UC Irvine

Atmospheric aerosol particles affect air pollution, public health, and radiative forcing of climate. Their phase state and morphology have far ranging effects on gas uptake, particle phase rate of reaction and aging. Recent reports have demonstrated that atmospheric particles may exist in semi–solid or glassy solid states. The phase transition between these two states occurs at the glass transition temperature (Tg). The goal of Chapters 2 and 4 of this work was to predict the phase state and viscosity of secondary organic aerosol (SOA) mixtures. A modeling approach to predict viscosity as a function of ambient temperature and relative humidity (RH) that uses a parameterization for estimating Tg of organic compounds from molecular composition is presented. The method is applied to α–pinene and isoprene SOA using marker compounds and toluene and diesel-derived SOA using high–resolution mass spectrometry (HRMS) data. Predictions agree well with measured viscosities. Finally, the viscosity of biomass burning SOA is predicted with two sets of HRMS data collected using different ionization techniques, electrospray ionization and atmospheric pressure photoionization, on the same samples. The use of different ionization techniques leads to a difference of ~102 in predicted viscosity at low RH.

Next, measurements of the reactive uptake of dimethylamine (DMA) by ammonium sulfate (AS) and mixed AS–sucrose particles at different RH are simulated with the kinetic multi–layer model of gas particle interactions (KM-GAP) in Chapter 3. Investigations into the role of amines in new particle formation and nano–particle growth are essential to our understanding of cloud condensation nuclei. KM-GAP is well suited to elucidating the mechanisms underlying amine uptake as it explicitly treats the temporal evolution of all species from the gas phase to the particle. Particle mass growth over time and its humidity dependence is successfully reproduced for all experimental samples. Uptake in the mixed AS–sucrose particles is limited by diffusion of DMA and AS through a viscous sucrose–rich shell at lower RH. Uptake coefficients increase when RH increases, but decrease when the molar fraction of sucrose increases at fixed RH. The model is extrapolated to emulate atmospheric conditions. At RH greater than or equal to 70% for liquid particles, amine uptake can lead to a mass increase of approximately 20 to 60%.