The reaction of nitrogen dioxide at low concentrations with natural waters

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 (H_NO2) 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 $\text{-d[NO}{}_2\text{]}{}_{\mathrm{aq}}\text{dt}\text{=2·k}{}_2\text{·[NO}{}_2\text{]}{}^2{}_{\mathrm{aq}}$

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·H_NO2²) was 1.8 × 10⁴M s⁻¹ atm⁻², the best-fit values for k₂ and H_NO2 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.