Carbon Dioxide Diffusivity in Single, Levitated Organic Aerosol Particles

The diffusivity of molecules relevant to condensed-phase chemistry within viscous secondary organic aerosol (SOA) remains highly uncertain. Whereas there has been an effort to characterize water diffusivity as well as the diffusivity of larger compounds, data are lacking almost entirely for small molecules, such as carbon dioxide (CO₂). Here we use photochemically generated CO₂ in single particles of aqueous citric acid as a SOA proxy, levitated in an electrodynamic balance, to deduce CO₂ diffusivity in the particle with unprecedented accuracy. For medium viscosities at intermediate relative humidities (∼25-40% RH), we find CO₂ diffusivities D_CO ≈ 10^-14 m² s^-1, agreeing with the Stokes-Einstein relationship based on current viscosity data but 10 times lower than that for water. Conversely, under dry high-viscosity conditions, we find that D_CO ≈ 10^-16 m² s^-1, which is 10 times higher than for water. We infer that the chemical degradation of atmospheric SOA particles will likely not be limited by CO₂ diffusivity.