A new measurement method for nitrogen oxides in the air using an annular diffusion scrubber coated with titanium dioxide

A new convenient measurement method of nitrogen oxides (NOx) in the ambient air was developed. The collection of NOx is performed by an annular diffusion scrubber coated with a mixture of titanium dioxide (TiO2) and hydroxyapatite (Ca10(PO4)6(OH)2) and the analysis is carried out by ion chromatography with conductivity detection. Under ultraviolet light (UV) illumination, TiO2 produces reactive oxygen species such as super oxide (O2−), hydroxyl radical (OH•) and peroxyhydroxyl radical (HO2•), by which nitric oxide (NO) is oxidized to nitrogen dioxide (NO2), and is further oxidized to nitric acid (HNO3). The yielded HNO3 and NO2 are effectively adsorbed on the surface of TiO2 and hydroxyapatite. The collection efficiencies of NO and NO2 by the annular diffusion scrubber coated with the catalysts under UV illumination are higher than 98%, respectively, at the air flow rate of 0.2–1.0 l min−1. After the collection of NOx, by feeding deionized water into the annular diffusion scrubber, HNO3 and NO2 which adsorbed on the catalysts are extracted as forms of nitrite ion (NO2−) and nitrate ion (NO3−). The extraction efficiencies of NO and NO2 are almost 100%. The activity of the washed catalysts can be completely recovered by drying with the purified air. Further, a simultaneous separated measurement of NO and NO2 can be performed by utilizing the UV illumination dependence. This method was applied to the measurement of NOx in the ambient air. The NOx concentration measured by this method was in good agreement with that obtained using the chemiluminescence NOx analyzer.