Ammonia (NH
3) and short-chain alkyl-amines (R
3N) are alkaline gases that play a crucial
role in the formation and growth of secondary aerosols. Heterogeneous uptake into
atmospheric particles is the dominant removal mechanism of gaseous NH
3 and R
3N. To
date, no experimental data are available on the simultaneous uptake of both gases. While
properties of ammonium sulfates formed during these atmospheric processes have been
well explored, aminium salts often lack a complete characterization, especially in terms of
their hygroscopic parameters.
In this study, the simultaneous uptake of NH
3 and dimethylamine (dimethylammonium,
DMA) into sulfuric acid (H
2SO
4) was measured for different gas molar ratios and relative
humidities. Subsequently, water activities of five R
3N and thei...[
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Ammonia (NH
3) and short-chain alkyl-amines (R
3N) are alkaline gases that play a crucial
role in the formation and growth of secondary aerosols. Heterogeneous uptake into
atmospheric particles is the dominant removal mechanism of gaseous NH
3 and R
3N. To
date, no experimental data are available on the simultaneous uptake of both gases. While
properties of ammonium sulfates formed during these atmospheric processes have been
well explored, aminium salts often lack a complete characterization, especially in terms of
their hygroscopic parameters.
In this study, the simultaneous uptake of NH
3 and dimethylamine (dimethylammonium,
DMA) into sulfuric acid (H
2SO
4) was measured for different gas molar ratios and relative
humidities. Subsequently, water activities of five R
3N and their mixtures with H
2SO
4 were
studied, in order to investigate the difference of hygroscopic characteristics of the formed
aminium and ammonium sulfates. The tendency of R
3N to stick to various surfaces and to
volatilize from their sulfate salt solutions posed major challenges to the practical
implementation of the experiments and were thoroughly addressed in the experimental
design.
The uptake of DMA and NH
3 was strongly dependent on the particle phase state and
extent of neutralization. Uptake into concentrated H
2SO
4 droplets was initially similarly
effective, followed by a favorable uptake of NH
3 into still acidic droplets. At low relative
humidity, the phase transition of partially neutralized particles prevented further DMA uptake, while NH
3 partitioning continued to neutralize the particle. Eventually, NH
3
displaced dimethyl-aminium from the solid neutralized particles to form ammonium
sulfate crystals. Yet, the presence of aminium ions suppressed crystallization, requiring
more NH
3 to partition into the particle before phase change occurred.
At higher relative humidity the particle remained as droplets and DMA and NH
3 did not
compete until the particle was fully neutralized, where DMA (as the stronger base) started
to displace part of the ammonium ions from the droplet.
Investigations of the hygroscopicity of aminium sulfates revealed that water activities of
aminium-to-sulfate ratios of 1:1 (the bisulfate salts) and lower, showed great similarity
with ammonium bisulfate. Aminium sulfates, however, were significantly more
hygroscopic than ammonium sulfate. Thus, the lower the extent of neutralization of the
sulfate, the more similar are the hygroscopic parameters of aminium and ammonium
sulfates.
The extent of neutralization of H
2SO
4 and the particle phase state (solid or liquid)
determine the uptake (and displacement) of R
3N relative to NH
3. The resulting amount of
aminium sulfate relative to ammonium sulfate in the particle, plus the achieved
neutralization ratio in turn significantly influence the hygroscopic properties of the
particle and its phase state.
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