Heterogeneous Interaction of H2O2 with TiO2 Surface under Dark and UV Light Irradiation Conditions

The heterogeneous interaction of H2O2 with TiO2 surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H2O2 ([H2O2](0) = (0.17-120) X 10(12) molecules cm(-3)), irradiance intensity (J(NO2) = 0.002-0.012 s(-1)), relative humidity (RH = 0.003-82%), and temperature (T = 275-320 K). Under dark conditions, a deactivation of TiO2 surface upon exposure to H2O2 was observed, and only initial uptake coefficient of H2O2 was measured, given by the following expression: gamma(0)(dark) = 4.1 X 10(-3)/(1 + RH0.65) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO2 surface, gamma(ss) (UV), was found to be independent of RH and showed a strong inverse dependence on [H2O2] and linear dependence on photon flux. In addition, slight negative temperature dependence, gamma(ss)(UV) = 7.2 x 10(-4) exp[(460 +/- 80)/T], was observed in the temperature range (275-320) K (with [H2O2] approximate to 5 x 10(11) molecules cm(-3) and J(NO2) = 0.012 s(-1)). Experiments with NO addition into the reactive system provided indirect evidence for HO2 radical formation upon H2O2 uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H2O2 with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO2) to be in the range of hours during daytime, i.e.,comparable to H2O2 photolysis lifetime (similar to 1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H2O2 uptake on mineral aerosol on the HOx chemistry in the troposphere.