posted on 2015-12-17, 00:22authored byGregory
P. Schill, David O. De Haan, Margaret A. Tolbert
In this study, we have explored the
phase behavior and the ice
nucleation properties of secondary organic aerosol made from aqueous
processing (aqSOA). AqSOA was made from the dark reactions of methylglyoxal
with methylamine in simulated evaporated cloud droplets. The resulting
particles were probed from 215 to 250 K using Raman spectroscopy coupled
to an environmental cell. We find these particles are in a semisolid
or glassy state based upon their behavior when exposed to mechanical
pressure as well as their flow behavior. Further, we find that these
aqSOA particles are poor depositional ice nuclei, in contrast to previous
studies on simple mixtures of glassy organics. Additionally, we have
studied the effect of ammonium sulfate on the phase, morphology, and
ice nucleation behavior of the aqSOA. We find that the plasticizing
effect of ammonium sulfate lowers the viscosity of the aqSOA, allowing
the ammonium sulfate to effloresce within the aqSOA matrix. Upon humidification,
the aqSOA matrix liquefies before it can depositionally nucleate ice,
and the effloresced ammonium sulfate can act as an immersion mode
ice nucleus. This change in the mode of nucleation is accompanied
by an increase in the overall ice nucleation efficiency of the aqSOA
particles.