10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Glass Forming Properties of Secondary Organic Aerosol Tracers and Surrogates Examined by Thin Film Dielectric Relaxation Spectroscopy
YUE ZHANG, Shachi Katira, Jason Injae Jung, Peyton Spencer, Andrew Lee, Andrew Lambe, Wen Xu, Leonid Nichman, Manjula Canagaratna, Zhenfa Zhang, Avram Gold, John Jayne, Jason Surratt, Timothy Onasch, Douglas Worsnop, Paul Davidovits, David Chandler, Charles Kolb, University of North Carolina at Chapel Hill
Abstract Number: 1643 Working Group: Aerosol Physics
Abstract Particulate matter (PM) has important effects on the climate, human health, and visibility. Field measurements from different parts of the world show organic components comprise more than half of the total PM mass loadings. Recent studies have shown that some organic particles can transform from a liquid/semi-solid phase state to a glassy phase state as temperature, humidity, and composition change. Glass transitions from liquid/semi-solid to solid phase states have important implications for reactivity, growth, and cloud formation (cloud condensation nuclei and ice nuclei) capabilities of secondary organic aerosols. Glassy particles are less likely to undergo heterogeneous reactions, thereby inhibiting particle growth due to the slow diffusion rates of surface reactants. Glassy particles are also more prone to form ice crystals, thus favoring cirrus clouds, compared with non-glassy particles.
The small size and relatively low mass concentration of SOA in the atmosphere make it difficult to monitor atmospheric SOA glass transitions using conventional methods. To circumvent these difficulties, we have adopted a new technique for measuring glass forming properties of atmospherically relevant organic aerosols. Aerosol particles to be studied are electrostatically deposited in the form of a thin film onto a quartz surface with interdigitated electrodes. The thin films were then analyzed by dielectric spectroscopy, which provides dipole relaxation information related to the motion of the molecules as a function of temperature (373 to 233K) at atmospherically relevant cooling rates. The glass transition temperatures (Tg) of organic aerosol components (glycerol and citric acid) are obtained using this technique, and the results agree well with available literature data.
The glass transition temperatures of isoprene SOA components, including isoprene-derived epoxydiols (IEPOX) and 2-methyltetrols, were measured at three atmospherically relevant cooling rates, 2 K/min, 5K/min, and 10 K/min. The glass transition temperatures of these two compounds range from 162-166 K and 240-245 K, respectively. The results indicate that increasing the cooling rate can reduce the glass transition temperatures of isoprene SOA tracers by 4-5 K. This temperature difference corresponds to an 800-meter height in the ambient atmosphere for the corresponding updraft induced cooling rates. These results underscore the importance of atmospheric variables (such as updraft rates) of organic particles on their phase state and subsequent multiphase reactivity and cloud formation properties.
The glass transition temperatures of water-organic binary mixtures were measured as five mixing ratios. The Gordon-Taylor equation is applied to fit the glass transition temperatures of the mixture, where 136 K is the Tg of pure water. The parameterization of the Gordon-Taylor equation is applied to calculate the effect of relative humidity (RH) and water content on the glass transition temperature of organic aerosols, assuming a growth factor of 1.2 at 90% RH. The glass transition temperature of organic aerosols drops by 15-20 K as the relative humidity changes from <5% to 90%.
In summary, the data obtained with broadband dielectric spectroscopy can be used to characterize glass transitions for both simulated and ambient organic aerosols. The glass transition temperatures of organic aerosols as a function of water content and cooling rates are derived, and their climate effects are interpreted.