Single particle measurements of mixing between mimics for biomass burning and aged secondary organic aerosols

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. This exchange may be impeded in highly viscous, semi-solid, or glassy particles due to diffusion limitations. Here we describe experiments on carefully prepared particle populations representing highly viscous or potentially glassy aged organic particles (non-volatile sugars C-13-glucose, sucrose, and raffinose with ammonium sulfate seeds) and fresh biomass burning particles (erythritol with black carbon seeds) to develop a model phase space for organic aerosol systems and better understand when particle phase state impedes mixing. Our hypothesis is that these limitations are alleviated at some relative humidity threshold, which increases with decreasing ambient temperatures. We quantify the mixing state of these particle populations from 10-25 degrees C and 5-90% RH using an Aerosol Mass Spectrometer (AMS) combining Event Trigger (ET) and Soot Particle (SP) modes. The observed single particle mass spectra are aggregated in short time slices and used to perform a linear combination of relevant reference spectra to determine the contributions each constituent has on the resulting particle signal. Our results suggest that the non-volatile sugar particles have little to no diffusive limitations to mixing at the conditions tested.

Automated summary

Gas-phase exchange between aerosols is an important mechanism of mixing particles. However, diffusion limitations can impede this exchange in highly viscous or glassy particles. This study examines the mixing state of different particle populations and suggests that diffusion limitations can be alleviated at a relative humidity threshold, which increases with decreasing ambient temperatures. Non-volatile sugar particles show little to no diffusion limitations under tested conditions.