Effect of humidity on the production of ozone and other radicals by low-pressure mercury lamps

An ultraviolet (UV) process using a low-pressure mercury lamp is affected by ambient humidity. It is due to strong influence of humidity on the production of ozone and other radicals by the UV light. In this paper, a photochemical reaction model under the irradiation of a low-pressure mercury lamp is developed, and the effect of humidity on the production of ozone and other radicals [O, O ( D 1 ) , O2(a1Δg), O 2 ( b 1 Σ g + ) , OH, HO2, H, and H2O2] by a low-pressure mercury lamp is discussed using the reaction model. The validity of the reaction model is confirmed by comparing the ozone densities calculated using the model with experimentally measured ozone densities, and they showed good agreement. The reaction model shows that the ozone density decreases with increasing humidity for three reasons: (i) attenuation of 185 nm light due to absorption by H2O, leading to a decreased O atom production by O2 + hν(185 nm) ⟶ O + O which is required to produce ozone by O + O2 + M ⟶ O3 + M; (ii) ozone destruction by O 3 + h ν ( 254 nm ) ⟶ O ( D 1 ) + O 2 ( a 1 Δ g ) , where the resulting O ( D 1 ) partly reacts with H2O before converting back to O3 after quenching to O; and (iii) an ozone destruction cycle OH + O3 ⟶ HO2 + O2 and HO2 + O3 ⟶ OH + 2O2. The effect of humidity on the densities of other radicals is also discussed using the reaction model.