Rapid NO2 formation in diluted petrol-fuelled engine exhaust—A source of NO2 in winter smog episodes

Substantially enhanced rates of oxidation of nitric oxide have been found in petrol engine exhaust diluted with air in the dark. The main product of oxidation is nitrogen dioxide, but up to 20% of the oxidised NO can be in the form of other NOy species. Similarly enhanced NO oxidation rates are also found in the presence of petrol vapour. The kinetics of NO oxidation are close to zero order in NO, the rate being appreciably faster than the known thermal reaction 2NO + O2 → 2NO2. Attempts to isolate the chemical species responsible for this rate enhancement from petrol vapour and engine exhaust have led to identification of 1-methyl-1,3-cyclopentadiene and 1,3-cyclohexadiene. A mechanism has been developed in which addition of NO2 to a conjugated diene causes initiation of a free-radical chain oxidation. A simple numerical model incorporating this process is capable of explaining the observed rate enhancements.