society

Special Issue: Marine Boundary Layer: Ocean Atmospheric Interactions Received: Revised: Accepted: Published: 10390

January 8, 2012 July 19, 2012 September 7, 2012 September 20, 2012 dx.doi.org/10.1021/es300073u | Environ. Sci. Technol. 2012, 46, 10390−10404

Environmental Science & Technology

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oceans may have suppressed atmospheric CO2 by 10−20 ppm in glacial versus interglacial periods,10.11,12 However there is little direct evidence that there is a causual relationship between high dust inputs and low CO2 levels and the suppression might be small.13,14 Artificial iron fertilization, mimicking dust input, has been shown to trigger a localized growth response in some marine ecosystems.15,16 A small number of case studies, which have tried to link natural dust deposition events to algal blooms have had mixed success, mostly because other driving factors could not be excluded from being the cause of the observed blooms.17,16 Modeling and experimental studies on the other hand have shown that atmospheric input can regionally dominate the surface ocean budget of iron.18,19 Thus, although uncertain, there is reason to believe that temporal and spatial changes in the magnitude and pattern of dust input to the ocean can affect the distribution and intensity of biological productivity and hence climate. In addition, changes in atmospheric dust as a result of human activities or climate may exhibit a large climate forcing with an estimated amplitude of 0.6 Wm2− between high and low dust decades.3 Because the primary origin of the marine dust input is from deserts, often even inner-continental regions, the characterization of dust transport to the oceans requires a global understanding of the dust cycle. There has been considerable progress over the past decade in identifying major dust source regions and the major transport pathways.20−23 Various satellite aerosol products show that the arid regions of Northern Hemisphere (North Africa, the Middle East, and Eastern Asia) are the largest and most persistently active sources; smaller, less active sources are found in South America, South Africa, and Australia.24,20,25,26 Another important feature of the dust cycle is the periodic nature of dust rise and atmospheric transport. Satellite measurements, supported by a limited database of actual aerosol measurements, show that the atmospheric concentration of dust can vary by orders of magnitude on time scales ranging from hours to decades.27−29 Sporadic large dust storms have the potential to dominate the annual or even multiannual dust flux in oceanic receptor regions. Once dust is deposited in the ocean, its impact on marine productivity will depend on a number of factors - most notably the amount of dust deposited to the ocean and the bioavailability of the elements present in the dust. This is one of the most poorly understood aspects of the entire global dust cycle.7 At present we do not know which properties control bioavailability. In the absence of such knowledge, the solubility of dust-elements is used as a proxy for bioavailability.30 Solubility is affected by many factors, including the mineralogy of the dust and the chemical processing of the dust particles (e.g., by SO2 and its oxidation products and sunlight) as they are transported through the atmosphere, a process that can last hours to weeks.31−33 Finally, solubility of dust-elements should be under conditions seawater pH and in the presence of organic ligands.34 In order to make research needs obvious to the reader we first shortly review the current state of knowledge of measuring and modeling dust emission, transport, deposition, iron bioavailability and dust−ocean−climate feedbacks. After this we discuss the experimental and modeling tools that are available to improve understanding of dust and bioavailable iron in the marine boundary layer and surface ocean waters. The ultimate goal is to identify and justify research needs to improve estimates of dust impact on ocean productivity.

Critical Review

THE DUST CYCLE

Emission of Dust Relevant for Transport to Oceanic Regions. The amount of dust, and its embedded iron, which is deposited to the ocean is ultimately related to the emission process of dust and its mineral composition from land surfaces. However, the global dust cycle cannot be adequately represented in the models without having reasonable information on the geographic distribution of dust sources. Satellite data are used to detect sources,20 including small-scale structures (“hot spots”) that could substantially contribute to global dust emissions.25,28 The importance of small-scale dust sources dispersed in large-scale basins has been well documented in extensive field studies in various regions including the Southwestern United States.35 One of the uncertainties connected to sources is the contribution of dust due to human activities. Mahowald et al.3 have shown that dust load may have doubled in the 20th century due to anthropogenic activities. In a new global high-resolution (0.1°) data set,23 by mapping sources based on MODIS Deep Blue, a distinction between natural, anthropogenic and hydrologic dust sources was made; they estimated that the anthropogenic sources account for 25% of global dust emissions. The necessary condition for dust emission is that the soil surface is dry and sparsely vegetated. The emission process starts if the near-surface wind exceeds a threshold value, above which soil particles begin to move in saltation horizontal flux.36,37 A small part of it generates the vertical flux available for further atmospheric transport.38 Another factor affecting the erosion thresholds is soil moisture, which affects the cohesion forces between the soil grains and thus increases the erosion thresholds.39 Mahowald et al.40 and Shao and Wyrwoll41 provide extensive review of the dust emission process. Factors of a significant influence on dust emissions include geographic variation of surface soil grain size distributions.42 Measurements of the dependence of the emitted dust particle size distribution on wind speed show contradictory results. Some theoretical models of dust emission43,44 consider that the size of emitted dust aerosols decreases with wind speed. On the other hand, recently formulated model of dust emission45 predicts that the emitted dust particle spectrum is independent of the wind speed. The particle size spectrum of the emitted dust controls the fraction that will be transported long distances. Over the source regions, the size distribution of mineral dust particles varies over a wide size range, from ∼0.1 to over 100 μm diameter, depending on wind speed and soil characteristics. The coarser particles are primarily deposited near the source regions. However, to characterize long-range transported dust, precise particle size measurements for less than about 10 μm are still a challenge.46 Atmospheric Dust Deposition Fluxes to the Ocean. Dust deposition in the marine boundary layer takes place through gravitational settling, turbulent dry deposition and wet scavenging by rain. Models yield a wide range of estimates of the ratios of wet-to-dry deposition.47 An earlier work in 200148 estimated that wet deposition accounts for only 30% of the total deposition, while some models show that wet deposition contributes 75−95% of the dust deposition to the ocean.49 In remote ocean regions the wet deposition pathway often dominates.50 Because the magnitude of the deposition flux is assumed to be proportional to dust concentrations, which can 10391

dx.doi.org/10.1021/es300073u | Environ. Sci. Technol. 2012, 46, 10390−10404


Critical Review

transport of Saharan dust to South America, and show strong critical differences in the southern hemisphere.82 Soluble or Bioavailable Dust Fractions. The impact of dust deposition on ocean primary productivity will depend on many factors: the composition of the dust at its source,83 the chemical processing of the particles as they are transported in the atmosphere, the processes that deposit the dust to the ocean, and the composition of the seawater into which the dust is deposited. The majority of studies on dust soluble components have focused on Fe, driven by the realization that Fe can be a limiting nutrient in large areas of the global ocean.6 Baker and Croot84 recently reviewed the factors that control Fe solubility. Here we summarize the factors most likely to affect soluble dust components in the atmosphere, including more recent results for Fe and other dust components. The initial composition of transported dust will depend on the mineralogical composition of source soils, which vary regionally and yield different iron content in dust.85,86,31,83 Models currently use large-scale inventories of soil properties, for example, UN Food and Agriculture Organization soil maps, to incorporate such variability. Many models49,40,87 typically use a constant Fe fraction in soil of 3.5% as a zero approximation, although this is recognized to be unrealistic. The assumption of a spatially heterogeneous fraction of iron at source only changes iron deposition amounts by less than 50%, which is smaller than other current uncertainties in the iron cycle.58 Newly developed global high resolution soil mineralogy data sets88 will allow the more accurate modeling of the Fe fractions in dust. Such maps, however, are based on bulk soil samples, while the mineralogy of erodible and transported dust may be very different from the parent soil. Fractionation of mineral phases (and the associated elements) during the dust mobilization phase can result in a mineralogical composition of the longrange transported dust that is different from that of the source terrains.89,90 Results of laboratory experiments performed by Mackie and co-workers91 suggest that particle abrasion during dust uplift results in the generation of finer particles than present in the parent soil, but does not increase the total amount of readily released (soluble) Fe. Iron in soils is primarily insoluble ferric iron (Fe(III)),92 with an average solubility in agricultural soils of less than 0.1%, (at an extraction pH of 4.65).93 In contrast, the measured solubility of iron in aerosols can be much higher, up to 80%.94 These differences can be attributed to a variety of factors including processes occurring in the day-to-week long atmosphere during transit. During this time dust particles will be exposed to sunlight and to reactive gases (e.g., SO2, NOx) and to their oxidation products, which tend to be acidic (e.g., H2SO4, HNO3). Chemical reactions can take place on the surface of the dust particles95 or while cycling through cloud droplets,96 modifying solubility of dust-elements.97 The mechanisms leading to increased aerosol iron solubility include photoreductive processes promoted by oxalate or other organic substances,98 in-cloud processing99 and acidic attack by anthropogenic compounds.100 Many of these processes can be expected to increase the solubility of other dust components also, but detailed studies on other components are rare in comparison to Fe. Shi et al.96 propose that repeated cycling of dust particles through clouds results in dissolution of surfacebound Fe, which reprecipitates as more soluble Fe oxyhydroxide nanoparticles on evaporation. The assessment of the actual solubility of dust−Fe is complicated by a significant presence of anthropogenic aerosols

be measured more reliably, concentration and deposition measurements are complementary for estimating the distribution of dust deposition. Our knowledge here, over the ocean, is based upon a few limited experimental data sets which are to date mostly discontinued: The SEAREX network was established on islands in the Pacific in the early 1980s and that operated for about a decade.51,52 The AEROCE network was established in the North Atlantic in the late 1980s and continued in operation into the mid-to-late 1990s.53,54 Only recently a few new monitoring stations, equipped with sophisticated instrumentation, have been established, often under the umbrella of the Global Atmospheric Watch program of the World Meteorological Organization (WMO GAW) (e.g., Malta and Izaña (Tenerife)). In addition, research programs in Africa and Asia (e.g., AMMA,55 SAMUM,56 ACE-ASIA57), and ship measurements58 have provided shorter-term information. Dry deposition presents a special challenge because there are almost no direct measurements of dry deposition flux. Typically both wet (automatic collector) deposition and bulk deposition are measured and the difference is reported as “dry deposition” which is unlikely to be very accurate, because of the differences between the aerodynamics and surface properties of such collectors and those of natural surfaces (e.g., the ocean surface). Few attempts exist to measure dry deposition on wet surfaces.59−62 Most estimates of dry deposition are based on aerosol concentration measurements multiplied by a dry deposition velocity. Models use parametrizations of varying complexity, from prescribed dry deposition velocities out of old parametrizations63 to more sophisticated approaches based on the resistance concept.64−66 The origin of large dust particles in remote oceanic sites, contributing considerably to dry deposition of dust, has not been clarified.67 Cloud processing of smaller particles to form larger particles might be responsable, which would require reliable measurements of rare large particles. In addition to dust deposition estimates from surface measurements, data from ground based and satellite remote sensing (AOD, size distribution, vertical distribution, aerosol type) are increasingly used to constrain models68 and their dust budget, hence the dust flux to the oceans.69−71 However, careful evaluation of the data is necessary to separate dust from the contributions of other aerosol components, especially sea salt, anthropogenic pollution and biomass burning aerosols.72−74 Mass fluxes derived from ocean sediment trap measurements are also used to provide additional information on atmospheric dust deposition, provided that the location is not strongly affected by material advected from continental slopes.75 Models use different schemes to estimate wet deposition, depending among others on assumptions about the wetability of the dust particles linked to the presence of hygroscopic material on dust (e.g., sulfates, nitrates) and the quality of the precipitation fields in the model. Aerosol indirect effects taken into account may change the amount of precipitation itself.76−78 As a consequence, model intercomparisons show a wide divergence in estimates of the wet deposition rates of dust.47,79−81 However, the wide range of remote to downwindof-deserts oceanic total deposition flux values is relatively well reproduced.47 In general models largely overestimate in remote sites the very low dust deposition, they miss the winter 10392



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from combustion processes and other human activities.101 Aerosol measurements on Bermuda show a clear impact from North American industrial Fe sources as evidenced by the correlation with pollution elements such as V.102,103 A recent model estimate stresses the large uncertainties in the global deposition of both combustion−Fe and dust−Fe, especially those of anthropogenic Fe. Emissions of reactive gases and secondary aerosols from anthropogenic sources have increased since preindustrial times and are expected, as an indirect effect, to have enhanced the solubility of micronutrients in dust particles, in particular downwind of industrialized regions.104 A recent compilation of measurements of aerosol Fe solubility measurements from a wide variety of sources shows there to be a broadly hyperbolic relationship between fractional Fe solubility and total aerosol Fe concentration (with high solubility at low total concentration and vice versa).94 This relationship appears to be consistent with the higher fractional solubility of anthropogenic Fe sources,102 but may also arise as a result of other processes associated with chemical processing and transport of dust in the atmosphere.84 Similar hyperbolic relationships have also been observed for aluminum, silicon and phosphorus,105 but not for manganese.106 Several other micronutrients contained in dust have been suggested to be important for marine biogeochemistry, such as phosphorus,107,108 or eg cobalt109 with no firm conclusions on the relevance for dust-climate feebacks so far. There is currently a lack of standardized methods and suitable reference materials for use in studies of aerosol dust solubility, and several workers have noted that this hinders progress in the field.84,110,111 The extent to which this lack of intercomparability distorts our view of dust solubility is not clear at present. Aguilar-Islas et al.110 reported that although different leaching solutions released different amounts of Fe from individual aerosol samples, there was greater variability in Fe amounts released from different samples with a given leach solution. Sholkovitz et al. 94 found that hyperbolic Fe solubility−total Fe concentration relationships were present for all of the extraction methods used in their extensive database. However, initial results from the GEOTRACES aerosol intercomparison study show that there are significant differences between methods for several trace elements, including Fe.111 Once dust is deposited in the ocean the impact on ocean productivity will depend on the fraction of its components that are bioavailable. However the specific properties of these elements that render them ″bioavailable″ to marine organisms are not entirely clear.84 Dissolved inorganic forms of Fe (especially Fe(II)) appear to be bioavailable,100,92 whereas iron(III) bound to organic ligands (of both marine112 and atmospheric113 origin) and colloidal Fe are available to different phytoplankton taxa to different extents. The incomplete understanding of trace element bioavailability in dust compounds the difficulties in standardizing solubility studies mentioned above, and also leads to a similar diversity of approaches to the treatment of iron solubility in modeling studies. Existing models of atmospheric processing focus on the iron solubility of hematite,114−116 and some reduce complexity by assuming that all iron is hematite.87 Experimental data suggests smaller particles have higher fractions of soluble iron,117,118 which is consistent with their having a longer lifetime and thus experiencing more atmospheric processing.49 This can also be explained by the larger surface area per mass of smaller particles

(that favors acidic coatings by sulfates and nitrates), or by highly soluble particles created by combustion, which favors the creation of smaller articles.87,58 Recently, it was shown that most bioavailable iron in dust is not in the form of oxides like hematite, which represent a large fraction of the total iron (50− 80% on average).119 Instead, the clay component, comprising 20−50% of the total iron, accounts for more than 90% of the soluble iron. Among the clays, illite and smectite account for about 65% and 27% of the soluble iron, respectively, while kaolinite accounts for about 2% in the samples analyzed in their study. Others have cautioned that the presence of Fe oxide impurities in standard clay minerals, and the variability between standard clays and these phases in the environment120 as well as differences between Fe solubility between commercial and soil Fe oxides,121 make the estimates of Journet et al. somewhat uncertain. A few global modeling studies have attempted to test simplified processing mechanisms. Because of the complexity of aerosol-phase iron chemistry and dissolution mechanisms coupled with the limited amount of field data available to constrain models, there is still no clear consensus regarding the importance of these mechanisms in soluble iron evolution.104 Introduced parametrizations are based on first order decay equations describing the evolution of iron solubility with different levels of complexity.49,115,116,122 Some models showed that because of the limited observations, many different mechanisms are equally likely to match available observations.49,115,116 In one model122 results compare well with observed mineral dust concentrations and deposition values, as well as available measurements of the soluble iron fraction within the North Atlantic basin. Only one study123 developed an extensive mechanism relying on a description of dust heterogeneous chemistry, deliquesced particle pH evolution and mineral dissolution to predict the evolution of the soluble iron under different ambient conditions. Feedback Processes Involving Dust Fluxes, Marine Biogeochemistry and Climate. Most of the evidence for links between atmospheric mineral dust inputs and marine biogeochemistry responses comes from broad-scale observations. Examples include observations that nitrogen fixing organisms (e.g., Trichodesmium) are much more abundant in the tropical North Atlantic (where there is a strong dust input) than in the tropical South Atlantic (where dust inputs are low);124−126 iron-limited HNLC (high-nitrate, low-chlorophyll) waters are generally located in areas far removed from major dust source regions (most notably the southern ocean)7 and that the dust supply in southern ocean is linked to glacial and interglacial cycles and seems to be related to changes in atmospheric CO2 concentrations9 and dust events are linked to increases in marine biomass.127 Smaller scale experiments (e.g., bottle incubations of phytoplankton and mesoscale ocean fertilization experiments) have also provided good evidence that Fe additions can have significant impacts on marine biogeochemistry in some ocean regions.124,16 However in ondeck incubation studies where dust additions were conducted, the results have often resulted in rather different responses in chlorophyll concentrations and productivity changes from those obtained by direct addition of “pure” Fe, P, or Fe, and P.15,123 Recent sediment trap studies show an increased flux of biogenic particles that is apparently a response of primary producers to fertilization by mineral dust inputs.75 The interpretation of sediment traps with dust is difficult because 10393



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Figure 1. (a) Sites for proposed long-term marine atmospheric measurement network located on multimodel median dust deposition field from 12 AeroCom models;47 WMO-GAW station identifier codes: AMS: Amsterdam, BMD: Bermuda, CGO: Cape Grim, CHI: Chatham, CRO: Crozet, CVO: Cape Verde, FGU: French Guyana, GSN: Gosan, ICE: Iceland, KER: Kerguelen, KEY: Miami, KGI: King George, MAR: Marion, MCO: Maldives, MID: Midway, NFI: Norfolk, IZO: Izaña, RPB: Barbados, PSA: Palmer, RUN: La Réunion, STY: Falkand Islands. (b) Nitrate concentrations (μM) in the surface (0−1 m) waters of the ocean;168 overlaid dots represent area where iron is the most important factor limiting growth rates for diatoms in present conditions, reproduced after Figure 4 in Krishnamurthy et al.19.

the dust acts as a ballast to enhance sedimentation rates.128 Enhanced biogenic particle fluxes recorded in traps at times of elevated mineral dust loadings may indicate significant enhancement of primary production,129 even when this production is undetected by satellite-retrieved chlorophyll-a. Nonetheless, satellite-based observations of chlorophyll-a and primary productivity in the surface ocean have been successfully used in specific case studies.130 Finally, the use of ocean thermodynamic and marine ecology models provides valuable information on where and how much marine primary production triggered by atmospheric input of mineral dust and iron may be important, revealing also where phosporous input might be more important.19,108 However, estimates of the impact of dust on ocean CO2 uptake in HNLC regions are highly uncertain because of intermodel differences in simulated dust emission and deposition fluxes47 in addition to the uncertainties in the chemical speciation of the micronutrients in the dust and the response of oceanic microorganisms. Recent studies suggest that even a doubling

of deposition of desert dust and a doubling of the solubility of iron (4× increase in soluble iron deposition) over the 20th century may not have substantially altered the carbon budget.3 In part, this is hidden by the large anthropogenic input of carbon through fossil fuel burning. However, it is also likely that the enormous dust input into the oceans may impact productivity and indirectly the nitrogen cycle of the ocean, in particular if we assume that higher iron leads to higher nitrogen fixation.131,132 Research Needs. Despite progress in recent years in this field, there continue to be unacceptably large uncertainties in estimates of the impact of dust on marine eco-systems and climate. The current research strategy is based largely on individual efforts and occasional coordinated field campaigns. While these provide essential information on dust processes, they cannot replace the island based long-term records, which have been discontinued. As stated earlier, the networks in operation in the 1980s until the mid-late 1990s have largely ceased operations. The poor state of such network activities, if 10394



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ocean productivity. These sites can also help to develop a global aerosol data set that can be used for better constrain global chemical transport and climate models. These measurements will also serve as a first order data set for the testing and development of satellite remote sensing algorithms. On the other hand remote sensing and modeling products should play an important role in establishing a refined priority list in station placement.140 Long-Term Marine Atmospheric Measurement Sites. Several criteria are used to let us propose here sampling sites (see Figure 1): the geographical and meteorological relationship to known dust sources or susceptibility of ocean to dust impact; the presence of other supporting observations (e.g., chemical, physical, meteorological); and the existence of supporting infrastructure. In many cases, the selection is limited because there are few islands that are ideally located. This listing is not intended to be complete. There are many sites located on continental shores or proximate off-shore islands that make important measurements; similarly we do not include sites in coastal waters and seas (e.g., the Mediterranean) where there is well-documented active research supported by, and integrated with, national and international programs.107 It is anticipated, hopefully correctly, that measurements from these various networks will eventually be integrated either through formal or informal structures. Instead we focus here on relatively remote island locations. Note that none of the following stations currently can be called operational, with commitments to measure in the long term dust and marine boundary layer characteristics, and if there are measurements ongoing then they differ considerably from that at other stations. South Atlantic. Falkland Islands (STY 51.27S, 60.59W; Palmer Station (PSA 64.77S, 64.05W); and King George Island (KGI 62.22S, 58.98W), Antarctica. The Falklands are ideally located to measure dust transport from southern South America, in particular, Patagonia which is believed to have been a major dust source during glacial periods as reflected by great increases in dust concentrations in Antarctic ice cores at those times.141 The University of East Anglia has recently established a site in the western Falklands on Carcass Island with a clean air sector directly exposed to southern South America. Palmer is a well established site for polar atmospheric chemistry studies. Aerosol measurements have been made in the past at Palmer and also at King George Island.142 South Pacific. Norfolk (NFI 25.05S, 167.98E) and Chatham Islands (CHI 43.92S, 176.50W). These islands are well suited to monitor Austrialian dust transport, an important source for the South Pacific. Norfolk would capture dust transport from Australia to the northeast and Chatham Island the transport to the southeast. The University of Miami had operated long-term sites on these islands in the past.51,143 Logistics are good for Norfolk, less so for Chatham, as is local support for operations. Indian Ocean. Hanimaadhoo Island (MCO 6.78S, 73.18E), Maldive Islands. As a follow-up on the INDOEX program,144 a Climate Observatory was established at Hanimaadhoo Island in the Republic of the Maldives in conjunction with the Atmospheric Brown Cloud project.145 The site is strongly impacted by the annual monsoon cycle. In the Northeast Monsoon it receives large quantities of dust and pollution primarily transported from the Indian subcontinent, the Middle East, and North Africa. During the Southwest Monsoon the air is extremely clean. This would be an ideal region to study the effects of pollution on dust Fe solubility and the seasonal

continued, will preclude scientific progress in understanding the controlling feedbacks between the dust cycle and climate in the decades to come. The large-scale temporal variability of dust transport and deposition in oceanic regions is currently not systematically observed. On a larger scale, there are a number of satellite systems that yield a global picture of aerosol and dust distributions, for example, MODIS and MISR.133,134 While these are very useful, they do not provide sufficiently specific information on aerosol properties (e.g., composition and size) nor deposition that would be needed to assess ocean impacts.26 In addition, satellite monitoring activities may also be discontinued with then even larger problems to ensure measurements of robust trends. Moreover, the satellite systems lack the sensitivity to make accurate measurements over those large areas of the world ocean where concentrations might be very low but nonetheless important, for example, the HNLC areas of the southern ocean. Lidar-instrumented satellites, such as CALIPSO, can provide measurements of aerosol vertical profiles and it can crudely discriminate among various aerosol types including dust, smoke, and pollution.135 However the algorithms are subject to considerable uncertainty136 and dust measurements in the MBL are particularly suspect especially when sea salt column loads are high. Similarly, ground-based upward-looking sensors such as the multispectral sun photometers used in AERONET137 and lidars138 provide important information of the temporal-spatial variability of aerosol types but lack specificity on other aerosol properties. Indeed the coupling of ground based in situ aerosol measurements coupled with remote sensing tools and modeling are needed to develop an accurate global scale picture of aerosol transport in general and dust in particular.138 To address these issues requires a research strategy that emphasizes a long-term, internationally coordinated network of surface and column atmospheric measurements carried out on selected islands and exposed coastal sites. Such a network should be coordinated with the Global Atmospheric Watch program of the World Meteorological Organization (WMOGAW). It would benefit the understanding of marine boundary layer processes in general. This effort must be complemented by other focused research activities. In the following sections we discuss scientific and technological considerations for both a long-term network and advances in in situ measurement technologies, assuming that strategies for remote sensing are discussed and developed elsewhere.139 In order to properly address the impact of the iron present in mineral dust on marine productivity, network sites and intensive campaigns should be located as much as possible in those ocean areas where there are high nitrate levels in surface ocean but where productivity (using chlorophyll as an indicator) is low, so-called HNLC regions; these are primarily found in remote ocean regions. However, in these remote areas dust levels are usually very low. Under such conditions it is a challenge to make measurements, especially of more sophisticated parameters such as bioavailable iron, dust size distribution or dust dry deposition. To address this problem we propose a two-level strategy: to make more systematic dust measurements at network sites in remote areas following a simple protocol, and to also carry out intensive processoriented campaigns in marine regions downwind of major dust source areas. Finally it is also important to establish some control aerosol network sites in regions where aerosol (and dust) concentrations are not believed to play a major role in 10395



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one may characterize the transition regions between major wind systems (i.e., in the Atlantic, between the trade winds and the westerlies). Local support is available through the University of Miami, the Bermuda Institute of Ocean Sciences (BIOS), and the Izaña Atmospheric Research Centre. Several French, German and American institutions cooperate on Cape Verde Island under the Surface Ocean Lower Atmosphere study. French Guyana (FGU 5.15N, 52.65W). A station here would enable the characterization of low-latitude dust transport, which is prominent in satellite products. A study carried out 30 years ago in Cayenne153 showed that in late winter and spring the transport of African dust to South America was as great as that taking place in the summer months at Barbados. In the recent AeroCom model intercomparison,47 the models missed or greatly underestimated this transport. Comparisons between Izana and Sao Vicente in the eastern Atlantic and Barbados and Miami and French Guyana in the West will serve to characterize the seasonal characteristics of dust transport and any associated changes in properties. French Guiana has good infrastructure especially in the vicinity of the space station at Kourou, which could provide high-quality scientific support. Iceland (ICE 63.40N, 20.29W). A station here would play an important role in characterizing the response of high-latitude deserts to climate change. Studies on Heimaey, Iceland, that began in 1991 show that there is considerable dust activity on Iceland, much of it linked to proglacial soil deposits.154 These sources are expected to become more active and increasingly important as an iron source in the coming decades as the glaciers retreat. Logistics and scientific support are excellent in Iceland. North Pacific. Midway (MID 28.22N, 177.36W). Midway is ideally situated to monitor the transport of dust from Asian sources to the central North Pacific. Measurements made by the University of Miami starting in 1981 and ending in 2001 clearly show the strong spring cycle of Asian dust transport along with the transport of high concentrations of pollutants.51 The presence of both dust and pollutants in the same air mass could have a significant effect on Fe solubility in contrast to African dust, which is usually associated with relatively low concentrations of pollutants. High concentrations of “industrial” iron would be expected in these samples. Midway is now a wildlife refuge under the U.S. Department of the Interior; logistics and local technical support should be good. Gosan, Korean (GSN 34.28N, 126.17E). This site was established as an ACE-Asia supersite.155 Situated on Jeju Island, this site is ideal for monitoring the regional background atmosphere in the East Asian region because there are no local industrial sources. It was initially established as an ACE-Asia supersite155 and, as such, has good facilities and infrastructure. The Aleutian Islands, Alaska or Whistler Peak, Canada. Measurements would characterize the westward transport from Asian sources to the high latitudes. The University of Miami operated a station on Shemya for several years in the early 1980s.51,143 Coordinated Atmospheric and Marine Experiments. Large, coordinated multidisciplinary field experiments are required to properly address the many complex and interlinked processes involved in characterizing the role of dust fertilization. In situ marine experiments will be logistically difficult, requiring access to the study site by a relatively large number of researchers, potentially for many weeks at a time. A program of this scope would require one or more large research

variability of dust deposition impacts on local water biogeochemistry. This site has good facilities and support infrastructure. Impact of anthropogenic sources and continental outflow on aerosol Fe solubility has been studied from the Bay of Bengal and the Arabian Sea (two important regions of the Indian Ocean). Although the mass concentration of aerosol−Fe over the two oceanic regions is not significantly different, the fractional solubility is 1 to 2 orders of magnitude higher over the Bay of Bengal (1.5−25%) compared to the Arabian Sea (0.02−4%). This spatial variability in the fractional solubility of Fe is attributable to differences in the nature of mineral dust transported to the two oceanic regions (alluvial dust to the Bay of Bengal vis-à-vis coarse dust from desert regions to the Arabian Sea). Furthermore, the role of anthropogenic sources (emissions from biomass burning and fossil-fuel combustion) in enhancing the fractional solubility of aerosol Fe is discernible based on the chemical composition of fine mode aerosols.146 The Southern Indian Ocean. There are not many good options in this region. Logistics are difficult and, except for Cape Grim, we would not expect much dust transport to these sites. Nonetheless some studies would be warranted because of the huge area of ocean with HNLC in the high latitude southern ocean (Figure 1). Cape Grim (CGO 40.68S, 144.69E). Measurements at Australia’s Cape Grim Baseline Air Pollution Station in remote northwestern Tasmania clearly show the impact of Australian dust emissions. This site also has a highly capable staff that carries out a highly developed protocol.147 Reunion Island (RUN 21.17S, 55.83E). Although there is not much dust in this region, measurements would provide data on Indian Ocean background aerosol and also transport out of Southern Africa, depending on season. Marion Island/Prince Edward Island (MAR 46.91S, 37.73E); Crozet (CRO 46.41S, 51.76E), Kerguelen (KER 49.34S, 69.33E). These islands are located in a band of strong westerlies (the “roaring forties”).Weather is frequently rainy and often severe; consequently, the air is expected to be very clean, perhaps with occasional impacts from southern Africa. Nonetheless measurements from one or more of these station would be important because they lie on the northern edge of the HNLC region (Figure 1). Unfortunately logistics are difficult for these islands. Amsterdam Island (AMS 37.83S, 77.56E). The site has a good aerosol and deposition measurement record dating from the 1980s. The site is ideally located and representative for central Indian Ocean conditions.148 North Atlantic. Barbados (RPB 13.16N, 59.43W). Studies on Barbados149 and Miami150 have shown large changes in dust transport over the past decades. The 2007 IPCC assessment151 suggests that large areas of northern North Africa will become drier in the future; but the models could not agree on the future direction of change for a large area of North Africa. Barbados is a vitally important station to monitor this source and the transport to the Atlantic. Logistics are excellent and local technical support is available through scientific institutions on the island. Bermuda (BMD 32.26N, 64.87W); Miami (KEY 25.73N, 80.16W); Izañ a, Tenerife (IZO 28.31N, 16.50W; Sao Vicente, Cape Verde Islands (CVO 16.86N, 24.87W). These stations would provide data on the latitudinal distribution of dust transport, necessary to assess aerosol nutrient inputs to the central Atlantic.152 Bermuda also serves to characterize the transport of dust and especially “industrial” iron from North America to the central Atlantic.152 With Miami150 and Bermuda 10396



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Sediment traps will be an essential component in targeted intensive deposition studies in order to quantify the actual deposition rates of dust particles in the water column. The interpretation of trap measurements is complicated by the fact that many factors control particle transport in the ocean, such as displacements by currents and biological processes (e.g., aggregation, disaggregation), which also affect trap efficiency. At the least, the lithogenic flux in open ocean regions can be used as an estimate for dust deposition to the oceans. Longterm sediment trap arrays are currently in place at sites that are influenced by dust outbreaks (e.g., Kiel 276 at 33°N and 22°W; Tropical Eastern North Atlantic Time-Series Observatory at 17.59°N and 24.25°E; and the Bermuda Atlantic Time Series Study at 31.67°N and 64.17°W). Some of these sites are associated with atmospheric observatories. Observations at oceanic and atmospheric observatories together with satellite and modeled dust occurrence and deposition data will help to understand the biogeochemical response to dust input. Atmospheric Measurements. Not all of the measurements mentioned in Table 1 are simple, and the quality of the experimental observations of some aerosol properties still requires attention. The World Calibration Centre for Aerosol Physics under the WMO-GAW program has developed ‘standard’ (recommended) methods for the measurements of some aerosol properties (e.g., the size distribution of ultrafine particles157). However, no reference method exists for bulk dust mass concentration or the mass concentration or size distribution of coarse particles, nor wet deposition of dust. Such information is vital for making good estimates of the deposition and solubility of dust. Underestimates in dry deposition of dust occur when they are inferred from sizesegregated aerosol measurements that do not include the very coarse size fraction In such cases the measurements will miss the few large particles (>20um) that are the major contributors to the mass flux, especially in regions proximate to, and downwind of, dust sources. There is a clear need for improved comparable wet deposition measurements. Insoluble dust particles and very low concentrations in rainwater require sophisticated comparable analytical methods. There is a more urgent need to improve estimates of dry deposition fluxes of mineral matter to the ocean. Much of our understanding rests on a paper by Slinn and Slinn,65 in which they theoretically examined the controlling processes for deposition to water surfaces. The most recent comprehensive reviews of dry deposition processes158,159 provide a good overview of the difficulties and show how little progress has been made in this field. The situation is especially difficult for mineral dust because of the size distribution over the oceans, which ranges primarily from ∼0.1 to ∼20 μm diameter, with the mass median diameter a few μm.46,160 We are unaware of any past or present efforts to measure the deposition rates of natural dust particles in the open-ocean environment. This involves the development of new technologies. Such measurements are also needed to improve the estimates of the size-dependent dry deposition velocity used in models. The Fe-bearing mineral phases present in dust particles over the oceans need better quantification.161 Size-segregated mineralogy and solubility should also be measured because chemistry occurring during transit could affect dust composition and element solubility. Measurement intercomparisons such as GEOTRACES111 should be initiated to make iron solubility data consistent. Dedicated intensive field observations

vessels and easy access to a well-equipped marine laboratory. An intermediate step is to use an in situ mesocosm approach as deployed in the Mediterranean,156 although even here there are substantial logistical issues to address. The choice of study sites for such experiments will also be problematic. Areas which are frequently subject to dust inputs may already have sufficient supplies of dust-borne trace elements, and thus would not show a strong response to a given deposition event, whether natural or deliberately introduced. The response of areas with less frequent (and possibly weak) dust inputs may be dependent on seasonality in dust inputs and phytoplankton community composition. In some circumstances it may be desirable to study the first deposition of dust to waters after a prolonged absence of dust. Perhaps the most dramatic results might be expected from studying the deposition of dust to a region that very rarely receives dust. In practice such an experiment would be very difficult to plan and conduct for a natural dust event because we are not able to reliably predict individual dust outbreak events on relevant time scales, and thus cannot schedule research vessels. One potential solution to this problem would be to perform “deliberate dust release” experiments of the type already conducted for mesoscale iron enrichment experiments in the remote ocean coupling research also to marine carbon cycle studies.16 A summary of both the atmospheric and marine measurements that should be made during such experiments is given in Table 1. The proposed marine measurements recognize the central role of iron as a micronutrient in the oceans, but also highlight the need to study other dust components, particularly the other micronutrient trace elements (e.g., Zn, Co, Cu, Cd, Ni). The studies should be combined with nitrogen fixation studies. Table 1. Ideal Measurements at Sampling Sites atmospheric measurements aerosol concentration and size distribution (TSP, PM10, PM2.5),, altitude resolved aerosol properties (lidar measurements), aerosol optical depth (sun photometer, microtops) size dependent total suspended particulates (TSP), PM10, PM2.5, mineral aerosol dust and suite of micronutrients and (Fe, Al, Mn, Ti, Co, chemistry PO4, Ca, Si), water-soluble iron and phosporus (by more and rainfall than one method), potential toxic trace metals (Zn, Pb, Cd, chemistry Ni, V), oxalate and other organic acids, elemental carbon, organic carbon and diagnostic ratios: NO3/SO4, NH4/SO4, NH4/NO3, OC/K+, OC/EC, N-org/P-org. mineralogy clays minerals, iron oxides (hemeatite, goethite, amorphous iron oxide) aerosol mass mixing state of dust, nitrate, sulfate, and elemental carbon spectrometry deposition wet deposition (preferably with an automatic wet-only fluxes collector or at least bulk) fluxes of nutrients (N, P, and Fe) (Fe as well as inorganic and organic P and N). Direct measurement of dry-deposition, and through model based 210-Pb fluxes and constituent to 210Pb ratio. surface ocean measurements physical salinity, temperature, density, mixed layer depth, photosynthetically active radiation (PAR), dust deposition fluxes based on sediments trap measurements chemical pCO2, POC, PON, DOC, CDOM, inorganic N and P, chlorophyll-a, DMS, N2O, dissolved Fe, Al, and Fe-binding ligands, N-14/N-15 measurements biological biomass, taxonomic composition (cyanobacteria vs microalgae), primary productivity and nitrogen-fixation, bacterial productvity, carbon-based models coupled with sediment trap data 10397



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try112 and chemical reactions with anthropogenic compounds.100,104,102 A main chemical parameter influencing the solubility and dissolution of the dust particles and thus the precipitation components is the pH value.96 While acting as a CCN the dust particles experience on average ten evaporation and condensation cycles of water vapor.165 Due to anthropogenic inputs the aerosol water can be highly acidic,166,114 which leads to a cycling of the dust particles through different pH regimes. Large variations of pH result in formation of nanosize Fe particles and thereby affecting Fe solubility.96 Nevertheless (warm) cloud processing has to be always combined with photoreduction and/or organic complexation that also influences Fe solubility.167 Because of this complexity it is necessary to figure out the dominant mechanisms which have to be included into models and/or to find methods to describe the overall mechanism. Field data to investigate and validate the dust chemistry and dissolution mechanisms and their importance is therefore needed. As stated before it is essential that the wet deposition is well characterized in the models. Taken into account that dust particles interact with clouds and therefore influence the type and amount of precipitation this is accompanied with many complex interactions as well as feedback mechanisms.76−78 This implies a critical need for more comparisons of model results and observations related to deposition to determine if dust transport and regional and global scale models can correctly simulate dust deposition processes. The documentation of model experiments is often incomplete and difficult to retrieve for those not directly involved in the experiments. The documentation of dust model simulations may be further improved by systematically making use of supplements in publications. This should be accompanied by making available in an open manner model fields in the form of CF-compliant netCDF data sets via ftp sites or via common databases such as AeroCom, SDS, ICAP, or other international model intercomparisons. Any dust publication of relevance for marine biogeochemistry should be accompanied by essential model fields, such as monthly surface concentrations, emissions, dry and wet deposition fluxes, column loads, and 3D mass mixing ratios of individual dust tracers as well as dust aerosol optical depth.

and laboratory-based studies are required to better understand the processes affecting dust composition and solubility in the marine receptor region. Although individual event-based studies provide critical information, long-term time-series observations new measurement protocols should be developed to provide conclusive data on trends of iron solubility. Atmospheric Model Development. Results from regional or global dust models are often utilized to obtain estimates of oceanic dust deposition fluxes.40 Much focus of dust modeling has been on description of conditions in the source regions that are not well characterized.162,69,163 In this regard, the influence of changing land-use practices and variability of vegetation cover should be included in models. Also, parametrizations of desert dust emissions may need to be made specific for the relevant processes before improved future predictions can be made. Particular focus should extend to understanding controls of dust sources and transport in the southern hemisphere, which are less well-known than the major dust sources in the northern hemisphere.164 An understanding of the mineralogical composition of source soils and how the composition of the mobilized soil dust is related to that of the parent soil would ultimately allow better quantification of the actual iron deposition. However, while realistic simulations of dust emission fluxes are a prerequisite for correctly reproducing transported dust, it is crucial to describe wet and dry dust deposition processes correctly for quantification of dust deposition fluxes. One goal would be to identify whether the model size distributions or removal parametrizations are the cause of differences between models. Assimilation of satellite data into models may provide improved AOD dust fields. However AOD is most sensitive to the concentration of fine particles while most of the dust mass resides in coarse particles. Thus coordinated measurements of dust size distributions in surface aerosols and in deposited dust need to be related to AOD. While data assimilation can improve our understanding of aerosol abundances and distribution, without such measurements we cannot expect improvements in model-estimated deposition rates. A recent comparison81 of global dust model results with wet deposition measurements and a dust model study in the framework of AeroCom47 showed that most models reproduce the seasonality of deposition and the dominance of wet deposition over many parts of the world ocean, but tend to overestimate it in regions where it is not the dominant removal mechanism. Modeled total dust deposition is often overestimated in Europe, North Atlantic, and the Indian Ocean, but underestimated in the Pacific and South Atlantic Ocean. In HNLC regions dust deposition data are particularly scarce, and thus dust deposition fluxes from models remain largely unconstrained. Particular care should be taken to understand the processes controlling dust deposition fluxes, for example, washout processes. Only then the changing dust deposition fluxes observed in sediment records or changes in dust in future climate projections can be interpreted correctly. Moreover it will be necessary to understand and include the dust processing into models to be able to describe the amount of dissolved iron into the ocean in dependence on the mineralogical composition and the atmospheric processing experienced instead of using one fixed value for each amount of dust. That requires the consideration of dust processing mechanisms, namely aerosol processing in clouds,99 cloud chemistry including dissolution kinetics,98,120,34 photochemis-

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CONCLUDING REMARKS The long-range transport and deposition of mineral dust to the ocean has implications to chemical processing of dust, modifying the surface properties and hence influencing the atmospheric radiative forcing on a regional scale. In addition, air−sea deposition of mineral dust is a major source of nutrients to the world oceans. Measurements on various times scales have shown significant correlation among dust emissions, transport, and deposition with climate change. Models have serious limitations in reproducing these relationships due to lack of incorporating changes caused by present-day anthropogenic activities. Therefore, while models demonstrate reasonable performance under current conditions, the predictive capacity of models for future scenario seems highly uncertain. Although mineral dust is a perennial constituent of the earth’s atmosphere and its constant deposition to the ocean, our inability to adequately model the global dust cycle is one of the primary uncertainties in developing future climate scenarios. 10398



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The lack of simultaneous and high quality measurements on chemical composition of aerosols, required for model verification, is a serious problem. There is an urgent need for the long-term measurements in the marine atmospheric boundary layer through a network of study sites spread across different oceans. The most logical and economical strategy would be to reactivate some of the monitoring stations that have been operational for several decades. This would have the added benefit of adding to the long-term record at these sites, many of which were operational in the late 1970s and early 1980s. Since then, short-term climatic variability has been reasonably well documented. By extending the aerosol record at these sites, we would develop data that would be ideal for testing the ability of models to reproduce the trends in dust with climate change. Although this paper has focused on dust, there are other aerosol species that play an important role, for example, sulfate and black carbon in radiative forcing and nitrogen species (e.g., nitrate, ammonium). Many of these species were measured as a part of the earlier ocean network studies and should become part of any future effort network of field based measurements. Because of the large areas of HNLC waters in the southern ocean, that region would be most sensitive to dust−Fe impact. Thus data from long-term observing programs in this region would be valuable especially at sites downwind from known dust source areas in South America, Australia, and South Africa. The network sites proposed in Figure 1 should be regarded as potential site locations, a selection which would require further refinement during implementation. The network studies will, of necessity, follow a relatively simple protocol. The network activities should be harmonized with aerosol studies carried out at more advanced research and monitoring sites. Further work on the adoption of reference methods for some key properties of aerosol dust is necessary. Complementing intensive field campaigns are needed to study processes of dust deposition and fractional solubility of iron in marine regions downwind of North Africa, south and southeast Asia, which are major sources of dust and pollutants. Future studies with respect to the impact of dust on marine biogeochemistry should focus on the following scientific problems:

Critical Review

AUTHOR INFORMATION

Corresponding Author

*Phone: +47 2296 3330; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper resulted from the deliberations of GESAMP Working Group 38, The Atmospheric Input of Chemicals to the ocean with input from members of the WMO Sand and Dust Storm Warning and Advisory System WMO-SDS. We thank the Global Atmosphere Watch (GAW) and the World Weather Research Programme (WWRP) of the World Meteorological Organization (WMO), the International Maritime Organization (IMO), and the Swedish International Development Agency (SIDA) for support. The authors are grateful to the participants of the joint workshop in Malta, 7-9 March 2011 in between the GESAMP Working Group 38 and the WMO-SDS that has led to the discussion foundations of the present paper.

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REFERENCES

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• Particle-size and dust mineralogy characterization to assess the impact of oceanic deposition of dust-derived iron. • The relative contributions of wet and dry deposition to the ocean surface. • The fractional solubility, and hence bioavailability, of Fe (and other micronutrients and pollutants such as P, Zn, Cu) from dust inputs. • With improved knowledge of the dust input, the response of the marine ecosystem requires quantification. • The selective response of oceanic biomass and different phytoplankton communities needs to be studied to further clarify the importance of feedback processes between dust, the oceanic carbon cycle and climate. Finally, institutional support for assessment activities, model and measurement intercomparison, and appropriate identification and documentation of models and measurements, remains a high priority. This is necessary to ensure an improved understanding of the role of dust in both the marine atmosphere and marine ecosystems on longer time scales. 10399



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