Reactive Uptake of HONO to TiO2 Surface: “Dark” Reaction

International audienceThe interaction of HONO with TiO2 solid films was studied under dark conditions 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 reactive uptake of HONO to TiO2 was studied as a function of HONO concentration ([HONO)0 = (0.3–3.3) × 1012 molecules cm–3), water concentration (RH = 3 × 10–4 to 13%), and temperature (T = 275–320 K). TiO2 surface deactivation upon exposure to HONO was observed. The measured initial uptake coefficient of HONO on TiO2 surface was independent of the HONO concentration and showed slight negative temperature dependence (activation factor = −1405 ± 110 K). In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficient: γ0 = 1.8 × 10–5 (RH)-0.63 (calculated using BET surface area, 40% uncertainty) at T = 300 K. NO2 and NO were observed as products of the HONO reaction with TiO2 surface with sum of their yields corresponding to nearly 100% of the nitrogen mass balance. The yields of the NO and NO2 products were found to be 42 ± 7% and 60 ± 9%, respectively, independent of relative humidity, temperature, and concentration of HONO under experimental conditions used. The contribution of aerosol to the total HONO loss in the boundary layer (calculated with initial uptake data for HONO on TiO2 surface) showed the unimportance of this process in the atmosphere. In addition, the diffusion coefficient of HONO in He was determined to be DHONO-He = 490 ± 50 Torr cm2 s–1 at T = 300 K