Temperature, Humidity, and Composition - Dependence of Secondary Organic Aerosol Viscosity

Principal Investigator(s):
Paul Ziemann, The Regents of the University of Colorado, Boulder

The overall objective of this research proposal is to develop and constrain computationally efficient expressions to assign the phase state and viscosity for secondary organic aerosol (SOA) over a wide range of atmospheric temperatures (20 < T < -60ºC), relative humidity (0 < RH < 100%), and SOA composition. These expressions are intended to be used in large scale models to delineate regions where gas-to-particle partitioning of SOA is slow (or fast), SOA aerosol water content negligible (or substantial), and heterogeneous ice nucleation on SOA surfaces is favored (or suppressed). The specific objectives are as follows: (1) measure the effects of molecular structure, temperature, and relative humidity on the viscosity of organic aerosol formed via controlled laboratory experiments, (2) assess the critical viscosity where semi-solid SOA surfaces lose their heterogeneous ice nucleation properties as RH increases, and (3) develop computationally inexpensive expressions to demarcate regions where aerosols are liquid, semisolid, glassy, and/or ice nucleation active.

The objectives of the proposed research will be addressed through laboratory studies on model compounds and on lab-generated chemically-complex SOA. Particle generation and composition analysis will be performed at CU Boulder using environmental chambers and spectroscopic methods, respectively. Evaluation of particle phase will utilize the NC State controlled particle coagulation system that can probe aerosol viscosity as a function of T and RH using the shape relaxation technique. Low temperature studies of ice nucleation will utilize the “long” version of the Colorado State Continuous Flow Diffusion Chamber, fitted with a Particle Phase Discriminator.

New measurements of aerosol viscosity as a function of T and RH will be obtained for complex model SOA systems designed to systematically vary molecular structure and inorganic content. A simple scheme for predicting particle phase state and viscosity as a function of T and RH suitable for inclusion in large scale models is proposed and will be tested. More fundamental approaches to predict particle phase state and viscosity based on group contribution methods will also be evaluated. New data on the ice nucleation properties of SOA at cirrus temperatures will be generated. The ice nucleation data will be parameterized using standard formulations in a format that directly feeds into regional and global model simulations. The studies will identify if a critical equilibrium viscosity exists below which heterogeneous ice nucleation is not possible. Experimental results will identify the potential role of molecular structure on ice nucleation. The experiments and modeling efforts will add to a body of literature that is grappling with competing models that hold vastly different views about the underlying process of ice nucleation. Specifically, there are competing and mutually exclusive ideas about how ice nucleation proceeds on organic particles, including ice nucleation induced by immutable special sites that are distributed stochastically over a surface (active-site model), ice nucleation induced at a random location determined by a rate equation and the free energy of the surface (classical heterogeneous nucleation theory), and ice nucleation induced by a compressed organic film on a liquid droplet.