Are primary quantum yields of NO2 photolysis at λ≤398 nm smaller than unity?

Primary quantum yields Phi of NO2 photolysis at wavelengths lambda less than or equal to 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 Phi 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 + NO2 (+ M) --> NO3 (+ M), NO3 + NO --> 2 NO2, and O --> NO2 --> O-2 + 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 300-398 nm. This leads to a revised recommendation of quantum yields Phi for the range 300-430 nm at 298 K and 248 K. A revision of the limiting low pressure rate constant at 298 K of the reaction O + NO2 + N-2 --> NO3 + N-2 of (1.6+/-0.2) x 10(-31) [N-2] cm(3) molecule(-1) s(-1) is also recommended (to be employed together with F-c = 0.6 and a limiting high pressure value of 2.2 X 10(-11) cm(3) molecule(-1) s(-1)).