AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
What is the Meaning of “Non-liquid” or “Liquid” Applied to Secondary Organic Material? Highlighting Differences in the Effects of Absorbed Water on Physical Properties Compared to Chemical Reactivity
SCOT MARTIN, Yong Jie Li, Pengfei Liu, Zhaoheng Gong, Yan Wang, Adam Bateman, Harvard University
Abstract Number: 58 Working Group: The Role of Water in Aerosol Chemistry
Abstract Ammonia uptake was studied for secondary organic material (SOM) of variable viscosity, ranging from non-liquid to liquid physical states. The SOM was produced in aerosol form from six precursors, including three terpenoid and three aromatic species. The viscosity of the hygroscopic SOM was adjusted by increasing the relative humidity (RH) in steps of 10% from <5% to >90% RH at 293 +/- 2 K in a plug-flow configuration. In a follow-on continuously mixed flow reactor, the aerosol was exposed to 5 ppm NH3 for average reaction times of 30, 370, or 5230 s. The ammonium-to-organic ratio of mass concentrations was < 0.03 for toluene SOM below a threshold RH. Above this threshold RH, ammonium-to-organic ratio increased to > 0.03 for toluene SOM, implicating an increase in NH3 chemical diffusivity in the SOM and suggesting a transition to a less diffusion-limited regime. The transition with respect to chemical diffusivity was <5% RH for SOM derived from isoprene (i.e., liquid SOM even to low RH), 40% RH for SOM derived from alpha-pinene, toluene, m-xylene, and 1,3,5-trimethylbenzene, and 80% for SOM derived from beta-caryophyllene. These values related to chemical reactivity differ from the RH transitions for physical behavior. For instance, for alpha-pinene-derived SOM the transition for chemical reactivity of 40% RH can be compared to the transition from rebounding to non-rebounding particles of 80% RH, calling attention to the limitations in inferring chemical behavior from physical behavior. The implication is that chemical transport models directly calibrated to the RH transitions for chemical reactivity could be more accurate than models calibrated to surrogates for reactivity based on physical properties.