AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Ammonia Uptake by Pure and Secondary Organic Aerosol Particles: Dependence on Particle Phase
SCOT MARTIN, Mikinori Kuwata, Ronan Lebouteiller, Harvard University
Abstract Number: 86 Working Group: Aerosol Chemistry
Abstract Chemical aging of organic aerosol particles, which is induced by uptake of reactive molecules, is known to alter their important characteristics, including hygroscopicity and optical properties. Recent studies demonstrated that secondary organic materials (SOM) particles are likely semisolid, raising a question if the uptake process is restricted by slow bulk diffusion. In this study, phase dependent ammonia uptake by pure and secondary organic aerosol particles was investigated. Crystalline pure organic particles were generated using a newly-developed particle generator, and its phase was regulated by controlling water exposure history. Particles were exposed different concentration levels of ammonia in a mixing reactor, and the particle phase ammonium was subsequently quantified using a high-resolution time-of-flight aerosol mass spectrometer (AMS). In addition, comparison of the AMS data with mobility size distribution data provides qualitative information on particle phase and morphology. No measurable ammonia uptake was observed for crystalline particles of the pure organic test compounds, while ammonia uptake was observed for aqueous and semisolid particles. Ammonia uptake by semisolid particles was significantly smaller than that observed for aqueous particles, likely due to slow mass transfer processes. In addition, ammonia uptake by SOM particles originated from biogenic precursors, those are known for their solid-like mechanical properties, were generated using the Harvard Environmental Chamber (HEC). Ammonia uptake by biogenic SOM particles was not a fast process at low relative humidity (RH) condition, consistent with the result for solid and semisolid pure organic particles. Ammonia uptake was enhanced at high RH, likely due to decreased viscosity. The results presented here demonstrate that uptake of reactive species by semisolid SOM particles is limited by slow bulk diffusion.