A modeling study of the effect of nitric acid on cloud properties

The effect of nitric acid (HNO3) on cloud microphysical and radiative properties is studied using an adiabatic cloud parcel model for a range of aerosol size distributions, different water vapor mass accommodation coefficients, and HNO3 concentrations. Results show that HNO3 not only increases cloud drop number concentration N-d, but also leads to significantly broader droplet size spectra at both the small- and large-size ends. The broader spectra are generally the result of competition for H2O and HNO3 among the polydisperse droplets. The increase in the number of activated cloud droplets in the presence of HNO3, and the deactivation of some of the small cloud droplets due to the outgasing of HNO3, lead to spectral broadening at the small- size end. At the large-size end the broadening is caused by an increase in the driving force for growth. For small drops the driving force tends to be decreased by the presence of HNO3. Although N-d increases with increasing HNO3 concentration, the increases in cloud optical depth and albedo due to HNO3 cannot necessarily be predicted by the commonly used relationships for cloud optical properties. The dependence of the cloud optical depth on N-d to the one-third power is shown to be an overestimate because droplet spectra are significantly broadened by HNO3. We show that broadening effects due to HNO3 and other chemical or microphysical factors need to be considered when estimating cloud optical properties and their effect on climate.