A comprehensive study on hygroscopic behaviour and nitrate depletion of NaNO3 and dicarboxylic acid mixtures: implications for nitrate depletion in tropospheric aerosols
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Published:2022-08-29
Issue:16
Volume:22
Page:10955-10970
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Ma Shuaishuai,Li Qiong,Zhang Yunhong
Abstract
Abstract. The nitrate depletion and HNO3 release in internally mixed
nitrate and dicarboxylic acid (DCA) particles have been widely detected in
field and laboratory studies. Nevertheless, considerable discrepancies are
still present among these measurements, and the influencing factors for this
acid-displacement reaction have not yet been elucidated. In this work, the
hygroscopic growth and chemical composition evolution of mixtures of
NaNO3 and DCAs, i.e. oxalic acid (OA), malonic acid (MA), and glutaric
acid (GA), were measured using attenuated total reflectance Fourier
transform infrared spectroscopy (ATR-FTIR) and vacuum FTIR techniques. The
nitrate depletion from NaNO3–OA mixtures was observed in both the
measurements, owing to the relatively higher acidity of OA compared to MA
and GA. At the same time, the NaNO3 phase state was found to act as a
key regulator of nitrate depletion. Amorphous NaNO3 solids at relative
humidity (RH) <5 % were inert to liquid OA. With increasing RH,
the mixtures experienced three interesting stages of phase changes showing
different HNO3 release rates; e.g. at around 15 % RH, the slow
HNO3 release was detected by the vacuum IR spectra, potentially
indicating the transformation of amorphous solids to semisolid NaNO3.
In the second stage (sudden RH increase from ∼15 % to
61 %), the HNO3 release rate was increased by about an order of
magnitude. When NaNO3 deliquescence occurred in the third stage, this
displacement reaction proceeded due to more available NO3- ion
formation. Compared to OA, MA and GA reacted with nitrate only in vacuum
FTIR measurement, while in ATR-FTIR measurement, the mixtures tended to be
effloresced completely without nitrate depletion. Further, the influences of
ambient pressure, chemical composition, and water activity on HNO3
release rates were estimated via Maxwell steady-state diffusive mass
transfer equation. The results showed that weaker acidity of MA and GA as
well as relatively lower HNO3 diffusion rate in the ambient gas phase
mainly contributed to the unobserved nitrate depletion in ATR-FTIR
measurement. Our findings reveal that chemical component, phase state, and
water activity of particles, as well as HNO3 gas-phase diffusion, play
crucial roles in HNO3 release from nitrate and DCA mixtures. This work
may provide a new perspective on nitrate depletion in the ageing processes
during transport of tropospheric aerosols.
Funder
National Natural Science Foundation of China
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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