AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Dependence of Water Transport Kinetics, Reactive Uptake Coefficients and Effective Volatility of Semi-Volatile Organic Components on Organic Aerosol Viscosity
JONATHAN P. REID, Frances Marshall, Young-Chul Song, Stephen Ingram, Allen E. Haddrell, David Topping, Jacqueline Hamilton, University of Bristol
Abstract Number: 95 Working Group: Aerosol Chemistry
Abstract The prevalence of secondary organic aerosol (SOA) in the atmosphere is well established, with chemical components spanning wide ranges in volatility, functionality, molecular weight and hygroscopicity. Departures from equilibrium behaviour have been observed in phase state and gas-particle partitioning of volatile and semi-volatile components, with anticipated impacts on ambient particle mass concentrations, ice nucleation efficiency, oxidation kinetics and optical properties. Improvements to predictions of non-equilibrium behaviour in ambient aerosol are dependent on refined representations of the microphysical properties and processes that aerosol undergo. In particular, the relationship between the viscosity of an aerosol particle and the diffusivity of species within the particle has been the focus of much recent work.
We will report direct measurements of the diffusion constants of water and semi-volatile organic compounds (SVOCs) in surrogates of atmospheric organic aerosol, examining the correlation of these mass transport coefficients with direct and independent measurements of particle viscosity. Briefly, single aerosol particles are trapped in either optical traps or an electrodynamic balance and the time-dependent response of particle size to changes in relative humidity and temperature examined to explore the mass transfer kinetics. Parallel measurements examine the coalescence of pairs of aerosol particles with the timescale for relaxation in shape allowing the determination of particle viscosity. In particular, we will discuss the correlation between diffusion constants of molecular species in aerosols of complex composition, the suppression in the vapour pressure of SVOCs and the reactive uptake coefficient of ozone with the viscosity of the particle phase. These measurements highlight that the Stokes-Einstein equation, relating viscosity with diffusion constants, can be used to provide a good representation of aerosol microphysics under certain conditions but can be many orders of magnitude in error in other instances.