Sources of nitrogen dioxide in winter smog episodes

Highly elevated concentrations of nitrogen dioxide have been observed in urban air in winter episodes in the UK, and cannot readily be explained by current knowledge of emissions and atmospheric chemistry, since formation from NO is thought to be limited by the availability of atmospheric ozone. Measurements of NO2NOx ratios in a road tunnel, and in a wind tunnel into which exhaust gases are introduced show ratios of less than 10%, and primary emissions and/or rapid oxidation of exhaust gas plumes cannot wholly explain elevated concentrations of NO2 in urban air. Studies in a laboratory reactor reveal that the rate of the NO + O2 reaction can be enhanced in the presence of other vehicle exhaust constituents to an extent which may account for the episodic peak concentrations of nitrogen dioxide.