Mixing times of organic molecules within secondary organic aerosol
particles: a global planetary boundary layer perspective
Adrian M. Maclean1, Christopher L. Butenhoff2, James W. Grayson1, Kelley Barsanti3, Jose L. Jimenez4, and Allan K. Bertram11Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada 2Dept. of Physics, Portland State University, Portland, Oregon 3Department of Chemical and Environmental Engineering and Center for Environmental Research and Technology, University of California, Riverside 4Cooperative Institute for Research in the Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
Received: 16 Mar 2017 – Accepted for review: 22 Mar 2017 – Discussion started: 27 Mar 2017
Abstract. When simulating the formation and life cycle of secondary organic aerosol (SOA) with chemical transport models, it is often assumed that organic molecules are well mixed within SOA particles on the time scale of 1 h. While this assumption has been debated vigorously in the literature, the issue remains unresolved in part due to a lack of information on the mixing times within SOA particles as a function of both temperature and relative humidity. Using laboratory data, meteorological fields and a chemical transport model, we determine how often mixing times are < 1 h within biogenic SOA in the planetary boundary layer (the region of the atmosphere where SOA concentrations are on average the highest). Based on laboratory viscosity measurements, we show that the mixing times are < 1 h most of the time (≥ 94 % of the occurrences) when the SOA concentrations are significant. In addition, we show that a reasonable upper limit to the mixing time for most locations is 30 min. Additional measurements are needed to explore further the effect of oxidation level, oxidation type, and gas-phase precursor on the viscosity and diffusion within biogenic SOA; nevertheless, based on the available laboratory data, the assumption of well mixed SOA in chemical transport models seems reasonable for biogenic SOA in most locations in the planetary boundary layer. On the other hand, slow diffusion in biogenic SOA may still be important in the PBL for heterogeneous chemistry. Slow diffusion in biogenic SOA will also be more important in the free troposphere where both the temperature and RH are lower than in the PBL. Mixing times within anthropogenic SOA can be longer than mixing times within biogenic SOA, at least a room temperature, but additional studies of viscosities or diffusion rates of organic molecules within anthropogenic SOA as a function of both temperature and RH are needed to better constrain how often mixing times are > 1 h within anthropogenic SOA in the PBL.
Maclean, A. M., Butenhoff, C. L., Grayson, J. W., Barsanti, K., Jimenez, J. L., and Bertram, A. K.: Mixing times of organic molecules within secondary organic aerosol
particles: a global planetary boundary layer perspective, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-247, in review, 2017.