JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D17210, doi:10.1029/2009JD013616, 2010 Particle‐resolved simulation of aerosol size, composition, mixing state, and the associated optical and cloud condensation nuclei activation properties in an evolving urban plume Rahul A. Zaveri,1 James C. Barnard,1 Richard C. Easter,1 Nicole Riemer,2 and Matthew West3 Received 25 November 2009; revised 3 April 2010; accepted 12 April 2010; published 11 September 2010. [1] The recently developed particle‐resolved aerosol box model PartMC‐MOSAIC (Particle Monte Carlo model–Model for Simulating Aerosol Interactions and Chemistry) was used to rigorously simulate the evolution of aerosol mixing state and the associated optical and cloud condensation nuclei (CCN) activation properties in an idealized urban plume. The model explicitly resolved the size and composition of individual particles from a number of sources and tracked their evolution due to condensation, evaporation, coagulation, emission, and dilution. The ensemble black carbon (BC)–specific absorption cross section increased by 40% over the course of 2 days due to BC aging by condensation and coagulation. Threefold and fourfold enhancements in CCN/CN ratios were predicted to occur within 6 h for 0.2% and 0.5% supersaturations (S), respectively. The particle‐resolved results were used to evaluate the errors in the optical and CCN activation properties that would be predicted by a conventional sectional framework that assumes monodisperse, internally mixed particles within each bin. This assumption artificially increased the ensemble BC‐specific absorption by 14–30% and decreased the single scattering albedo (SSA) by 0.03–0.07, while the bin resolution had a negligible effect. In contrast, the errors in CCN/CN ratios were sensitive to the bin resolution for a chosen supersaturation. For S = 0.2%, the CCN/CN ratio predicted using 100 internally mixed bins was up to 25% higher than the particle‐resolved results, while it was up to 125% higher using 10 internally mixed bins. Neglecting coagulation overpredicted aerosol water content and number concentrations (<0.2 mm), causing errors in SSA from −0.02 to 0.035 and overprediction of CCN concentrations by 25–80% at S = 0.5%. Citation: Zaveri, R. A., J. C. Barnard, R. C. Easter, N. Riemer, and M. West (2010), Particle‐resolved simulation of aerosol size, composition, mixing state, and the associated optical and cloud condensation nuclei activation properties in an evolving urban plume, J. Geophys. Res., 115, D17210, doi:10.1029/2009JD013616. 1. Introduction natural aerosols, including their number concentration, particle size and size‐dependent composition and mixing [2] Uncertainties in the direct and indirect radiative for- state, optical properties, solubility, and the ability to serve as cings by anthropogenic aerosols estimated by the current nuclei of cloud particles [Ghan and Schwartz, 2007]. generation of global climate models greatly exceeds that of [3] Anthropogenic and natural aerosol particles range from all other forcing mechanisms combined [Houghton et al., a few nanometers to a few microns in size and can be 2001; Forster et al., 2007]. Among other key advances, composed of a wide variety of primary and secondary spe- the next generation of climate models requires improved cies. Primary species are those that are directly emitted into physical and chemical properties of both anthropogenic and the atmosphere such as soot from fossil fuel combustion, industrial particles, cooking particles, sea salt, dust, biomass ‐ 1Atmospheric Sciences and Global Change Division, Pacific Northwest burning particles, etc. Secondary species are various low National Laboratory, Richland, Washington, USA. volatile and semivolatile gas phase oxidation products or 2Department of Atmospheric Sciences, University of Illinois at Urbana‐ direct emissions such as sulfuric acid, nitric acid, ammonia, Champaign, Urbana, Illinois, USA. ‐ 3 and myriad low volatile and semivolatile organic com- Department of Mechanical Science and Engineering, University of pounds that can partition into the particle phase. Secondary Illinois at Urbana‐Champaign, Urbana, Illinois, USA. aerosol particles can also form via homogeneous nucleation Copyright 2010 by the American Geophysical Union. of low volatility gases such as sulfuric acid and large 0148‐0227/10/2009JD013616 molecular weight oxidized organic species. D17210 1of19 D17210 ZAVERI ET AL.: PARTICLE‐RESOLVED MIXING STATE MODELING D17210 [4] Primary particles and particles formed via homoge- sectional model consisting of any number of size distribu- neous nucleation are initially “externally mixed” (i.e., each tions, size bins in each distribution, and components in each type of primary aerosol has a distinct size distribution and distribution and first performed global simulations of aerosol composition). Upon interacting with semivolatile gases in the mixing state using 18 size distributions, with 17 size bins atmosphere via condensation (and evaporation) and with other per distribution. Transfer of particles across bins due to particles of different compositions via coagulation, these condensation and coagulation was calculated with the particles undergo changes in size and chemical composition moving center approach of Jacobson [1997]. Subsequently, and gradually become “internally mixed” to varying degrees. Jacobson [2003] also developed numerical techniques for This process of evolution of aerosol size, composition, and cloud and precipitation development from multiple aerosol mixing state is referred to as “aging” of aerosols. Evidence size distributions. Recently, Bauer et al. [2008] developed a from laboratory, field measurements, and modeling studies treatment of mixing state that used a two‐moment repre- shows that optical properties [Jacobson,2001;Schnaiter et sentation of the 16 modes of different types. Also, Oshima et al., 2003; Kleinman et al., 2007] and cloud condensation al. [2009] applied a mixing‐state resolved version of the nuclei activation properties [Cantrell et al., 2001; Mochida et MADRID (Model of Aerosol Dynamics, Reaction, Ioniza- al., 2006; Kuwata et al., 2007; Medina et al., 2007; Cubison et tion, and Dissolution) sectional box model [Zhang et al., al., 2008; Furutani et al., 2008] are strongly dependent on 2004] to evaluate the effects of aerosol mixing state on aerosol size, composition, and mixing state. optical and CCN activation properties. Their model used a [5] Accurately simulating evolution of aerosol size, com- two‐dimensional aerosol representation of 40 size sections position, and mixing state and the associated optical and ranging from 0.0215 to 10 mm, with black carbon (BC) mass cloud condensation nuclei (CCN) activation properties in a fractions divided into 10 even sections between 0% and mixture of different types of primary and secondary aerosols 100%. Coagulation was ignored, and the transfer of particles is a numerically difficult and computationally expensive across the two‐dimensional bins due to condensation was problem. Sources of uncertainties and errors in predicting calculated with the moving center approach of Jacobson these quantities include (1) incomplete understanding of the [1997]. While considerable progress has been made in this various processes and knowledge gaps in the fundamental area, a rigorous simulation of the evolution of aerosol properties of aerosols and (2) various assumptions and mixing state and its influence on optical and CCN activation numerical simplifications that are typically made in aerosol properties has not been done. models for computational efficiency. In this paper, we focus [7] A computationally efficient approach that can accu- our attention on the effects of assumptions and simplifica- rately represent important aspects of the aerosol mixing state tions made in the representation of aerosol mixing state. is ultimately needed for use in global models. A model that From a computational standpoint, if an aerosol particle can rigorously resolves and simulates the full aerosol mixing contain varying fractions of N chemical components (e.g., state (one that is free of the artificial mixing of composition N = 10 with sulfate, nitrate, chloride, carbonate, ammonium, and numerical diffusion in sectional and modal models) is sodium, calcium, primary organics, secondary organics, also necessary to evaluate and improve simpler and more black carbon), then the mixing state is an N‐dimensional efficient approaches. To this end, we recently developed a space and the size‐resolved particle composition distribution particle‐resolved aerosol box model by coupling the com- is a multivariate function. Thus, ideally an N‐dimensional prehensive aerosol chemistry model MOSAIC (Model for fixed grid sectional model is required to accurately resolve Simulating Aerosol Interactions and Chemistry) [Zaveri et composition of any type of particle that can form due to al. 2008a] with the stochastic aerosol coagulation module condensation and heterocoagulation. A sectional model Particle Monte Carlo model (PartMC) [Riemer et al., 2009]. consisting of, say, 10 size sections (which is fairly coarse) The coupled Lagrangian box model, referred to as PartMC‐ then translates into 10N grid points in the size and species MOSAIC, was applied to study the evolution of soot mixing mass dimensions. However, many comprehensive sectional state due to coagulation and condensation in an idealized aerosol modules have typically considered only one particle urban plume