Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia arethought to be the dominant processes responsible for new particle formation (NPF) in the cold temperaturesof the middle and upper troposphere. Ions are also thought to be important for particle nucleation inthese regions. However, global models presently lack experimentally measured NPF rates under controlledlaboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here withdata obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets)chamber, we present the first experimental survey of NPF rates spanning free tropospheric conditions. Theconditions during nucleation cover a temperature range from 208 to 298 K, sulfuric acid concentrationsbet ween 5 × 105and 1 × 109cm3, and ammonia mixing ratios from zero added ammonia, i.e., nominally purebinary, to a maximum of ~1400 parts per trillion by volume (pptv). We performed nucleation studies under pureneutral conditions with zero ions being present in the chamber and at ionization rates of up to 75 ion pairscm3s1to study neutral and ion-induced nucleation. We found that the contribution from ion-inducednucleation is small at temperatures between 208 and 248 K when ammonia is present at several pptv or higher.However, the presence of charges significantly enhances the nucleation rates, especially at 248 K with zeroadded ammonia, and for higher temperatures independent of NH3levels. We compare these experimental datawith calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporationrates obtained from quantum chemistry.