Uncertainty in aerosol hygroscopicity resulting from semi-volatile organic compounds

We present a novel method of exploring the effect of uncertainties in aerosol properties on cloud droplet number using existing cloud droplet activation parameterisations. Aerosol properties of a single involatile particle mode are randomly sampled within an uncertainty range and resulting maximum supersaturations and critical diameters calculated using the cloud droplet activation scheme. Hygroscopicity parameters are subsequently derived and the values of the mean and uncertainty are found to be comparable to experimental observations. A recently proposed cloud droplet activation scheme that includes the effects of co-condensation of semi-volatile organic compounds onto a single lognormal mode of involatile particles is also considered. In addition to the uncertainties associated with the involatile particles, concentrations, volatility distributions and chemical composition of the semi-volatile organic compounds are randomly sampled and hygroscopicity parameters are derived using the cloud droplet activation scheme. The inclusion of semi-volatile organic compounds is found to have a significant effect on the hygroscopicity and contributes a large uncertainty. For non-volatile particles that are effective cloud condensation nuclei, the co-condensation of semi-volatile organic compounds reduces their actual hygroscopicity by approximately 25 %. A new concept of an effective hygroscopicity parameter is introduced that can computationally efficiently simulate the effect of semi-volatile organic compounds on cloud droplet number concentration without direct modelling of the organic compounds. These effective hygroscopicities can be as much as a factor of two higher than those of the non-volatile particles onto which the volatile organic compounds condense.