Uptake of organic vapours and nitric acid on atmospheric freshly nucleated particles

Abstract. Sulfuric acid, ammonia, and amines are believed to be key contributors to the initial steps in new particle formation in the atmosphere. However, other compounds such as organic compounds or nitric acid are believed to be important for further growth at larger sizes. In this study, we investigate the potential uptake of first-generation oxidation products from α-pinene (pinic and pinonic acid) and isoprene (trans-β-IEPOX, β4-ISPOOH, and β1-ISOPOOH), a potential highly oxidised molecule (HOM), formic acid, and nitric acid. The uptake is probed onto (SA)10(base)10 freshly nucleated particles (FNPs), where SA denotes sulfuric acid, and the bases are ammonia (AM), methylamine (MA), dimethylamine (DMA), or trimethylamine (TMA). The addition free energies were calculated at the ωB97X-D3BJ/6-311++G(3df,3pd)//B97-3c level of theory. We find favourable addition free energies of −8 to −10 kcal mol−1 for the HOM, pinic acid, and pinonic acid on the less sterically hindered (SA)10(AM)10 and (SA)10(MA)10 FNPs. This suggests that isoprene oxidation products do not contribute to the early growth of FNPs, but the α-pinene products do, in accordance with their expected volatilities. Calculating the second addition of a pinic acid molecule or pinonic acid molecule on the (SA)10(AM)10 FNPs, we find that pinic acid maintains its large addition free energy decrease due to its two carboxylic acid groups interacting with the other monomer, as well as the FNP. The pinonic-acid addition free energy drops to −3.9 kcal mol−1 due to the weak interactions between the FNP and its carbonyl group and the lack of monomer–monomer interactions. Calculating the addition free energy under realistic atmospheric conditions, we find that the FNPs studied are too small (1.4 nm) to support the growth of the studied uptake monomers. We find that the accretion product pinyl diaterpenylic ester (PDPE; C17H26O8) yields an addition free energy value of −17.1 kcal mol−1. This suggests that PDPE can overcome the strong Kelvin effect of a 1.4 nm FNP and lead to spontaneous uptake under ambient conditions.