The reaction of nitrogen dioxide at low concentrations with natural waters

Abstract

Abstract The reaction of nitrogen dioxide in air with water, and with a range of aqueous solutions, has been studied by measuring the loss of nitrogen dioxide from the gas-phase after bubbling through a fixed volume of solution. Concentrations of NO2 between 10 and 40 ppbv (1 ppbv = 10−9 atm) were generated using a permeation tube. The characteristic mixing time of the reactor (60 s) was estimated by measuring the increase in conductivity of water as an air stream containing 1% CO2 was passed through the reactor. All measurements were made at 10°C. Values for the second-order reaction rate constant (k2) and the Henry's Law coefficient (HNO2) were estimated from a least-squares fit to an equation which explicitly included the rate of mixing in the reactor. The second-order rate constant was defined from the equation -d[NO 2 ] aq dt =2·k 2 ·[NO 2 ] 2 aq There was no evidence of a parallel first-order reaction of NO2 with water or solutes. The overall reaction rate coefficient for the second-order reaction with water (relative to the gas phase) at 10°C (k2·HNO22) was 1.8 × 104M s−1 atm−2, the best-fit values for k2 and HNO2 were (6.0±2.0) × 106 M−1 s−1 and (5.5±0.6) × 10−2 M atm−1, respectively. Synthetic sea water, made from NaCl alone, or including the other major ionic constituents, showed similar reaction rates to deionized water. A solution of sea sal salt gave an overall reaction rate of 7.4 × 104 M s−1 atm−2, and a sample of coastal sea water gave an overall reaction rate of 15 × 104 M s−1 atm−2, on the assumption of uniform concentration of NO2(aq) in the test solution, which may not be valid. The apparent eight-fold increase in reaction rate for sea water relative to deionized water is, however, not sufficient to increase significantly the rate of removal of NO2 from the atmosphere above the ocean such that atmospheric transport limits the process. Uptake of NO2 is limited by aqueous-phase mixing in the sea surface, with overall deposition velocities unlikely to exceed 0.1 mms−1.