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Primary quantum yields Φ of NO 2  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 NO 2  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 + NO 2  (+ M) → NO 3  (+ M), NO 3  + NO → 2 NO 2 , and O + NO 2  → 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 Φ 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 + NO 2  + N 2  → NO 3  + N 2  of (1.6±0.2)× 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 × 10 -11  cm 3  molecule -1  s -1 ).

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