Diffusion Coefficients and Phase Behavior of Biomass Burning Organic Aerosol

F. K. A. GREGSON, N. G. A. Gerrebos, C. E. Schwartz, S. Kamal, C. Carlsten, A. K. Bertram, University of British Columbia

     Abstract Number: 25
     Working Group: Biomass Combustion: Outdoor/Indoor Transport and Indoor Air Quality

Abstract
Emissions produced through biomass burning are among the largest contributors to worldwide organic aerosol, thereby significantly impacting human health and the climate. An improved understanding on the phase behaviour and diffusion coefficients within biomass burning organic aerosol (BBOA) is needed to understand this impact. Diffusion coefficients can affect the growth rates and size distributions of atmospheric aerosol as well as controlling heterogeneous reaction rates and the rate of whitening of the brown carbon in BBOA. The phase behaviour, i.e. how many phases are present in the particles and their morphology, is important for the equilibrium partitioning of semi-volatile organic compounds (SVOCs), heterogeneous reaction rates and whitening rates, as well as influencing the ability of BBOA to activate into cloud droplets.

BBOA emissions can be compositionally complex due to evolution in the atmosphere including through dilution, oxidation and partitioning of SVOCs. As a bottom-up approach towards understanding the properties of particles present in forest fire plumes, we study lab-generated proxies of primary emitted BBOA. In a controlled furnace tube, wood smoke particles are generated from heating wood under smoldering conditions at varying heating temperatures. We observe the phase behaviour of the sampled BBOA by exposing the particles to varying relative humidity (RH) within a flow cell coupled to an optical microscope. We also measure the diffusion coefficients of organic molecules in the collected woodsmoke particles using fluorescence recovery after photobleaching (FRAP). We show that the phase behaviour of primary emitted woodsmoke aerosol varies with RH, with some particles undergoing liquid-liquid phase separation at a separation RH (SRH). We report the diffusion coefficients of the single phase below the SRH, or of the two separate phases above the SRH. These results have important implications for understanding the affects of woodsmoke particles on air quality, the climate and human health.