Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part 1: Treatment of inorganic electrolytes and organic compounds in the condensed phase

Abstract

Abstract. Air quality models that generate the concentrations of semi-volatile and other condensable organic compounds using an explicit reaction mechanism require estimates of the physical and thermodynamic properties of the compounds that affect gas/aerosol partitioning: vapour pressure (as a subcooled liquid), and activity coefficients in the aerosol phase. The model of Griffin, Kleeman and coworkers (e.g., Griffin et al., 2003;Kleeman et al., 1999) assumes that aerosol particles consist of an aqueous phase, containing inorganic electrolytes and soluble organic compounds, and a hydrophobic phase containing mainly primary hydrocarbon material. Thirty eight semi-volatile reaction products are grouped into ten surrogate species which partition between the gas phase and both phases in the aerosol. Activity coefficients of the organic compounds are calculated using UNIFAC. In a companion paper (Clegg et al., 2008) we examine the likely uncertainties in the vapour pressures of the semi-volatile compounds and their effects on partitioning over a range of atmospheric relative humidities. In this work a simulation for the South Coast Air Basin surrounding Los Angeles, using lower vapour pressures of the semi-volatile surrogate compounds consistent with estimated uncertainties in the boiling points on which they are based, yields a doubling of the predicted 24-h average secondary organic aerosol concentrations. The dependency of organic compound partitioning on the treatment of inorganic electrolytes in the air quality model, and the performance of this component of the model, are determined by analysing the results of a trajecCorrespondence to: S. L. Clegg (s.clegg@uea.ac.uk) tory calculation using an extended version of the Aerosol Inorganics Model of Wexler and Clegg (2002). Simplifications are identified where substantial efficiency gains can be made, principally: the omission of dissociation of the organic acid surrogates; restriction of aerosol organic compounds to one of the two phases (aqueous or hydrophobic) where equilibrium calculations suggest partitioning strongly in either direction; a single calculation of activity coefficients of the organic compounds for simulations where they are determined by the presence of one component at high concentration in either phase (i.e., water in the aqueous phase, or a hydrocarbon surrogate compound P8 in the hydrophobic phase) and are therefore almost invariant. The implications of the results for the development of aerosol models are discussed.

General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales

“…env.uea.ac.uk/aim/aim.php/) was developed as the benchmark code for evaluation of the activity coefficients in the partitioning module. Combination of inorganic and organic activity coefficients was implemented (Clegg et al, 2008b) as well as user specification of organic compounds/surrogate properties and vapour pressure estimation. The E-AIM model can now calculate densities (Clegg and Wexler, 2011a, b) and particle surface tensions (Dutcher et al, 2010) and reference thermodynamic data for inclusion of amines (Ge et al, 2011) Topping et al (2009) and Topping et al (2011b) for inorganics and organics respectively.…”

Computation of liquid-liquid equilibria and phase stabilities: implications for RH-dependent gas/particle partitioning of organic-inorganic aerosols

“…Therefore, vapour pressure estimation methods based on molecular structures and boiling point temperatures (if available) are often the only way to obtain p • j for such semivolatile compounds. This constitutes one of the major sources of uncertainty in gas/particle partitioning computations (Camredon and Aumont, 2006;Clegg et al, 2008;Hallquist et al, 2009;Barley and McFiggans, 2010).…”

“…While the pure compound vapour pressures of water, glycerol, and 1,6-hexanediol are based on measurements and therefore assumed accurate, no measurements are available for 1,2,5,8-octanetetrol and 1,2,10-decanetriol. For this reason, we use the molecular structurebased vapour pressure estimation methods accessible at the Extended AIM Aerosol Thermodynamics Model (E-AIM) website (Clegg et al, 2008) (http://www.aim.env.uea.ac.uk/ aim/aim.php). That is: boiling point estimation by Nannoolal et al (2004) A semi-closed system of six components at constant temperature and gas-phase volume is 6-dimensional, with five degrees of freedom regarding mixture compositions.…”

Hygroscopicity distribution concept for measurement data analysis and modeling of aerosol particle mixing state with regard to hygroscopic growth and CCN activation

“…Similar distribution functions can also be defined and applied with other hygroscopicity parameters such as equivalent ion densities Wex et al, 2007) or equivalent soluble fractions of ammonium sulfate or bisulfate as used in many earlier studies (Gunthe et al, 2009 and references therein). Moreover, similar formalisms could also be based on the van't Hoff factor (McDonald, 1953) or the product of the stoichiometric dissociation number and osmotic coefficient of the solute (Robinson and Stokes, 1959;; and references therein) averaged over all chemical components of a particle according to mixing rules (e.g., the Zdanovski-Stokes-Robinson approximation), or more advanced models taking into account complex solute interactions and concentration dependencies (e.g., the extended aerosol inorganic model of Clegg et al, 2008).…”