Heterogeneous reaction of NO with hematite: The effects of UV irradiation, O and relative humidity.

Heterogeneous reactions on mineral dust surfaces are increasingly considered important in the removal of gaseous pollutants and the formation of secondary aerosols. Although the heterogeneous reaction of NO on the hematite surface has been investigated in many previous studies, little is known about the reaction of NO with hematite under ambient conditions. In this work, heterogeneous reactions of NO with hematite at 298 K were investigated via a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The influence of UV illumination, relative humidity (RH) and O on the uptake coefficients and adsorption amount of NO, as well as the nitrate formation on the hematite surface, has been analyzed comprehensively. UV irradiation shows a significant effect on the true uptake coefficient (γ), which increases from 2.00 × 10 to 4.76 × 10 in the N stream and 1.32 × 10 to 4.07 × 10 in the air stream under dry conditions (∼0.3 % RH). RH (in the range of 0-67 %) exhibits an inhibitory effect on the adsorption of NO on the hematite surface because of the competition between NO and water molecules, that is, γ and adsorption amount of NO decrease with an increase in RH under both the dark and light reaction conditions. Meanwhile, both the γ and adsorption amount of NO on hematite decrease in the air stream compared to those in N conditions. In addition, the results from the DRIFTS experiments indicate that the presence of UV irradiation promotes the conversion of NO to nitrate and both the RH and O suppress the nitrate formation. From this research, the heterogeneous reactions between NO with mineral dust under ambient conditions will be better understood.