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Characteristics and mixing state of amine-containing particles at a rural site in the Pearl River Delta, China

Abstract. Particulate amines play an important role for the particle acidity
and hygroscopicity and also contribute to secondary organic aerosol mass. We
investigated the sources and mixing states of particulate amines using a
single-particle aerosol mass spectrometer (SPAMS) during summer and winter
2014 at a rural site in the Pearl River Delta, China. Amine-containing
particles accounted for 11.1 and 9.4 % of the total detected
individual particles in summer and winter, respectively. Although the
increase in amine-containing particle counts mostly occurred at night, no
obvious correlations between amine-containing particles and ambient relative
humidity (RH) were found during the sampling period. Among the three markers
we considered, the most abundant amine marker was
74(C2H5)2NH2+, which was detected in 90
and 86 % of amine-containing particles in summer and winter, followed by
amine marker ions of 59(CH3)3N+, and
86(C2H5)2NCH2+ which were detected in less
than 10 % of amine-containing particles during sampling period. The
amine-containing particles were characterized by high fractions of
carbonaceous marker ions, carbon–nitrogen fragments, sulfate, and nitrate in
both summer and winter. More than 90 % of amine-containing particles were
found to be internally mixed with sulfate throughout the sampling period,
while the percentage of amine particles containing nitrate increased from
43 % in summer to 69 % in winter. Robust correlations between the peak
intensities of amines, sulfate, and nitrate were observed, suggesting the
possible formation of aminium sulfate and nitrate salts. Interestingly, only
8 % of amine particles contained ammonium in summer, while the percentage
increased dramatically to 54 % in winter, indicating a relatively
ammonium-poor state in summer and an ammonium-rich state in winter. The
total ammonium-containing particles were investigated and showed a much
lower abundance in ambient particles in summer (3.6 %) than that in winter
(32.6 %), which suggests the ammonium-poor state of amine-containing
particles in summer may be related to the lower abundance of
ammonia/ammonium in gas and particle phases. In addition, higher abundance of
amines in ammonium-containing particles than that of ammonium in
amine-containing particles suggests a possible contribution of
ammonium–amine exchange reactions to the low abundance of ammonium in
amine-containing particles at high ambient RH (72 ± 13 %) in
summer. The particle acidity of amine-containing particles is estimated via
the relative acidity ratio (Ra), which is defined as the ratio of the
sum of the sulfate and nitrate peak areas divided by the ammonium peak area.
The Ra was 326 ± 326 in summer and 31 ± 13 in winter,
indicating that the amine-containing particles were more acidic in summer
than in winter. However, after including amines along with the ammonium in
the acidity calculation, the new Ra′ values showed no seasonal change
in summer (11 ± 4) and winter (10 ± 2), which suggests that
amines could be a buffer for the particle acidity of ammonium-poor
particles.
