Ozone decomposition on Saharan dust: an experimental investigation

The heterogeneous reaction between O3 and authentic Saharan dust surfaces was investigated in a Knudsen reactor at≈ 296 K. O3 was destroyed on the dust surface and O2 was formed with conversion efficiencies of 1.0 and 1.3 molecules O2 per O3 molecule destroyed for unheated and heated samples, respectively. No O3 desorbed from exposed dust samples, showing that the uptake was irreversible. The uptake coefficients for the irreversible destruction of O3 on (unheated) Saharan dust surfaces depended on the O3 concentration and varied between 3.5 × 10−4 and 5.5 × 10−6 for the initial uptake coefficient (γ0 ≈ 3 × 10−5 at 30 ppbv O3 STP) and between 4.8 × 10−5 and 2.2 × 10−6 for the steady-state uptake coefficient (γss ≈ 7 × 10−6 at 30 ppbv O3 STP). At very high O3 concentrations the surface was deactivated, and O3 uptake ceased after a certain exposure period. Sample re-activation (i.e. de-passivation) was found to occur over periods of hours, after exposure to O3 had ceased, suggesting that re-activation processes play a role both in the laboratory and in the atmosphere.