The abundance and basicity of a stabilizing base have shown to be key factors in sulfuric acid driven atmospheric new-particle formation. However, since experiments indicate that a low concentration of ammonia enhances particle formation from sulfuric acid and dimethylamine, which is a stronger base, there must be additional factors affecting the particle formation efficiency. Using quantum chemistry, we provide a molecular-level explanation for the synergistic effects in sulfuric acid−dimethylamine−ammonia cluster formation. Because of the capability of ammonia to form more intermolecular interactions than dimethylamine, it can act as a bridge-former in sulfuric acid−dimethylamine clusters. In many cluster compositions, ammonia is more likely to be protonated than dimethylamine, although it is a weaker base. By nanoparticle formation rate simulations, we show that due to the synergistic effects, ammonia can increase the particle formation rate by up to 5 orders of magnitude compared to the two-component sulfuric acid−amine system.
The size-resolved composition of nanoparticles formed and grown through acid−base reactive uptake has been studied in the laboratory by reacting gas-phase nitric acid (HNO 3) and dimethylamine (DMA) in a flow tube under dry (<5% RH) and humid (∼55% RH) conditions. Sizeresolved nanoparticle composition was measured by a thermal desorption chemical ionization mass spectrometer over the diameter range of 9−30 nm. The nanoparticle geometric mean diameter grew in the presence of water compared to dry conditions. Acid/base ratios of HNO 3-DMA particles at all measured sizes did not strongly deviate from neutral (1:1) in either RH condition, which contrasts with prior laboratory studies of nanoparticles made from sulfuric acid (H 2 SO 4) and base. Theoretical methods were used to investigate the underlying chemical processes that explain observed differences in the compositions of HNO 3-DMA and H 2 SO 4-DMA particles. Calculations of HNO 3-DMA cluster stability indicated that a 1:1 acid/base ratio has >10 7 smaller evaporation rates than any other acid/base ratio in this system, and measured nanoparticle composition confirm this to be the most stable pathway for growth up to 30 nm particles. This study demonstrates that nanoparticle formation and growth via acid−base reactive uptake of HNO 3 and DMA follow the thermodynamic theory, likely because of both components' volatility.
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