Photoformation rate, steady-state concentration and lifetime of nitric oxide radical (NO) in a eutrophic river in Higashi-Hiroshima, Japan

• Nitrite is a major source of NO photoformation in the Kurose River.
• OH is not a major sink of NO in the Kurose River, scavenging only ⩽0.0011%.
• Steady-state concentration of NO in the Kurose River is about 10−11 M.
• The lifetime of NO in the Kurose River ranges from 0.05 to 1.3 s.

Monthly measurements (January–December 2013) of the photoformation rate, steady-state concentration and lifetime of nitric oxide radical (NO) in the Kurose River in Higashi-Hiroshima City, Japan, were obtained. Each month, river water samples were collected at six different stations (upstream to downstream). NO was quantified using 4, 5-diaminofluorescein-2 (DAF-2) as a probe and triazolofluorescein (DAF-2T) as a standard. Results show that NO photoformation rate ranged from 0.01 to 35.4 (×10−10 M s−1). The radical steady-state concentration in the river ranged from 0.02 to 68.5 (×10−11 M). There was a strong correlation (r2 = 0.95) between NO photoformation rate and the nitrite concentration in the river suggesting that this anion is a major NO precursor. On average, 98% of the photoformed NO came from river nitrite, and this was calculated using the photoformation rate constant {5.7 × 10−5 M(NO)s−1 M(NO2−)−1} of NO from the anion concentration found in the study. The NO lifetime ranged from 0.05 to 1.3 s in the river and remained fairly stable in the upstream and downstream samples. The OH radical, which was quantified during the study, had a photoformation rate of 0.01–13.4 (×10−10 M s−1) and a steady-state concentration of 0.04–119 (×10−16 M) with a lifetime that ranged from 0.3 to 22.5 (×10−6 s). OH only accounted for ⩽0.0011% of the total NO scavenged, showing that it was not a major sink for river NO.