Cluster-to-particle transition in atmospheric nanoclusters

Abstract. The formation of molecular clusters is an imperative step leading to the formation of new aerosol particles in the atmosphere. However, the point at which a given assembly of molecules represents an atmospheric molecular cluster or a particle remains ambiguous. Applying quantum chemical calculations, we elucidate this cluster-to-particle transition process in atmospherically relevant sulfuric acid–base clusters. We calculate accurate thermodynamic properties of large (SA)n(base)n clusters (n=1–15), with SA being sulfuric acid and the base being either ammonia (AM), methylamine (MA), dimethylamine (DMA) or trimethylamine (TMA). Based on our results, we deduce property-based criteria for defining freshly nucleated particles (FNPs), which act as a boundary between discrete cluster configurations and large particles. We define the onset of FNPs as being when one or more ions are fully solvated inside the cluster and when the gradient of the size-averaged binding free energy approaches zero. This definition easily allows the identification of FNPs and is applicable to particles of arbitrary chemical composition. For the (SA)n(base)n clusters studied here, the cluster-to-particle transition point occurs around 16–20 monomers. We find that the formation of FNPs in the atmosphere depends greatly on the cluster composition and atmospheric conditions. For instance, at low temperature (278.15 K) and high precursor concentration (AM =10 ppb and MA =10 ppt), the SA–AM and SA–MA systems can form clusters that grow to and likely beyond ∼ 1.8 nm sizes. The SA–DMA system forms clusters that grow to larger sizes at low temperature (278.15 K), independent of the concentration (DMA =1–10 ppt), and the SA–TMA system (1:1 acid–base ratio) can only form small clusters that are unable to grow to larger sizes under the studied conditions.