Heterogeneous chemistry of nitryl halides in relation to tropospheric halogen activation

Abstract

The heterogeneous chemistry of nitryl chloride and nitryl bromide by salt containing solutions was studied as a function of temperature in the range from 275 to 293 K with the wetted-wall flowtube combined with FTIR and mass spectrometry detection. Uptake coefficients and values of the product Hk(1/2) on these saline solutions have been determined. For nitryl halides interacting with NaI and NaBr solutions, the values of the product Hk(1/2) are respectively 4384.7 ± 326.7 and 103.1 ± 18.7 M atm-1 s(-1/2) for nitryl chloride at 275 K and 544.2 ± 94.7 and 47.7 ± 15.2 M atm-1 s(-1/2) for nitryl bromide at 278 K. When reacting with NaI or NaBr solutions, these heterogeneous reactions release, as major products, the molecular forms of the halogen i.e., respectively I2 and Br2. A simplified reaction scheme explaining the formation of these products is presented and is inserted into a model simulating the chemistry in the marine boundary layer. The modelling effort showed Cl and BrO atoms concentrations up to 5 x 104 and 1.8 x 106 molecules cm-3 respectively, which are comparable to values actually measured in field campaigns.