An Experimental and Modeling Study of Nanoparticle Formation and Growth from Dimethylamine and Nitric Acid

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 (HNO₃) and dimethylamine (DMA) in a flow tube under dry (<5% RH) and humid (∼55% RH) conditions. Size-resolved 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 HNO₃-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 (H₂SO₄) and base. Theoretical methods were used to investigate the underlying chemical processes that explain observed differences in the compositions of HNO₃-DMA and H₂SO₄-DMA particles. Calculations of HNO₃-DMA cluster stability indicated that a 1:1 acid/base ratio has >10⁷ 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 HNO₃ and DMA follow the thermodynamic theory, likely because of both components' volatility.