AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Effect of Relative Humidity on the Evaporation Kinetics of Alpha-Pinene Secondary Organic Aerosol
ELLIS SHIPLEY ROBINSON, Neil Donahue, Carnegie Mellon University
Abstract Number: 652 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract The effect of relative humidity (RH) on the evaporation kinetics of secondary organic aerosol (SOA) from alpha-pinene ozonolysis was investigated using a two-chamber dilution method. Recent studies have pointed to the slow evaporation of alpha-pinene SOA as indicative of a highly-viscous physical state, and we employ the plasticizing effects of water to test this theory. Our hypothesis was that if particle-phase diffusion were responsible for slow evaporation this diffusion limitation would be eliminated at high relative humidity, where the SOA particles are known to take up substantial water. The evaporation rate, however, does not correlate with RH, and thus the data do not support the idea that diffusion limitations are controlling the evaporation timescale.
Our two-chamber dilution approach allows for near-instantaneous dilution of both particles and gases, which eliminates wall-bound particles as a source of confusion for in-chamber dilution studies. Upon SOA formation, aerosol suspensions were transferred from the reaction chamber into a clean smog chamber, instantaneously diluting the gas-particle mixture by a large, but finite, amount. Measurements of the particle size distribution were taken with a Scanning Mobility Particle Sizer (SMPS) to determine mass-loss due to evaporation as a function size. The chemical composition of the SOA was measured using a High-Resolution Aerosol Mass Spectrometer (HR-AMS).
We present here a new experimental technique for performing dilution experiments on aerosols. Additionally, we demonstrate that a highly viscous, or “glassy,” physical state is not responsible for the observed SOA evaporation rate.