Abstract. An improved understanding of the fate and properties of atmospheric aerosol
particles requires a detailed process-level understanding of fundamental
factors influencing the aerosol, including partitioning of aerosol
components between the gas and particle phases. Laboratory experiments with
levitated particles provide a way to study fundamental aerosol processes
over timescales relevant to the multiday lifetime of atmospheric aerosol
particles, in a controlled environment in which various characteristics
relevant to atmospheric aerosol can be prepared (e.g., high
surface-to-volume ratio, highly concentrated or supersaturated solutions,
changes to relative humidity). In this study, the four-carbon unsaturated
compound butenedial, a dialdehyde produced by oxidation of aromatic
compounds that undergoes hydration in the presence of water, was used as a
model organic aerosol component to investigate different factors affecting
gas–particle partitioning, including the role of lower-volatility
“reservoir” species such as hydrates, timescales involved in
equilibration between higher- and lower-volatility forms, and the effect of
inorganic salts. The experimental approach was to use a laboratory system
coupling particle levitation in an electrodynamic balance (EDB) with
particle composition measurement via mass spectrometry (MS). In particular,
by fitting measured evaporation rates to a kinetic model, the effective
vapor pressure was determined for butenedial and compared under different
experimental conditions, including as a function of ambient relative
humidity and the presence of high concentrations of inorganic salts. Even under
dry (RH<5 %) conditions, the evaporation rate of butenedial is
orders of magnitude lower than what would be expected if butenedial existed
purely as a dialdehyde in the particle, implying an equilibrium strongly
favoring hydrated forms and the strong preference of certain dialdehyde
compounds to remain in a hydrated form even under lower water content
conditions. Butenedial exhibits a salting-out effect in the presence of
sodium chloride and sodium sulfate, in contrast to glyoxal. The outcomes of
these experiments are also helpful in guiding the design of future EDB-MS
experiments.
Single-particle experiments measuring humidity and inorganic salt effects on gas-particle partitioning of butenedial
