AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Laboratory Study of Secondary Organic Aerosol Mixing at Low and High Relative Humidity: Implication for Moisture-induced Phase Change
QING YE, Ellis Shipley Robinson, Ryan Sullivan, Neil Donahue, Carnegie Mellon University
Abstract Number: 375 Working Group: Aerosol Chemistry
Abstract Whether secondary organic aerosol (SOA) from different sources can mix into a single phase via vapor uptake within atmospherically relevant timescales is a key question towards understanding how various sources of SOA combine to form air pollution. This mixing process is determined by the thermodynamic properties of SOA, e.g. volatility and activity coefficients of organics in mixtures, as well as kinetic properties, e.g. high viscosity that might hinder diffusion of guest vapors into the particles. This study aims to probe any moisture-induced phase change of SOA and how it affects this mixing process. Laboratory generated alpha-pinene-derived SOA and toluene-derived SOA are used as proxies for biogenic and anthropogenic SOA. A High resolution time-of-flight aerosol mass spectrometer coupled with a light-scattering module that is able to perform single particle measurement is used in this study to probe mixing behavior on a single-particle level. Our results show that under dry conditions, alpha-pinene-derived SOA can uptake vapor from d8-toluene-derived SOA, but alpha-pinene oxidation products do not show up in particles of pure d8-toluene derived SOA. In addition, toluene-derived SOA and d8-toluene-derived SOA do not exchange vapors when it is dry, even though toluene derived vapors do appear in alpha-pinene derived particles. These results demonstrate that alpha-pinene-derived SOA and toluene-derived SOA are thermodynamically miscible and that the toluene SOA system has sufficient volatility to drive mixing, but the toluene-derived SOA exists in a glassy state that impedes mixing. However, at elevated RH, the presence of mass from the guest vapors in toluene-derived SOA indicates the disappearance of diffusion limitation in the particle, suggesting a phase change from a glassy state to a softer, liquid state in the condensed phase. Our finding is crucial for improving description of SOA’s formation, and hence its impact on health and climate.