Mixing and Phase Behavior of Organic Particles

We have developed novel experiments aimed at understanding whether and how quickly organic aerosols (OA) mix using single-particle mass spectrometry, as different treatments of mixing in regional models significantly affect predicted mass and composition. First, we designed experiments that separate OA formation chemistry from thermodynamics to test whether two populations of particles equilibrate with eachother through the gas phase on experimental timescales. Singleparticle mass spectrometry measurements from the aerosol mass spectrometer (AMS) allowed us to quantify the extent of mixing that had occurred. We calibrated this technique using pure-component aerosols with known vapor pressure and phase state, the results of which agreed with a condensation-evaporation model. We then applied these techniques to three atmospherically-relevant situations to determine that: 1) anthropogenic secondary OA (aSOA) does not mix with a surrogate for hydrophobic primary OA (POA), 2) biogenic SOA (bSOA) does not mix with hydophobic POA, and 3) bSOA shows significant mixing with aSOA. The sum of these experiments show that these complex interactions can be measured for atmospherically important systems, a first step towards quantifying activity coefficients for complex OA mixtures. We also investigated mixing within individual particles, using mixed-particles of squalane (a surrogate for hydrophobic POA) and SOA from ↵-pinene + O3 that we determined to contain two separate phases. In these experiments, after formation of the mixed-particles, we perturbed smog chamber with a heat ramp. These data revealed that squalane is able to quickly evaporate from the mixed-particles, and that almost all of the SOA is comprised of material lower in volatility than squalane (a low-volatility constituent of pump oil). For this latter “comparative volatility analysis,” we had to correct for the highly variable collection efficiency (CE) of the mixed particles to correctly calculate the mass fraction of SOA remaining. One of the larger implications of this work is highly dependent on the particle morphology, which we were not able to determine definitively: if indeed the particles are coreshell with squalane inside a thick layer of SOA, our results show that diffusivity within SOA is not ultra-low. Lastly, we present work that furthers our understanding of single-particle CE in the AMS, a quantity especially important for experiments where particle phase is dynamic or there are two separate populations of particles. We report the particle CE of SOA, ammonium sulfate, ammonium nitrate, and squalane. We also determine that half of SOA particles that give meaningful signal, do so at a time later than would be predicted based on their optically-measured flight time through the