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.

The interaction of HO₂ radicals with solid films of Arizona Test Dust (ATD) was studied using a low-pressure flow reactor (1–9 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake coefficient of HO₂ was measured from the kinetics of HO₂ consumption on Pyrex rods coated with ATD as a function of HO₂ concentration ((0.35–3.30) × 10¹² molecule cm⁻³), relative humidity (RH = 0.02–94%), temperature (T = 275–320 K) and UV irradiance intensity (JNO₂ = 0–0.012 s⁻¹). The initial uptake coefficient was found to be independent of concentration of HO₂, temperature and irradiation conditions, and to decrease with increasing relative humidity: γ₀ = 1.2/(18.7 + RH_1.1) (with 30% estimated conservative uncertainty). The uptake coefficient was calculated using geometric surface area and should be considered as an upper limit of γ₀. An upper limit of 5% was found for the gaseous H₂O₂-forming pathway of the HO₂ reaction with ATD surface. The results of the measurements indicate that HO₂ loss on dust aerosol may be a non-negligible sink for HO_x species in the troposphere with the effect depending on specific local conditions.

The interaction of NO₂ with TiO₂ solid films was studied under UV irradiation using a low pressure flow reactor (1–10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The NO₂ to TiO₂ reactive uptake coefficient was measured from the kinetics of NO₂ loss on TiO₂ coated Pyrex rods as a function of NO₂ concentration, irradiance intensity (JNO₂ = 0.002–0.012 s⁻¹), relative humidity (RH = 0.06–69 %), temperature (T = 275–320 K) and partial pressure of oxygen (0.001–3 Torr). TiO₂ surface deactivation upon exposure to NO₂ was observed. The initial uptake coefficient of NO₂ on illuminated TiO₂ surface (with 90 ppb of NO₂ and JNO₂≅0.006 s⁻¹) was found to be γ₀ = (1.2±0.4) ×10⁻⁴ (calculated using BET surface area) under dry conditions at T = 300 K. The steady state uptake, γ, was several tens of times lower than the initial one, independent of relative humidity, and was found to decrease in the presence of molecular oxygen. In addition, it was shown that γ is not linearly dependent on the photon flux and seems to level off under atmospheric conditions. Finally, the following expression for γ was derived, γ = 2.3×10⁻³ exp(−1910/T)/(1 + P^0.36) (where P is O₂ pressure in Torr), and recommended for atmospheric applications (for any RH, near 90 ppb of NO₂ and JNO₂ = 0.006 s⁻¹).