Unexpected Properties of Atmospheric Reactions

Abstract

Numerical modelling of the chemistry of stratospheric ozone requires precise laboratory data on many free radical reactions under atmospheric conditions. Although these processes occur under conditions where Maxwell-Boltzmann energy distributions obtain, the wide range of pressures and temperatures (down to 200 K or below) covered has yielded many unexpected features. The rates of many reactions differ widely from those which might be predicted on the basis of transition state theory. In particular a number of atom transfer reactions involving HO and HO₂ exhibit parallel second and third order rate expressions with fairly similar small or negative temperature coefficients. These can be understood in terms of hydrogen-bonded or perhaps covalently bonded intermediates, but the quantitative interpretation of such pressure dependences is beyond the scope of theories which assume the randomisation of internal energy.