posted on 2019-05-31, 00:00authored bySabrina Chee, Nanna Myllys, Kelley C. Barsanti, Bryan M. Wong, James N. Smith
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.