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 H 2 SO 4 ‐HNO 3 ‐HCl‐NH 3 ‐H 2 O system using a single‐particle condensation model. Under conditions 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‐water‐ammonia condensation was shown to be rather slow and unable to produce cloud condensation nuclei (CCN) in the atmosphere. Under favorable conditions and after reaching a certain size, nuclei may start to absorb large quantities of nitric or hydrochloric acid together with ammonia, which produces CCN‐size particles over a timescale of a few minutes. The resulting CCN are susceptible to evaporation as the ambient conditions change but may become stable via cloud processing. The fast, HNO 3 ‐HCl‐NH 3 ‐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 condensable vapors, such as organics, or a different growth mechanism.