AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Temperature- and Humidity-Dependent Phase States of Secondary Organic Aerosols
SARAH SUDA PETTERS, Sonia Kreidenweis, Andrew Grieshop, Megan Claflin, Paul Ziemann, Markus Petters, Colorado State University
Abstract Number: 356 Working Group: Aerosols, Clouds, and Climate
Abstract Secondary organic aerosols (SOAs) can exist in amorphous semi-solid or glassy phase states whose viscosity varies with atmospheric temperature and relative humidity (RH). Here we report the viscosity and glass transition temperatures of 100 nm monodisperse SOA particles as a function of RH and temperature. SOAs were generated by dark ozonolysis of a series of monoterpenes in a laminar continuous-flow tube reactor. Monodisperse aerosol viscosity was measured online at different temperatures and RHs using the method of N.E. Rothfuss and M.D. Petters (doi:10.1080/02786826.2016.1221050). The method infers viscosity from the relaxation time scale of synthesized dimers. Glass transition temperatures were obtained by extrapolating the temperature-dependent viscosity in the range of measurement to 1012 Pa·s. The atomic oxygen-to-carbon (O:C) ratio was estimated using parametrizations of mass spectra from an Aerosol Chemical Speciation Monitor (ACSM). The SOA systems underwent an RH-controlled transition from glassy to semi-solid to liquid in a narrow temperature range, outside of which particles were either solid or liquid. Results suggest that pure SOA may be semi-solid or glassy even inside humid cool boundary layers. For each SOA system the weak dependence of viscosity on RH is likely due to the low hygroscopicity of the particles. Glass transition temperatures for five SOA systems varied by about 30°C. Based on prior work correlating functional group composition to viscosity, this is equivalent to the addition of one OH group. Glass transition temperatures decreased with increasing oxidation state, suggesting that molecules were either fragmented or oligomerized during oxidation. To our knowledge this work provides the first measurements of SOA glass transition temperatures using an online technique. SOA phase state diagrams presented in this work may help explain the evaporation and oxidative loss behavior of ambient and laboratory-generated aerosols.