A WINTER NO2 SMOG EPISODE IN THE UK

A major NO2 episode occurred in London, England, during December 1991. During a 4-day period, urban background concentrations of this pollutant were the highest recorded in the U.K. since measurements began in the 1970s. Analyses of a broad range of measured pollutants indicate that the episode was primarily caused by motor vehicle emissions occurring during an extended period of cold, stable, anticyclonic conditions. It is demonstrated that, for U.K. NO2 episodes, there is a strong relationship between episode duration and peak observed hourly average concentrations. The highest NO2 levels during the episode were observed during the nighttime. This observation is explainable, at least in part, by gas-phase oxidation of NO to NO2 at low temperatures. This may be an important mechanism for producing NO2 during episode periods in other temperate cities of the world.