Kinetics and products of HONO interaction with TiO2 surface under UV irradiation

Abstract

The interaction of HONO with UV irradiated TiO2 solid films was studied using a low pressure flow reactor (1–10 Torr of Helium) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to TiO2 was studied in absence of O2 in the reactor as a function of HONO concentration ([HONO]0 = (0.5–5.0) × 1012 molecule cm−3), relative humidity (RH = 0.001–60%) and temperature (T = 275–320 K). The measured initial uptake coefficient (γ0) of HONO on TiO2 surface was independent of the HONO concentration and showed slight inverse dependence on temperature (activation factor = −1390 ± 150 K) and relative humidity: γ0 = 6.9 × 10−4 (RH)−0.3 (at T = 280 K, calculated using BET surface area, 30% uncertainty). NO2 and NO were observed as products of the HONO reaction with UV irradiated 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 (48 ± 7) and (52 ± 8) %, respectively, independent of relative humidity, temperature and concentration of HONO under experimental conditions used. The HONO loss on mineral aerosol during daytime (calculated with uptake data for HONO on TiO2 surface in presence of UV irradiation) appears to be of minor importance compared with HONO photolysis in the atmosphere.