Chemical Reactivity and Non-Liquid/Liquid States of Secondary Organic Material (Invited Presentation)

The reactivity of secondary organic material (SOM) of variable viscosity, ranging from non-liquid to liquid physical states, was studied. The SOM was produced as particles of 50 to 150 nm in diameter in aerosol form from six precursors, including three terpenoid and three aromatic species. The viscosity of the hygroscopic SOM was adjusted by exposure to relative humidity (RH) from <5% to >90% RH in steps of 10% at 293 +/- 2 K. The aerosol was subsequently exposed to 5 ppm NH3 for mean reaction times of 30, 370, or 5230 s. The ammonium-to-organic ratio of mass concentrations in the particles increased monotonically from <5% RH to a limiting value at a threshold RH, implicating a switchover in the reaction kinetics from a system limited by diffusivity within the SOM for low RH to one limited by other factors, such as saturated uptake, at higher RH. The switchover RH for reactivity differed in all cases from the transition RH of a non-liquid to a liquid state. The substantial differences in RH for transitions in chemical reactivity compared to those of physical state imply that chemical transport models should not use the non-liquid to liquid transition of SOM as a one-to-one surrogate for SOM reactivity.