Increased steady state uptake of ozone on soot due to UV/Vis radiation

[1] Oxidative aging of soot particles in the atmosphere may contribute significantly to their climate and human health effects. With their large specific surface area, soot particles are an ideal substrate for heterogeneous reactions. However, these reactions are typically of limited relevance due to deactivation of the surface. Here we show that light in the UV-A and visible (Vis) range significantly enhances the reactivity of soot with ozone. In these laboratory experiments, soot from a propane burner was first “passivated” with ozone (ozone mixing ratio of 300 pbb) for 15 h before ozone uptake was measured in presence of UV radiation. Passivated soot was shown to be reactivated during irradiation with a 4 times higher steady state ozone uptake rate as compared to dark conditions. The uptake increased even by a factor of 15 when humidity (95% relative humidity) was introduced into the system. Possible mechanisms of ozone uptake are discussed. Further analysis using near edge X-ray absorption spectroscopy and contact angle measurements showed a decrease of oxygenated functional groups in the soot upon exposure to ozone and ozone in combination with light.