AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
The Role of Water in Controlling Heterogeneous Transformations of Viscous Oxygenated Organic Aerosol
JAMES F. DAVIES, Kevin Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 22 Working Group: The Role of Water in Aerosol Chemistry
Abstract Understanding the role played by water in the heterogeneous chemistry and physical dynamics of secondary organic aerosol is important given the large uncertainty in their influence on global climate. Water may influence reactivity directly, due to its role as a reactant, or indirectly, due to the dependence of particle phase and viscosity on water content. Chemical processing also leads to changes in hygroscopicity and, thus, understanding the dynamics of cloud formation is predicated on understanding the chemistry.
Aerosol flow-tube measurements in combination with high resolution mass spectrometry allow the chemical composition of an aerosol to be determined and monitored over the course of chemical processing. In this study, the influence of relative humidity (RH) on the OH-initiated oxidation of organic aerosol is explored and, through measurements on citric acid aerosol, the role of water on the chemistry of model oxygenated organic aerosol is investigated.
At high RH, the chemical decay kinetics exhibit exponential behavior, as expected for well-mixed droplets undergoing oxidative aging. Under low RH conditions, however, non-exponential decay kinetics are observed and a limited portion of the aerosol is reacted, defined by an accessible depth which scales with the viscosity, falling from around 50 nm at 50% RH to 7 nm at 20% RH. The effective reactive uptake coefficient, Gamma$_(eff), also exhibits a complex dependence on RH. We show that Gamma$_(eff) initially increases as the RH decreases from saturation, attributed to the decreasing concentration of water at the surface, before reaching a maximum at around 50% RH. A further decrease in RH results in a sharp decrease in Gamma$_(eff), attributed to the increasing viscosity of the aerosol. An exploration of oxidation products reveals selective reaction of the most mobile species, with functionalized products becoming less available for reaction due to their slower diffusion in viscous solution.