Using Model Particle Systems to Constrain Atmospheric Particle "Glassiness" and Mixing Limitations

Wildfires may constitute a positive feedback loop with climate change; the soot particles released in the smoke from these fires have been shown to have a forcing effect on climate, exacerbating the warmer and drier climate in arid regions, resulting in more destructive and more frequent wildfires. Particles released in these wildfires are primarily composed of black carbon and a complex mix of organic compounds. Fire plumes eventually combine with the ambient aerosol population. These initially distinct (externally mixed) aerosol populations are often assumed to quickly form internal mixtures. If ambient aerosols are not able to mix well chemically with fresh biomass burning aerosols, we may need to change our understanding of their health and climate effects. When distinct aerosol populations combine, mixing should happen on a time scale of a few hours if it is not inhibited by the physical characteristics of the particles. Gas-phase exchange between aerosol populations via evaporation and condensation of semi-volatile organics can be a major mechanism of mixing between accumulation-mode particles with slow coagulation. Viscous, semi-solid, or glassy particles may impede this by posing diffusion limitations to such mixing. Here we describe experiments on carefully prepared particle populations representing glassy aged organic particles and fresh biomass burning particles to develop a model phase space for organic aerosol systems and better understand when particle glassiness impedes mixing. We quantify the mixing state of these particle populations using an Aerosol Mass Spectrometer (AMS) in the Event Trigger (ET) and Soot Particle (SP) modes simultaneously. The ET mode of the AMS records single-particle mass spectral data by triggering data acquisition when desired mass-to-charge ratios are detected and the SP mode enables refractory black carbon particles to be characterized. Our results suggest that the non-volatile sugar particles present no diffusive limitations for mixing with erythritol under the conditions tested. Preliminary results suggest that some laboratory generated SOA particles have diffusive limitations to mixing. Our hypothesis is that these limitations are alleviated at some relative humidity threshold, which increases with decreasing ambient temperatures.