The sensitivity of Secondary Organic Aerosol component partitioning to the predictions of component properties - Part 2: Determination of particle hygroscopicity and its dependence on "apparent" volatility

A large number of calculations of absorptive partitioning of organic compounds have been conducted, making use of several methods to estimate pure component vapour pressures and activity coefficients (p(0) and gamma(i)). The sensitivities of the predicted particle properties (density, hygroscopicity, CCN activation potential) to the choice of p(0) and gamma(i) models and to the number of components used to represent the organic mixture have been systematically compared. The variability in theoretical hygroscopic growth factor attributable to the choice of estimation technique increases with decreasing mixture complexity. Generally there is low sensitivity to the choice of vapour pressure predictive technique. The inclusion of non-ideality is responsible for a larger difference in predicted growth factor, though still relatively minor. Assuming instantaneous equilibration of all semi-volatile on drying the aerosol to 0% RH massively increases the sensitivity. Without such re-equilibration, the calculated growth factors are comparable to the low hygroscopicity of organic material widely measured in the laboratory and atmosphere. Allowing re-equilibration on drying produces a calculated hygroscopicity greater than measured for ambient organic material, and frequently close to those of common inorganic salts. Such a result has substantial implications on aerosol behaviour in instruments designed to measure hygroscopicity and on the degree of equilibration of semi-volatile components in the ambient atmosphere. The impacts of this variability on behaviour of particles as cloud condensation nuclei, on predicted cloud droplet number and uncertainty in radiative forcing are explored. When it is assumed only water evaporates on drying, the sensitivity in radiative forcing, "Delta F" to choice of p(0) and gamma(i) estimation technique is low when the particle organic volume fraction is less than 55%. Sensitivities increase with decreasing component complexity. If all components re-equilibrate on drying, the sensitivity of Delta F increases substantially for organic volume fractions as low as between 16 and 22% depending on the complexity of the organic composition and assumed aerosol size distribution. The current study ignores the impact of predicted changes in particle size which will increase uncertainty in droplet number and forcing.