URBAN NO2 CONCENTRATIONS IN THE UK IN 1987

A national air quality monitoring network has been established in the U.K. by Warren Spring Laboratory, on behalf of the U.K. Department of the Environment, to determine compliance with the European Community Directive for nitrogen dioxide. Data from the six network sites for 1987 are presented and analysed. No site in the U.K. breached the NO2 Directive Limit Value during this year, though the closest approaches were at the two London stations. Annual average NO2 concentrations, which varied from 23 to 39 ppb, were consistent with the top five percentile of long-term measurements from a national survey of over 360 U.K. urban areas carried out in 1986. The temporal variability of NO2 concentrations was substantially lower over all time scales than that for NO: winter/summer ratios for all sites averaged 2.9 for NO and 1.3 for NO2. Most sites showed strong diurnal variations for NO which were primarily influenced by traffic emissions during rush hours, although these variations were less marked for NO2. A markedly non-proportional relationship between annual and daily average NO2 and NO(x) levels was observed, and this has important implications for its efficacy of possible NO(x) emission control strategies. An analysis of measured NO2/NO(x) ratios over weekends and weekday periods demonstrates, for instance, that a 20% emission cutback may result, typically, in a corresponding NO2 reduction of 12% at urban locations in the U.K. NO(x) reductions, however, exert a more proportional effect on NO2 concentrations at kerbside and rural locations.