GROWTH OF FRESHLY NUCLEATED PARTICLES IN THE TROPOSPHERE - ROLES OF NH3, H2SO4, HNO3, AND HCL

Growth of airborn, nanometer-size nuclei was examined in a H2SO4-HNO3- HCl-NH3-H2O system using a single-particle condensation model. Under c onditions typical of the lower troposphere, growth is driven initially by the flux of sulfuric acid, followed by thermodynamic equilibration by the more abundant water and ammonia vapors. Pure sulfuric acid-wat er-ammonia condensation was shown to be rather slow and unable to prod uce cloud condensation nuclei (CCN) in the atmosphere. Under favorable conditions and after reaching a certain size, nuclei may start to abs orb large quantities of nitric or hydrochloric acid together with ammo nia, which produces CCN-size particles over a timescale of a few minut es. The resulting CCN are susceptible to evaporation as the ambient co nditions change but may become stable via cloud processing. The fast, HNO3-HCl-NH3-driven condensation was estimated to occur frequently in continental aerosol systems. Marine CCN production cannot be explained by this process; it requires either the participation of other conden sable vapors, such as organics, or a different growth mechanism.