ATMOSPHERIC CHEMISTRY OF HONO

other reactions of N02 that could potentially occur in the atmosphere include the reaction with components of sea salt particles such as NaCl and NaBr (e.g., Robbins et al., 1959 Cadle and Robbins, 1960 Schroeder and Urone, 1974 Chung et al., 1978 Sverdrup and Kuhlman, 1980 Finlayson-Pitts, 1983 Zetzsch, 1987 Mamane and Gottlieb, 1990 Winkler et al., 1991 Junkermann and Ibusuki, 1992 Vogt and Finlayson-Pitts, 1994 Karlsson and Ljungstrom, 1995 Vogt et al., 1996 Peters and Ewing, 1996 De Haan et al., 1999)  

Both gaseous products, nitrosyl chloride and nitrosyl bromide, absorb light strongly in the visible (Chapter 4) and hence photolyze readily at dawn, producing atomic chlorine and bromine, respectively. 

However, these reactions are also second order in N02 and appear to be too slow at atmospheric NOz levels to be important (Vogt and Finlayson-Pitts, 1994 Peters and Ewing, 1996). There is one aspect of the mechanism that is quite interesting, however, in that the mechanism appears to be at least in part a stepwise process involving the formation of a radical anion intermediate, C1---N02 in the solid, which has been identified by electron paramagnetic resonance (EPR) in both the NaCl and NaBr reactions at room temperature (Wan et al., 1996). This intermediate appears to be remarkably stable and may be responsible for synergistic health effects observed when rats were exposed to a combination of N02 and NaCl aerosols (Last and Warren, 1987). 

N02 has also been observed to be taken up on mineral oxides of types commonly found in particles in the atmosphere. For example, Miller and Grassian (1998) exposed powders of A1203 and TiOz to N02 in both the presence and the absence of water on the surface. At low N02 concentrations (e.g., 5 mTorr), only N02 which was chelated to the metal ion was observed using FTIR for both dry and hydrated oxides. While nitrate was observed at higher N02 concentrations, these are much larger than would be encountered in the atmosphere. Whether the chelated N02 on the surface can react at a significant rate with various atmospheric gases is not clear. 

In short, while kinetics are very important in determining the importance of reactions in the atmosphere, other aspects such as the formation of reactive intermediates, and the physical and chemical nature of reaction surfaces, should also be taken into account. 

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