Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part 2: Predictions of the vapour pressures of organic compounds

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 vapour pressures of the organic compounds that partition between the aerosol and gas phases. The model of Griffin, Kleeman and co-workers (e.g., 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. In Part 1 of this work (Clegg et al., 2008) the thermodynamic elements of the gas/aerosol partitioning calculation are examined, and the effects of uncertainties and approximations assessed, using a simulation for the South Coast Air Basin around Los Angeles as an example. Here we compare several different methods of predicting vapour pressures of organic compounds, and use the results to determine the likely uncertainties in the vapour pressures of the semi-volatile surrogate species in the model. These are typically an order of magnitude or greater, and are further increased when the fact that each compound represents a range of reaction products (for which vapour pressures can be independently estimated) is taken into account. The effects of the vapour pressure uncertainties associated with the water-soluble semi-volatile species are determined over a wide range of atmospheric liquid water contents. The vapour pressures of the eight primary hydrocarbon surrogate species present in the model, which areCorrespondence to: S. L. Clegg (s.clegg@uea.ac.uk) normally assumed to be involatile, are also predicted. The results suggest that they have vapour pressures high enough to exist in both the aerosol and gas phases under typical atmospheric conditions.

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

“…The normal boiling points used in previous applications of the UCD-CACM model were obtained either from measurements or using the estimation software of Advanced Chemistry Developments (ACD) which is described in a manuscript by Kolovanov and Petrauskas (undated) 1 , (B. L. Hemming, personal communication). Full details, and a comparison with other boiling point and vapour pressure prediction methods, are given in Paper 2 (Clegg et al, 2008).…”

“…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 key thermodynamic properties of these compounds or surrogates -particularly their vapour pressures and activities in the aerosol liquid phase(s) -can be estimated using structure-based methods. We evaluate the uncertainties associated with the vapour pressures of the surrogate compounds in a second study (Clegg et al, 2008), hereafter referred to as Paper 2. This work focuses on the impact on calculated gas/aerosol partitioning of uncertainties in some of the key elements of the thermodynamic treatment of both inorganic and organic aerosol compounds, including: activity coefficients, aerosol water content, solids formation, dissociation equilibria, and the treatment of groups of organic compounds using a reduced number of surrogate species.…”

Solid state and sub-cooled liquid vapour pressures of substituted dicarboxylic acids using Knudsen Effusion Mass Spectrometry (KEMS) and Differential Scanning Calorimetry

“…The subset of vapour pressure estimation methods shown here are those from the E-AIM (Extended Aerosol Inorganics Model) website, http: //www.aim.env.uea.ac.uk/aim/aim.php (Wexler and Clegg, 2002;Clegg et al, 2008). The estimation methods used here first require a boiling point.…”

“…Here we compare our experimental results with the methods available on the E-AIM (Extended Aerosol Inorganics Model) website, http://www.aim.env.uea.ac.uk/aim/aim. php (Wexler and Clegg, 2002;Clegg et al, 2008). These methods use a normal boiling point (T b ) from which the vapour pressure at the required temperature is extrapolated using a vapour pressure equation (referred to here as the vapour pressure method).…”