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Salt Aerosols in the Atmosphere: 1. Model Development

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Salt Aerosols in the Atmosphere: 1. Model Development JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D3, PAGES 3805-3818, FEBRUARY 20, 1997 Modeling sea-salt aerosols in the atmosphere 1. Model development S. L. Gong and L. A. Bartie AtmosphericEnvironment Service, Downsview, Ontario, Canada J.-P. Blanchet Earth SciencesDepartment, University of Quebec at Montreal, Canada Abstract. A simulationof the processesof sea-saltaerosol generation, diffusive transport, transformation,and removal as a function of particle size is incorporatedinto a one- dimensionalversion of the Canadiangeneral climate model (GCMII). This model was then run in the North Atlantic betweenIceland and Ireland during the period of January- March. Model predictionsare comparedto observationsof sea-saltaerosols selected from a review of availablestudies that were subjectedto strict screeningcriteria to ensure their representativeness.The number and masssize distributionand the wind dependencyof total sea-saltaerosol mass concentrations predicted by the model comparewell with observations.The modeled dependenceof sea-saltaerosol concentration in the surface layer(X,/xg m -3) on 10-mwind speed (U•0, m s-•) is givenby X = beaU•ø.Simulations showthat both a and b changewith location.The value a and b range from 0.20 and 3.1 for Mace Head, Ireland to 0.26, and 1.4 for Heimaey, Iceland. The dependenceof X on surfacewind speedis weaker for smallerparticles and for particlesat higher altitudes.The residencetime of sea-saltaerosols in the first atmosphericlayer (0-166 m) rangesfrom 30 min for large particles(r = 4-8/xm) to -60 hoursfor smallparticles (r = 0.13-0.25/xm). Although somerefinements are required for the model, it forms the basisfor comparing the simulationswith long-term atmosphericsea-salt measurements made at marine baselineobservatories around the world and for a more comprehensivethree-dimensional modelingof atmosphericsea-salt aerosols. 1. Introduction can only be performed at certain times and locations,applica- tion of the experimentalresults is limited. It is vital to develop Sea-salt aerosolsplay a very important role in a variety of an aerosol transport model to predict its concentration and processesin the atmosphere.They influenceradiative transfer size distribution on a regional scale as a function of atmo- directlyby scatteringsolar radiation and indirectlyby altering sphericprocesses. Various modelshave been proposedin the cloud droplet size distributionand concentration,thereby in- literature [Gathman, 1982; Fairall and Davidson, 1986; Erick- fluencingthe albedo of marine boundary layer clouds.In ad- son and Duce, 1988; Fitzgerald,1992; van Eijk et al., 1992]. A dition, sea-saltaerosol particles are chemicalcarriers of spe- summaryof major functionsand assumptionsin these models ciescontaining C1, Br, I, and S and therefore play a role in the is shown in Table 1. All of these models are limited to the atmosphericcycles of these important elements.The halogens marine boundary layer and do not addressthe long-range Br and C1, once mobilized by heterogeneousreactions from transport of sea salt. sea-saltinorganic forms to reactive gaseousforms (e.g., Br2, A comprehensivesea-salt aerosol model coupledwith a one- C12)[e.g., Mozurkiewcz, 1995], can play a role in atmospheric dimensionalclimate model (FIZ-C) [Therrien,1993] is pre- ozonedepletion and destructionof light hydrocarbons[Jobson sentedin this paper. Using the meteorologicalconditions gen- et al., 1994]. erated by the FIZ-C, the model includes the following Numerous experimental investigationsof sea-salt aerosols processes:(1) sea-saltgeneration due to surfacewind; (2) have been conductedaround the globe in order to determine vertical transportby turbulenceand convection;(3) dry depo- their abundance and physical/chemicalproperties [Jacobs, sitionand gravitationalsettling; and (4) wet removalprocesses 1937; Woodcock,1953; Toba, 1965a, b; Lovett, 1978; Prospero, which includeboth in-cloudand below-cloudscavenging. The 1979; Podzimek,1980; Parungoet al., 1986; Marks, 1990; Ike- model doesnot at this time includechemical or physicaltrans- gami et al., 1994;McGovern et al., 1994]. Sea-saltaerosol con- formationthrough interactions with other aerosoltypes such as centrationis a strongfunction of the state of the sea surface, acidic sulphur. The particle size distribution is modeled by which is in turn determinedby meteorologicalconditions, es- representingthe sizespectrum as a seriesof discretesize bins. pecially surfacewind speed.Sea-salt particles are distributed from 0.02 to 60 txm with a bimodal size distribution in the 2. Physical Model submicronportion [Fitzgerald,1991]. Since the observations The fate of sea-saltaerosols, once they are injected into the Copyright1997 by the American GeophysicalUnion. atmospherefrom the ocean source,is governedby a seriesof Paper number 96JD02953. physicalprocesses such as transport,coagulation, dry and wet 0148-0227/97/96JD-02953509.00 removal,and chemicaltransformation. Transport of aerosolsis 3805 3806 GONG ET AL.: ATMOSPHERIC SEA-SALT MODELING, 1 Table 1. Summaryof Sea-SaltAerosol Models Removal Size Authors Dimension Domain Spectrum Transport Source Dry Wet Gathman [1982] ' one MBL no eddy Blanchard[1963] D • BC* IC* Fairall and Davidson[1986] one MBL yes eddy Fairall and Davidson[1983] BC? G? IC? Burk [1986] one MBL no eddy Monahahet al. [1986] D? BC* G? IC* Stramska[1987] one MSL yes eddy Monahah et al. [1986] D* BC* G? IC* Ericksonand Duce [1988] two surface no eddy empirical D? BC? G? IC* FitzgeraM[1992] zero MBL yes eddy Monahah et al. [1986] D* BC? G* IC? van Eijk et al. [1992] zero MBL yes eddy empirical D* BC* G* IC* MBL, marine boundarylayer; D, dry deposition;G, gravitationalfall; BC, below-cloudscavenging; IC, in-cloudscavenging. *Not included. ?Included. realized by turbulent diffusion,advection, and vertical convec- In (2), the aerosolconcentration change has been dividedinto tion of the atmosphere,which is representedin this model by tendenciesfor dynamics,surface, clear air, dry deposition,in- the FIZ-C parameterizationswhich are analogousto thosefor cloud and below-cloudprocesses. The dynamicsincludes re- moisture transport in the FIZ-C. A generalizedprognostic solvedmotion as well as subgridturbulent diffusionand con- massbalance equation for size i of typej aerosolparticle can vection.The surfaceprocesses include surface emission rate of be written as both natural and anthropogenicaerosols and serveas bound- ary conditionsfor the model. Particle nucleation,coagulation, Opxu + divpxuU = Su + I u (1) and chemicaltransformation are includedin clear-airprocess. Ot In the current version,coagulation is not includedfor sea-salt whereX'q is the mass mixing ratio of thei th sizerange aerosols aerosols.However, for purposesof predictingtotal mass of (kg/kgair),U isthe horizontal wind velocity vector (m s-•), p is sea-saltaerosols for comparisonwith observations,this is not a theair density (kg m-3), and Sq is the source and sink terms major drawback, since Na and C1 which dominate sea-salt whichmay includefollowing contributions: (1) surfacesource aerosolmass are not affectedsubstantially by coagulation.The (natural and anthropogenic);(2) clear air processes(particle aerosolmodel is structuredin such a way that any process which affects the concentration and size distribution of sea-salt nucleation, particle coagulation, and chemical transforma- aerosols can be modified or added to the model as new or tion); (3) in-cloud processes(activation of aerosols,attach- ment to clouds, and removal of aerosol-attachedclouds by better parameterizationsfor suchprocesses become available. hydrometeors);(4) dry deposition;and (5) precipitationscav- Somedetailed physical parameterizations for sea-saltaerosols enging,where particle nucleation,and chemicaltransforma- will be presentedin the followingsections. tion are processesnot requiredfor sea-saltaerosols. Term I u representsthe rate of intersectionaltransfer processwhich 2.1. Aerosol Growth With Relative Humidity moves the aerosol mass from one size bin to another. The Experimental evidence [Fitzgerald,1991] has shown that formation of new aerosol particles through coagulationor there exists a size distribution of sea-salt aerosols of radii breakingof the existingparticles is one intersectionaltransfer rangingfrom 0.02 to 60 •m. A knowledgeof size distribution process.Since only dry massof aerosolsis entered into the is necessaryin order to calculatethe effect of the aerosol on massbalance equation (1), the condensation/evaporationof climate as well as on chemicalreactions in the atmosphere. water on sea-saltaerosol particles will not contribute to the In this model, the aerosolsize distributionis simulatedby a intersectionaltransfer rate. However, condensation/evapora- method similar to that developedby Gelbardand Seinfeld tion may contribute to intersectional transfer rate of other [1980].The whole sizerange of aerosolparticles is dividedinto typesof aerosols,for example,sulphate aerosols when sulphu- a number of sections(or size bins), and each size sectionis ric acid vapor condenseson the existingaerosols. representedby one prognosticequation (i) which is a mass In accordancewith the FIZ-C convention,the prognostic continuityequation averagedfor that size range. The calcula- equation is rewritten in the form of tion of sea-salt generation is carried out by integrating the production over each size bin while the physicalproperties 0-7-= at +-bT +7F suchas dry depositionand wet removalrates are calculatedby dynamics surface clear air dry usingthe
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