AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Equilibration Time Scales of Secondary Organic Aerosol from Alpha-pinene Ozonolysis
RAWAD SALEH, Allen Robinson, Carnegie Mellon University
Abstract Number: 435 Working Group: Aerosol Chemistry
Abstract Most chemical transport models assume instantaneous equilibrium to represent gas-particle partitioning of atmospheric aerosols. Recently, this approach has been challenged by studies which suggest that biogenic secondary organic aerosol (SOA) cannot reach equilibrium within atmospheric timescales. The emergent hypothesis is that evaporation/condensation rates are limited by the mass transfer rate (diffusion) within the condensed phase, which is thought to be “glassy”. There are two issues that might bias these findings: 1) they are based on measurement of evaporation rates (not equilibration time scales), which are sensitive to the assumption on SOA volatility; and/or 2) the SOA loadings are much higher than typical atmospheric conditions (hundreds of micrograms/m$^3) .
Here, we investigate the equilibration time scales of SOA from alpha-pinene ozonolysis by measuring the dynamic response to a step-change in temperature (14 C to 30 C). Upon heating, equilibrium is disturbed, and the particles evaporate to restore equilibrium at the new temperature, which is said to be attained when evaporation ceases. This method requires no prior knowledge of volatility. Experiments were performed in a flow-tube, with SOA loading of 40-50 micrograms/m$^3, and in a smog chamber, with SOA loading of 5 micrograms/m$^3. Both experiments exhibited dynamic responses consistent with an accommodation coefficient on the order of 10$^(-1). For a wide range of typical atmospheric conditions, this translates into equilibration time scales on the order of 10 minutes.