Imaging Molecular Reaction and Diffusion in Organic Aerosol Particles

Atmospheric aerosol particles are composed of water and a variety of inorganic and organic components stemming from natural and anthropogenic sources Airborne organic matter, in particular, can be in a highly viscous or glassy phase with properties comparable to that of a solid. It was found that water, the atmospheric solvent, diffuses through a glassy organic particle quite slowly and establishes equilibrium with relative humidity only after timescales on the order of days In addition to water, diffusion of oxidizing trace gas molecules such as O 3 through highly viscous organic aerosol can be limited and may lead to an inhomogeneous O 3 concentration and reduced chemical reaction rates compared to what is expected when assuming a homogeneous mixture Reaction and diffusion kinetics of O 3 with organic shikimic acid particles was quantified in viscous organic matter using X-ray microspectroscopy however, any concentration gradients could not be visualized on spatial scale of tens of nanometers Visualizing chemical gradients due to gas-to-particle reaction would allow direct evidence of coupled molecular diffusion and reaction in aerosol particles for the first time, which we find necessary for accurate predictions of atmospheric aerosol chemical aging.