Abstract
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.
Highlights
Atmospheric aerosols are an important focus of environmental research due to their effect on atmosphere, climate and health (Pöschl, 2005)
The selected particles were counted with a condensation particle counter (CPC) and deposited via impaction on a silicon nitride (Si3N4) membrane supported by a Si window frame, which is attached to the removable front piece of the environmental microreactor
Note that the spectra are normalized to the averaged absorption between 310 and 320 eV, i.e., total carbon, so that the peak height of the 1s-π ∗ transition is proportional to the shikimic acid concentration in an individual particle and differences in particle size are accounted for
Summary
Atmospheric aerosols are an important focus of environmental research due to their effect on atmosphere, climate and health (Pöschl, 2005). Diffusion is significantly slower, which is likely to decrease the reaction rate, diminish water uptake, inhibit ice nucleation and influence the formation and growth of secondary organic aerosol (SOA) (Shiraiwa et al, 2011; Zobrist et al, 2011; Mikhailov et al, 2009; Murray, 2008; Perraud et al, 2012; Shiraiwa and Seinfeld, 2012). Formation of amorphous solids and semi-solids should severely slow down or even inhibit reactions This change in reactivity has important implications for the lifetime of particles and their constituents and the particles’ physical and chemical properties. We have utilized our microreactor to observe the ozonolysis of single shikimic acid particles in situ and under different relative humidities, to provide new insights into the dependence of reactivity on physical state.
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