AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Effect of Heterogeneous Uptake of Ammonia on the Properties of Secondary Organic Aerosols Particles
DAVID BELL, Alla Zelenyuk, Dan Imre, Scot Martin, Pacific Northwest National Laboratory
Abstract Number: 621 Working Group: Aerosol Chemistry
Abstract Particle phase and morphology can greatly impact the properties and transformations of secondary organic aerosol (SOA). For example, the rates of heterogeneous reactions strongly depend on particle phase and on whether the condensed-phase reacting substances are on the particle surface or inside the particle. In complex cases, a protective hardened surface layer could be present limiting contact with the gas phase.
We will present the results of a recent study, in which we characterized changes in the properties of α-pinene SOA particles as a result of exposure to ammonia at different relative humidity (RH).
We show that at low RH (<5% RH) the uptake of ammonia by α-pinene SOA particles is consistent with surface adsorption by highly viscous semi-solids, in agreement with previous studies. This surface limited process produces particles with size-dependent composition and density, with smaller particles, having larger fraction of reactive products.
We find that after ~4 hours of exposure to 10 ppm of ammonia, SOA particles developed a solid “crust”, which imposes diffusion limitations on gas-particle mass transfer as apparent from changes in SOA evaporation rates. In addition, we measured the coalescence rates of reacted and un-reacted SOA particles as function of particle size and exposure to ammonia. We find that the formation of this thin surface layer slows particle coalescence rates by a few orders of magnitude. As an aside, we note that the presence of this layer needs to be considered when using coalescence time to determine particle viscosity.
At high RH (>95%RH) all characterized particles are spherical, have orders of magnitude lower viscosity, and contain significantly higher fraction of reactive products.