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

• Published in: Environmental science & technology 1999-08, Vol.33 (15), p.2561-2565
• Main Authors: Arakaki, Takemitsu, Miyake, Takayuki, Hirakawa, Tsuyoshi, Sakugawa, Hiroshi
• Format: Article
• Language: English
• Subjects: 540120 - Environment, Atmospheric- Chemicals Monitoring & Transport- (1990-); AIR POLLUTION; Atmosphere; ATMOSPHERIC CHEMISTRY; CHALCOGENIDES; Chemical composition and interactions. Ionic interactions and processes; CHEMICAL REACTIONS; CHEMISTRY; Earth, ocean, space; ENVIRONMENTAL SCIENCES; Exact sciences and technology; External geophysics; HYDROGEN COMPOUNDS; HYDROXYL RADICALS; INORGANIC ACIDS; Meteorology; NITROGEN COMPOUNDS; NITROGEN DIOXIDE; NITROGEN OXIDES; NITROUS ACID; OXIDES; OXYGEN COMPOUNDS; PH VALUE; PHOTOCHEMICAL REACTIONS; POLLUTION; RADICALS
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es980762i

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 - (pK a = 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 least-squares best-fit analysis. We further performed a model calculation to elucidate the significance of N(III) photolysis in the atmospheric hydrometeors, assuming that the dissolution of gaseous HONO was the only source of N(III). We found that photolysis of aqueous-phase N(III) could play a significant role in initiating oxidation reactions in atmospheric hydrometeors (i.e., dew) exhibiting higher pH and N(III) concentrations.