THE DARK DECAY OF HONO IN ENVIRONMENTAL (SMOG) CHAMBERS

The dark decay of nitrous acid gas (HONO), added to filtered outside a ir in a glass/pyrex smog chamber, was studied. For the concentrations used in the study, in the range of 1-10 ppmv, a decay with a half-life value of hours was found. Since the decay rate increased with HONO co ncentration and was quite reproducible, it was analyzed as a second-or der reaction and a rate constant of (1.5 +/- 0.3) 10(-19) cm(3) molec( -1) s(-1) was obtained. This is very similar to a value obtained in a previous study in a chamber with similar wall material. High concentra tions of copper oxide aerosols and NaCl droplets did not measurably ac celerate the decay. There was some removal of HONO by reaction with Na Cl, as evidenced by the formation of aerosol-nitrate, but the contribu tion of this process to the total loss of HONO was negligible. Whereas initially only a limited amount of NO was formed as a result of the d ecay of HONO, the subsequent loss of HONO was via decomposition into N O and NO2 (and H2O). The final concentrations of the gases were in acc ordance with the equilibrium values calculated with the equilibrium co nstant from an earlier study. The decay was also studied at relative h umidities approaching saturation. It was found that the decay varied f rom experiment to experiment but that it was on average a factor of te n faster than at the standard relative humidity of 80%. The accelerati on is ascribed to catalysis of the decay adsorbed water at the walls o f the chamber. (C) 1997 Elsevier Science Ltd.