ON THE EXCHANGE OF NO3 RADICALS WITH AQUEOUS-SOLUTIONS - SOLUBILITY AND STICKING COEFFICIENT

The exchange of NO3 radicals with the aqueous-phase was investigated a t room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO3 was determined on 0.1 M NaC l solutions and was found to be gamma(NO3) greater than or equal to 2 x 10(-3) in good. agreement with recent studies. The Henry coefficient of NO3 was estimated to be K-H(NO3) = 1.8 M.atm(-1), with a (2 sigma) uncertainty of +/-3 M.atm(-1). From the upper limit for the Henry coe fficient (K-H = 5 M.atm(-1)) and available thermodynamic data, the red ox potential of dissolved NO3/NO3- is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates . The results are discussed in the light of a recent publication. Base d on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coeffici ent K-H and the rate coefficient k(10) of the potential reaction NO3 H2O --> HNO3 + OH. For K-H = 0.6 M.atm(-1) we find k(10) < 0.05 s(-1) .atm(-1), i.e., about 100 times smaller than what was suggested by Rud ich and co-workers. Because of its small solubility, heterogeneous rem oval of NO3 is only important under conditions where the dissolved NO3 is removed quickly from equilibrium, for example by reactions with Cl - or HSO3- ions in the liquid-phase. Otherwise, heterogenous removal s hould mainly proceed via N2O5.