AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Organic Aerosol Mixing Observed By Single Particle Mass Spectrometry
ELLIS SHIPLEY ROBINSON, Rawad Saleh, Neil Donahue, Carnegie Mellon University
Abstract Number: 684 Working Group: Aerosol Chemistry
Abstract Using single-particle mass spectra from the High-Resolution Time-of-Flight Aerosol Mass Spectrometer, direct measurements of vapor-driven mixing between organic aerosol populations are presented. Multiple-chamber mixing experiments between docosane and D46-docosane (22 C linear alkane, crystalline-solid at room temperature) showed no mixing at room temperature, while squalane (30 C branched alkane, liquid at room temperature) and D62-squalane mixed on the expected timescale from our condensational mixing model. Docosane and D46-docosane particles were driven to mix when the temperature of the mixing chamber was elevated above the melting point of docosane. In a mixing experiment at room temperature with populations of docosane and squalane, we show that docosane vapors were absorbed by squalane particles, but squalane vapors were not absorbed by docosane particles. We attribute these results to low-diffusivity in the crystalline docosane particles. Mixing experiments were also performed on surrogate systems for atmospheric primary and secondary organic aerosol (POA and SOA, respectively). SOA derived from D8-toluene oxidation (a surrogate for anthropogenic SOA (aSOA)) did not mix with squalane (a surrogate for hydrophobic POA (hPOA)), but does mix into SOA derived from alpha-pinene ozonolysis. Because aSOA/hPOA are not limited in volatility, we hypothesize that non-mixing in this system is due to immiscibility. In the aSOA/bSOA system, the presence of d8-toluene derived SOA molecules in the alpha-pinene derived SOA provides evidence that the diffusion coefficient in alpha-pinene derived SOA is high enough for mixing on the timescale of 1 minute. These observations are generally invisible to bulk aerosol measurements, which underscores the importance of single-particle composition data for understanding mixing processes.