Simulating the evolution of soot mixing state with a particle-resolved aerosol model

RIEMER ET AL.: PARTICLE-RESOLVED MIXING STATE MODELING 1 Simulating the evolution of soot mixing state with a particle-resolved aerosol model N. Riemer,1 M. West,2 R. A. Zaveri,3 and R. C. Easter3 Abstract. The mixing state of soot particles in the atmosphere is of crucial importance for assessing their climatic impact, since it governs their chemical reactivity, cloud con- densation nuclei activity and radiative properties. To improve the mixing state repre- sentation in models, we present a new approach, the stochastic particle-resolved model PartMC-MOSAIC, which explicitly resolves the composition of individual particles in a given population of different types of aerosol particles. This approach accurately tracks the evolution of the mixing state of particles due to emission, dilution, condensation and coagulation. To make this direct stochastic particle-based method practical, we imple- mented a new multiscale stochastic coagulation method. With this method we achieved optimal efficiency for applications when the coagulation kernel is highly non-uniform, as is the case for many realistic applications. PartMC-MOSAIC was applied to an ideal- ized urban plume case representative of a large urban area to simulate the evolution of carbonaceous aerosols of different types due to coagulation and condensation. For this urban plume scenario we quantified the individual processes that contribute to the ag- ing of the aerosol distribution, illustrating the capabilities of our modeling approach. The results showed for the first time the multidimensional structure of particle composition, which is usually lost in internally-mixed sectional or modal aerosol models. 1. Introduction Soot particles are an important constituent of the atmo- spheric aerosol, since they participate in tropospheric chem- istry [Saathoffet al., 2001] and affect human pulmonary health [Pope and Dockery, 1996]. Because of its ability to absorb light [Horvath and Trier, 1993], soot is also recog- nized as an important player in the aerosol radiative forcing of climate at global, regional, and local scales [Menon et al., 2002; Chung and Seinfeld, 2005; Roeckner et al., 2006]. The source of soot particles is the incomplete combustion of car- bon containing material, which means that except for natu- ral biomass burning all sources of soot are of anthropogenic origin [Penner, 1995]. The dominant removal process is wet deposition [Ducret and Cachier, 1992]. Soot particles can be transported over long distances reaching remote regions such as the Arctic [Clarke and Noone, 1985; Hansen and Nazarenko, 2004]. The initial composition of soot particles consists of black carbon and organic carbon. The precise mixture depends heavily on the source [Medalia and Rivin, 1982; Andreae and Gelenc´ser, 2006]. While freshly emitted soot particles are rather hydrophobic and present in an external mixture, their hygroscopic qualities can change due to coagulation with soluble aerosols, condensation of secondary organic and inorganic species, and photochemical processes [Weingart- ner et al., 1997]. These processes are usually referred to as “aging,” and they determine the particle growth in re- sponse to ambient relative humidity and the ability to be activated as cloud condensation nuclei. The aging processes also have a profound effect on the aerosol optical properties. 1Department of Atmospheric Science, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA 2Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA 3Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA For example, internally mixed soot shows greater absorptiv- ity compared to externally mixed soot. This effect on radia- tive properties has been studied by a number of investiga- tors, e.g. Ch´ylek et al. [1995]; Jacobson [2001]; Riemer et al. [2003]; Schnaiter et al. [2005]; Bond et al. [2006]. Measure- ments show that atmospheric soot particles are internally mixed with other aerosol species in varying proportions, and that the hydrophobic portion of the aerosols decreases signif- icantly as the distance from the sources increases [Andreae et al., 1986; Levin et al., 1996; Okada and Hitzenberger, 2001; Johnson et al., 2005; Cubison et al., 2008]. Since it is well recognized that soot particles contribute to both the direct and indirect/semi-direct climate effect [Lesins et al., 2002; Jacobson, 2000, 2002b; Nenes et al., 2002], an adequate representation of soot and its mixing state is sought for use in both global and regional mod- els, and the parameterization of soot aging is key to de- termining its atmospheric abundance. Many global models have simulated both (fresh) hydrophobic soot and (aged) hydrophilic soot, which can be considered as a minimal rep- resentation of the soot mixing state. Several of the models have assumed that the conversion from hydrophobic to hy- drophi

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