PH Dependent Photoformation of Hydroxyl Radical and Absorbance of Aqueous-Phase N(III) (HNO2 and NO2-)

Abstract

Ultraviolet−visible (UV−Vis) absorption spectra of aqueous-phase N(III) (HNO2 and NO2-) were studied for the typical pH ranges observed in the atmospheric waters. The molar absorptivity of HNO2 is larger than that of NO2- in UV-A regions. The N(III) molar absorptivity at a specific pH can be determined based on the molar absorptivities and acid−base equilibrium of HNO2 and NO2- (pKa = 3.27). Photochemical formation of hydroxyl radicals (OH radicals) was also studied in aqueous solutions of N(III) for pH values between 1.9 and 6.2 (seven pH values). The OH radical photoformation rate constants showed a distinctive pH dependence, approximately 10-fold higher at pH = 1.9 than at pH = 6.2. The pH-dependent OH radical photoformation also followed the speciation pattern of HNO2 and NO2-. Hydroxyl radical photoformation rate constants (±SE) were estimated to be (3.1 ± 0.08) × 10-4 s-1 for HNO2 and (3.2 ± 0.61) × 10-5 s-1 for NO2- for vernal equinox solar noon conditions at 34° N (Higashi-Hiroshima), using the le...