1 research outputs found
Volatility and Oxidative Aging of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity
The
microphysical structure and heterogeneous oxidation by ozone
of single aerosol particles containing maleic acid (MA) has been studied
using aerosol optical tweezers and cavity enhanced Raman spectroscopy.
The evaporation rate of MA from aqueous droplets has been measured
over a range of relative humidities and the pure component vapor pressure
determined to be (1.7 ± 0.2) × 10<sup>–3</sup> Pa.
Variation in the refractive index (RI) of an aqueous MA droplet with
relative humidity (RH) allowed the subcooled liquid RI of MA to be
estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth
are shown to be consistent with equilibrium model predictions from
previous studies. Simultaneous measurements of the droplet composition,
size, and refractive index have been made during ozonolysis at RHs
in the range 50–80%, providing insight into the volatility
of organic products, changes in the droplet hygroscopicity, and optical
properties. Exposure of the aqueous droplets to ozone leads to the
formation of products with a wide range of volatilities spanning from
involatile to volatile. Reactive uptake coefficients show a weak dependence
on ozone concentration, but no dependence on RH or salt concentration.
The time evolving RI depends significantly on the RH at which the
oxidation proceeds and can even show opposing trends; while the RI
increases with ozone exposure at low relative humidity, the RI decreases
when the oxidation proceeds at high relative humidity. The variations
in RI are broadly consistent with a framework for predicting RIs for
organic components published by Cappa et al. (J. Geophys. Res. 2011, 116, D15204). Once oxidized,
particles are shown to form amorphous phases on drying rather than
crystallization, with slow evaporation kinetics of residual water