The influence of physical state on shikimic acid ozonolysis: a case for in situ microspectroscopy
- 1Paul Scherrer Institute, Laboratory of Radio- and Environmental Chemistry, 5232 Villigen PSI, Switzerland
- 2Swiss Federal Institute of Technology Zurich, Institute for Atmospheric and Climate Science, 8092 Zurich, Switzerland
- 3Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Department of the Environment, 28040 Madrid, Spain
- 4University of Bern, Department of Chemistry and Biochemistry, 3012 Bern, Switzerland
- *now at: University of Helsinki, Viikki Teacher Training School, 00014 University of Helsinki, Finland
Abstract. Atmospheric soluble organic aerosol material can become solid or semi-solid. Due to increasing viscosity and decreasing diffusivity, this can impact important processes such as gas uptake and reactivity within aerosols containing such substances. This work explores the dependence of shikimic acid ozonolysis on humidity and thereby viscosity. Shikimic acid, a proxy for oxygenated reactive organic material, reacts with O3 in a Criegee-type reaction. We used an environmental microreactor embedded in a scanning transmission X-ray microscope (STXM) to probe this oxidation process. This technique facilitates in situ measurements with single micron-sized particles and allows to obtain near-edge X-ray absorption fine structure (NEXAFS) spectra with high spatial resolution. Thus, the chemical evolution of the interior of the particles can be followed under reaction conditions. The experiments show that the overall degradation rate of shikimic acid is depending on the relative humidity in a way that is controlled by the decreasing diffusivity of ozone with decreasing humidity. This decreasing diffusivity is most likely linked to the increasing viscosity of the shikimic acid–water mixture. The degradation rate was also depending on particle size, most congruent with a reacto-diffusion limited kinetic case where the reaction progresses only in a shallow layer within the bulk. No gradient in the shikimic acid concentration was observed within the bulk material at any humidity indicating that the diffusivity of shikimic acid is still high enough to allow its equilibration throughout the particles on the timescale of hours at higher humidity and that the thickness of the oxidized layer under dry conditions, where the particles are solid, is beyond the resolution of STXM.