A modeling study of aqueous production of dicarboxylic acids: 2. Implications for cloud microphysics

We estimate the effect of selected small organic species, present in aerosol particles as internal mixtures with sulphate, on cloud microphysical properties using a numerical model. The initial aerosol compositions were motivated by a prior model study in which it was shown that under certain conditions, small dicarboxylic acids, primarily oxalic and glutaric acids, can be efficiently formed via aqueous chemical processes in clouds and remain in the aerosol fraction upon drop evaporation. The simulations performed here separate the effects of the growth in particle mass via in-cloud oxidation from the change in composition of the resulting aerosol. Although the sulphate/organic mixed particles are somewhat less hygroscopic than pure ammonium sulphate, their activity as cloud condensation nuclei in a simulated constant updraft is similar, and the main influence of prior cloud processing of particles arises from the change in total mass and size distribution. We also performed a separate series of simulations initialized with an aerosol consisting of 90% adipic acid/10% ammonium sulphate, chosen to represent a lower limit on the mixed-particle hygroscopic behavior. If only the reduced solubility of the mixture, relative to that of pure sulphate aerosol, is considered, the drop number concentration is suppressed by up to 50%, depending on the choice of initial conditions. However, the suppression of surface tension due to the presence of the organics, modeled using two different approaches, leads to a compensating effect that can result in little net change to the drop concentration relative to that for the pure sulphate aerosol.