Ozone decomposition on Saharan dust: an experimental investigation

The heterogeneous reaction between O-3 and authentic Saharan dust surfaces was investigated in a Knudsen reactor at; approximate to296 K. O-3 was destroyed on the dust surface and O-2 was formed with conversion efficiencies of 1.0 and 1.3 molecules O-2 per O-3 molecule destroyed for unheated and heated samples, respectively. No O-3 desorbed from exposed dust samples, showing that the uptake was irreversible. The uptake coefficients for the irreversible destruction of O-3 on (unheated) Saharan dust surfaces depended on the O-3 concentration and varied between 15 X 10(-4) and 5.5 x 10 6 for the initial uptake coefficient (gamma(0) approximate to 3 x 10(-5) at 30 ppbv O-3 STP) and between 4.8 x 10(-5) and 2.2 x 10(-6) for the steady-state uptake coefficient (gamma(SS) approximate to 7 x 10(-6) at 30 ppbv O-3 STP). At very high O-3 concentrations the surface was deactivated, and O-3 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 O-3 had ceased, suggesting that re-activation processes play a role both in the laboratory and in the atmosphere.