Figure 2. NA condensation and hygroscopicity of a chemical system of
RNCs-sulfuric acid-nitric acid. (a) The moles of condensed nitrate
against temperature with varying mol fraction of MEA at fixed initial
water vapor of 0.07848 mol (equivalent to 60% RH at 263.15 K and
101,325 Pa in 1 m3). (b) the mole ratio of water and
sulfate in the condensed phases at different RH with varying
amine:ammonia mole ratios at 298K. The reference curve (red solid line)
only has ammonium sulfate [Chan and Chan, 2012]. All curves have the
same total moles of RNCs as 8.86895×10−8 and the same
initial moles of HNO3 and
H2SO4 as
1.11151×10−9 and 2.03777×10−11,
respectively. The percentage in the legends refers to the initial mole
fraction (mol %) of the amine(s) in the total RNCs.
In addition toTc ,
deliquescence relative humidity (DRH) may also play an important role in
the nucleation and growth of particles. The RH at a transition point, at
which the water starts to condense on the particles, is referred as DRH.
Generally, dry ammonium sulfate particles have a DRH
~79%. While ammonium nitrate may uptake water vapor at
much lower temperature, the condensation of ammonia and NA is likely
hindered at room temperature [Wang et. al., 2020]. Figure 2b shows
the water uptake by the condensed phases at 298 K in an ASN system with
additional amines (DMA and MEA representing Group I and II RNCs,
respectively). The chemical compositions of the RNCs and varying
amine:ammonia mol ratios relevant to the atmosphere were investigated.
In all cases, the sulfate all condensed as ammonium sulfate; therefore,
the mol ratio of aqueous water to
sulfate
is directly proportional to the level of water uptake by the condensed
phases. Since ammonia and NA do not condense at 298 K in the system of
ammonia-SA-NA, the DRH of the ASN system without amines is expected to
be 79 % (the DRH of ammonium sulfate) [Chan and Chan, 2012]. Adding
0.01 % or 0.1 % mole fraction of DMA to the system only lowered the
DRH by ~1 %. However, the presence of only 0.01 mol %
MEA can lower the DRH of the condensed phases to ~71 %
(Figure 2b, insert).
In contrast to the limited effects of DMA on particle DRH, MEA plays a
significant role in lowering the DRH at high concentrations. For
example, 0.1 mol % MEA can lower the DRH to 30 %, helping the
particles absorb water in a relatively dry environment. These
observations are consistent with the hygroscopic behavior of aminium
cations in aqueous solutions [Qiu and Zhang, 2012; Clegg et al., 2013;
Rovelli et al., 2017]. As shown in the Figure 2b insert, the system
with
0.01
% MEA and 0.1 % DMA shows somewhat enhanced water uptake by the
condensed phase than the cases with only 0.01 % MEA or 0.1 % DMA;
however, the effect is limited and the presence of 0.01 % MEA was the
main contributor to the lowered DRH. It seems that the effects of amines
on the hygroscopicity of particles formed in the RNCs-SA-NA system are
additive and dominated by Group II RNCs.