The heterogeneous reaction of ozone with soot aerosol

The loss of ozone on carbon soot aerosol was investigated in the temperature range 238-330 K for reaction times up to 72 h: 200 mu g m(-3) ozone in dry synthetic air is depleted with an initial reaction probability gamma = 1.2 x 10(-6) at 296 K. The reaction probability decreases with a characteristic time tau of about 12 h (1/tau = k(3) = (2.3 +/- 0.6) x 10(-5) s(-1)) due to surface passivation. The reaction rate has a positive temperature dependence, and a complex negative dependence on the ozone concentration. A minimum set of four quasi-elementary reactions is required to model the observed concentration and time dependencies of the ozone loss rate: rapid destruction of one monolayer equivalent of ozone on pristine surface sites (SS): SS + O-3 --> SSO + O-2 (I), ozone-induced recovery of reactive sites: SSO + O-3 --> SS + 2O(2) (or SS' + O-2 + CO2) (IIa, b), spontaneous recovery of reactive sites: SSO --> SS' + CO, and spontaneous site passivation: SSO --> SSp (III). The kinetic parameters gamma(2ab), k(2c), and k(3), which were determined in 11 independent experiments, lead to the following analytical expression for the effective reaction probability gamma(eff) which applies when the fast initial surface oxidation has reached a steady state: gamma(eff) = [2 gamma(2ab) + 2.6 x 10(15) k(2c)/([O-3][c])]exp (-k(3)t). This equation is valid over a wide range of atmospheric conditions. It implies that ozone depletion on dry soot aerosol is negligible both in the troposphere and lower stratosphere. (C) 1999 Elsevier Science Ltd. All rights reserved.