Are Primary Quantum Yields of NO 2 Photolysis at λ ≤ 398 nm Smaller than Unity ?

Primary quantum yields Φ of NO2 photolysis at wavelengths λ # 398 nm and at bath gas pressures below 1 bar are analyzed. Stern-Volmer plots for collisional photolysis quenching, from experiments at pressures between 10 and 1000 bar, do not indicate a substantial reduction of the quantum yield below unity for pressures below 1 bar. The consequences of the recently discovered fluctuations of specific rate constants k(E) for NO2 dissociation on collisional photolysis quenching are analyzed. These effects can lead to a small reduction of Φ at pressures below 1 bar which, however, is also smaller than the reduction reported in some experiments. Reanalysis of these experiments shows instead that, apart from experimental artifacts, the influence of the secondary reactions O 1 NO2 (1 M) → NO3 (1 M), NO3 1 NO → 2 NO2, and O 1 NO2 → O2 1 NO was underestimated. As a consequence, all experimental evidence so far is in favour of a low pressure primary quantum yield which is unity over the complete wavelength range 3002 398 nm. This leads to a revised recommendation of quantum yields Φ for the range 3002 430 nm at 298 K and 248 K. A revision of the limiting low pressure rate constant at 298 K of the reaction O 1 NO2 1 N2 → NO3 1 N2 of (1.660.2)310 [N2] cm molecule s is also recommended (to be employed together with Fc 5 0.6 and a limiting high pressure value of 2.2310 cm molecule s).