Uptake of Organic Vapors 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-ISOPOH), a potential highly oxidized 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 either 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 favorable addition free energies of −8 to −10 kcal/mol 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 due to the weak interactions between the FNP and its carbonyl group and the lack of monomer–monomer interactions. The high potential for pinic addition is confirmed by calculating the addition free energy at realistic atmospheric conditions. This means that pinic acid has the potential for organic growth on ∼2 nm FNPs, implying that other dicarboxylic acids could potentially also aid in the early growth.