Nitric acid, a highly soluble trace gas generated in the atmosphere by photochemical processes, affects the behavior of aerosol and cloud particles because of its strong affinity for water. The lowered water activity of solution droplets containing nitric acid enhances their condensational growth and retards their evaporation. Several numerical models have recently included the water-nitric acid interaction to demonstrate the impacts nitric acid uptake can have on cloud formation, the chemical properties and microstructure of aerosols, and particle lifetimes. However, the modeling of such phenomena involves several fundamental assumptions regarding both the kinetics of particle growth and the chemical equilibria in concentrated solutions. This study provides some much-needed verification of the physics underlying the phenomena.

The evaporation and growth of individual solution droplets influenced by nitric acid have been investigated experimentally under well-controlled environmental conditions in an electrodynamic levitation system. Droplets in the radius range of 15 to 30 microns were launched into a gaseous environment that was intentionally out of equilibrium with the droplet properties in order to learn quantitatively how they adjust to new conditions. The time-dependent sizes of the droplets were measured to high precision via Mie-scattering techniques and compared with calculations from a kinetic mass growth model coupled to a chemical thermodynamic model.

The results demonstrate that the calculated droplet sizes fit the data extremely well, thus validating the coupling of a detailed model of droplet chemistry with a commonly used kinetic model for mass and energy transfer. The model, once validated, serves as a valuable diagnostic tool that helps us understand the changing properties of the droplets. We find that adjustment to new environmental conditions occurs in two stages, one dictated by the transport of water, the other by the exchange of nitric acid. The overall kinetics of droplet evaporation and growth is, however, limited by the presence of the nitric acid, which equilibrates slowly compared to water.