Abstract
Abstract. The effect of inorganic semivolatile aerosol compounds on the cloud condensation nucleus (CCN) activity of aerosol particles was studied by using a computational model for a DMT-CCN counter, a cloud parcel model for condensation kinetics and experiments to quantify the modelled results. Concentrations of water vapour and semivolatiles as well as aerosol trajectories in the CCN column were calculated by a computational fluid dynamics model. These trajectories and vapour concentrations were then used as an input for the cloud parcel model to simulate mass transfer kinetics of water and semivolatiles between aerosol particles and the gas phase. Two different questions were studied: (1) how big a fraction of semivolatiles is evaporated from particles after entering but before particle activation in the DMT-CCN counter? (2) How much can the CCN activity be increased due to condensation of semivolatiles prior to the maximum water supersaturation in the case of high semivolatile concentration in the gas phase? Both experimental and modelling results show that the evaporation of ammonia and nitric acid from ammonium nitrate particles causes a 10 to 15 nm decrease to the critical particle size in supersaturations between 0.1% and 0.7%. On the other hand, the modelling results also show that condensation of nitric acid or similar vapour can increase the CCN activity of nonvolatile aerosol particles, but a very high gas phase concentration (as compared to typical ambient conditions) would be needed. Overall, it is more likely that the CCN activity of semivolatile aerosol is underestimated than overestimated in the measurements conducted in ambient conditions.
Highlights
Atmospheric aerosol particles are composed of a large variety of chemical compounds
The existing cloud condensation nucleus (CCN) model was modified to include diffusion of semivolatile gases from sample and sheath flows to chamber walls
The CCN model was used to produce aerosol trajectories inside the CCN column, which were later used as an input for the air parcel model to simulate growth of aerosol particles to cloud droplet sizes
Summary
Atmospheric aerosol particles are composed of a large variety of chemical compounds. Some of these can be considered to be nonvolatile, meaning that their vapour pressure is so low that, in practice, they are always found from the particle phase. The other group of compounds found from particles is semivolatile Depending on conditions such as temperature and relative humidity, these compounds may exist in a gas, liquid, or solid particle phase. The Köhler theory was modified to take into account the semivolatile gases present at trace-level concentrations in the atmosphere, and they have been found to affect the cloud condensation nucleus (CCN) activity of aerosol particles
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