Variations of cloud droplet concentrations and the optical properties of clouds due to changing hygroscopicity: A model study

The effects of the availability of gaseous nitric acid, hydrochloric acid, and ammonia on cloud droplet formation have been studied using an adiabatic air parcel model including detailed multicomponent condensation. The used preexisting lognormal particle distribution is bimodal in size and in hygroscopicity: four lognormal distributions were used. In each mode particles were assumed to be internally mixed; that is, they were composed partly of salt and partly of an insoluble substance. According to the simulations made, a clear positive correlation between condensable species concentrations and the number of activated cloud droplets exists. Thus trace gases can also influence the optical thickness and reflectance of clouds. The simulations show that ammonia enhances the condensation rate of acids and thus also their effect on the cloud droplet activation significantly. The change in optical thickness varies as a function of the number concentration of preexisting particles, having a maximum (Δτ/τ = 1.6) near a concentration of 1000–2000 cm−3.