Evolution of organic aerosol mass spectra upon heating: implications for OA phase and partitioning behavior C. D. Cappa1 and K. R. Wilson2 1Department of Civil and Environmental Engineering, University of California, Davis, CA 95616, USA 2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Abstract. Vacuum Ultraviolet (VUV) photoionization mass spectrometry has been used to
measure the evolution of chemical composition for two distinct organic
aerosol types as they are passed through a thermodenuder at different
temperatures. The two organic aerosol types considered are primary
lubricating oil (LO) aerosol and secondary aerosol from the α-pinene + O3 reaction (αP). The evolution of the VUV mass spectra for
the two aerosol types with temperature are observed to differ dramatically.
For LO particles, the spectra exhibit distinct changes with temperature in
which the lower m/z peaks, corresponding to compounds with higher vapor
pressures, disappear more rapidly than the high m/z peaks. In contrast, the
αP aerosol spectrum is essentially unchanged by temperature even
though the particles experience significant mass loss due to evaporation.
The variations in the LO spectra are found to be quantitatively in agreement
with expectations from absorptive partitioning theory whereas the αP
spectra suggest that the evaporation of αP derived aerosol appears
to not be governed by partitioning theory. We postulate that this difference
arises from diffusivity within the αP particles being sufficiently
slow that they do not exhibit the expected liquid-like behavior and perhaps
exist in a glassy state. To reconcile these observations with decades of
aerosol growth measurements, which indicate that OA formation is described
by equilibrium partitioning, we present a conceptual model wherein the
secondary OA is formed and then rapidly converted from an absorbing form to
a non-absorbing form. The results suggest that, although OA growth may be
describable by equilibrium partitioning theory, the properties of organic
aerosol once formed may differ significantly from the properties determined
in the equilibrium framework.
Citation: Cappa, C. D. and Wilson, K. R.: Evolution of organic aerosol mass spectra upon heating: implications for OA phase and partitioning behavior, Atmos. Chem. Phys., 11, 1895-1911, doi:10.5194/acp-11-1895-2011, 2011.