The effect of temperature and humidity on the reaction of ozone with combustion soot: Implications for reactivity near the tropopause

The effect of temperature (296-238 K) on the reaction of combustion soot (n-hexane) with ozone at low concentration (6-8 ppm) has been measured. Long optical path FTIR spectroscopy has revealed the rate law for ozone loss beyond initial stages, second order in O-3, to be the same over this range of conditions. The reaction rate is 3.5 times lower at 238 K than at 296 K, and reveals an activation energy of 12.9 +/- 0.5 kJ mol(-1). The effect of humidity on the reaction has been estimated using its recently determined rate law dependence (p(0.2)). These data, differing from O-3 reaction kinetics obtained from other types of carbonaceous particles used as surrogates for atmospheric soot, have implications for the role of combustion soot in atmospheric chemistry. Any involvement of aircraft soot in ozone depletion near the tropopause, for example, should be estimated using these temperature and humidity dependences.