Size resolved chemical composition of nanoparticles from reactions of sulfuric acid with ammonia and dimethylamine
Nanoparticle formation and growth driven by acid-base chemistry was investigated by introducing gas-phase sulfuric acid (H2SO4) with ammonia (NH3) or dimethylamine (DMA) into a flow tube reactor. A thermal desorption chemical Ionization mass spectrometer was used to measure the size-resolved chemical composition of H2SO4-DMA and H2SO4- NH3 nanoparticles formed under dry conditions and at 60% relative humidity. In contrast with predictions for bulk aqueous systems, nanoparticles showed a strong size-dependent composition gradient and did not always reach a fully neutralized state in excess of gas-phase base. Smaller particles were more acidic, with an acid:base ratio of 0.7 ± 0.1 and 1.3 ± 0.3 for 8.6 and 9.5 nm H2SO4-DMA particles formed under dry and humid conditions, respectively, and 3.1 ± 0.6 and 3.4 ± 0.3 for 7.5 nm H2SO4-NH3 particles formed under dry and humid conditions, respectively. The acidity of particles generally decreased as particles grew. H2SO4-DMA particles became fully neutralized as they grew to 14 nm, but H2SO4-NH3 particles at 12 nm were still acidic and were never observed to reach bulk sample thermodynamic equilibrium for the experimental conditions in this study. Thermodynamic modeling demonstrated that the observed trends can be reproduced by modifying acid dissociation constants to minimize acid-base chemistry, which may be caused by steric or mixing effects, and by considering volatilization of the neutral base.
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