AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Parameterizing Vapor Wall Loss Rate in a Teflon Chamber
XUAN ZHANG, Rebecca Schwantes, Hanna Lignell, Matthew Coggon, Richard Flagan, John Seinfeld, Caltech
Abstract Number: 15 Working Group: Aerosol Chemistry
Abstract Understanding the impact of secondary organic aerosol (SOA) on climate change and human health requires an accurate representation of SOA formation from biogenic and anthropogenic emissions. The chamber measured SOA yields from the photochemistry of individual volatile organic compounds (VOC) provide the standard parameterization in atmospheric 3-D models for simulating the SOA budget in regional and global scales. However, field measurements indicate that the SOA abundance is routinely underpredicted when the traditional chamber derived parameterizations are used. One potential reason for this low bias is the unaccounted chamber wall effects, in particular, the deposition of gas-phase SOA precursors onto chamber walls, other than partitioning onto existing particles. In this study, we report the wall loss rates of ~ 30 organic vapors generated from the photochemical oxidation of isoprene, toluene, alpha-pinene, and dodecane at 298 and 318 K. These organic vapors span a wide range in volatilities and oxidation states, thus representative of intermediate- and semi-volatile compounds produced under typical chamber experimental conditions. A vapor wall loss model is developed to describe the observed vapor-wall interactions. We find that the Teflon chamber walls serve as a large reservoir of absorbing organic materials. The partition of vapors on chamber walls is a reversible process, which might be explained by the equilibrium sorption mechanisms of glassy polymers. The accommodation coefficient of a specific vapor on chamber walls is related to its volatility: vapors with lower vapor pressures tend to be more accommodated by the Teflon walls. The impact of vapor wall loss on the SOA yield is evaluated by comparing the equilibrium time scales between vapor-wall and vapor-particle partitioning.