Mixing and phase partitioning of primary and secondary organic aerosols

Predicting primary and secondary organic aerosol (POA and SOA) concentrations requires understanding the phase partitioning of semi-volatile organic species. A well-mixed single phase organic aerosol can absorb greater amounts of semi-volatile species but little experimental evidence exists on the phase distribution of particulate organics. We investigated the phase partitioning and mixing of semi-volatile POA and SOA in a smog chamber. Particle time of flight (PToF) data from an Aerodyne aerosol mass spectrometer (AMS) were used to quantify the extent of mixing. The SOA plus motor oil and diesel fuel combination produced a weakly mixed system, in which two particulate organic phases coexist. However, the POA in diesel exhaust readily mixed with SOA, forming a single phase after one hour. Although both POA types contain semi-volatile components, there is a fundamental difference in their partitioning behavior with SOA. The high resolution AMS data reveal minor differences in composition between the two types of POA. This work provides further evidence that there exists a set of unidentified components that influence particulate mixing that affect OA formation and suggests the extent of absorbent phase mixing (strong versus weak) can be observed and quantified with PToF data.