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., Griffin et al., 2005) 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 are 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.
Highlights
A generalised scheme for including the organic components of aerosols in air quality and other atmospheric models, and used in the UCD-CACM model of Griffin, Kleeman and co-workers, is shown in Fig. 1 of Clegg et al (2008)
The methods assessed include both the UNIFAC-based approach of Asher and coworkers, and the Myrdal and Yalkowsky (1997) equation combined with the boiling point equation of Joback and Reid (1987) which we have found yields significantly higher Tb www.atmos-chem-phys.net/8/1087/2008/
The physical properties of polar multifunctional organic compounds, such as those that make up secondary organic aerosol (SOA), are among the most difficult to predict
Summary
A generalised scheme for including the organic components of aerosols in air quality and other atmospheric models, and used in the UCD-CACM model of Griffin, Kleeman and co-workers (where CACM stands for the Caltech Atmospheric Chemistry Mechanism), is shown in Fig. 1 of Clegg et al (2008). Complementary, approach which we adopt is to apply current predictive methods to both the surrogate organic compounds in the UCD-CACM model and the reaction products they represent This enables us (i) to establish approximate ranges of uncertainty of the vapour pressures of compounds present in the model; (ii) to assess the further approximations inherent in grouping multiple compounds into surrogates to which single values of fi and pi◦ are applied and, (iii) to determine (in Paper 1) the significance of uncertainties in terms of gas/aerosol partitioning and SOA formation. The results are relevant, first, to the general development of atmospheric aerosol models based upon an explicit chemistry and corresponding to Fig. 1 in Paper 1, highlighting particular areas in which a better quantitative understanding of the physical chemistry is needed They identify elements of the UCD-CACM model on which future work is likely to focus
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
