Heterogeneous reaction of ozone with hydrocarbon flame soot

Abstract

The reaction of ozone with toluene and kerosene flame soot was studied over the temperature range 240 to 350 K using a low pressure (a few Torr) flow reactor coupled to a modulated molecular beam mass spectrometer. A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well controlled and adjustable combustion conditions. Soot was found to be deactivated in reaction with ozone, the uptake coefficient (γ) being dependent on the time of exposure. The values of (1.8 ± 0.7) × 10⁻⁴ and (3.8 ± 1.5) × 10⁻⁴ independent of temperature in the range 240–350 K were determined for the initial uptake coefficient of ozone on toluene and kerosene soot, respectively. The process of soot ageing (deactivation) was parameterized, the uptake coefficient being expressed as a function of time and gas phase ozone concentration: γ = γ₀/(1 + γ₀k[O₃]t), with temperature independent values of k = (1.1 ± 0.4) × 10⁻¹⁰ and (6.2 ± 2.5) × 10⁻¹¹ cm³molecule⁻¹s⁻¹, for toluene and kerosene soot, respectively. From the soot surface saturation experiments the following maximum number of ozone molecules taken up were determined: ≃7 × 10¹⁴ for toluene and ≃9 × 10¹⁴ molecule cm⁻² for kerosene soot. Experiments on soot ageing confirmed that soot deactivation occurs under real ambient conditions. The present results support current considerations that heterogeneous loss of ozone on soot has negligible impact on ozone concentration throughout the atmosphere.