The annual survey of urban NO2 concentrations, carried out for the DETR by AEA Technology, draws on data from more than 1,200 local authority monitoring sites using diffusion tubes. The first three surveys showed no improvement in NO2 levels between 1993-95, despite the mandatory introduction of catalytic converters on new cars from the start of 1993 (ENDS Report 266, pp 10-11 ).
The new report for 1996 shows a faint improvement on the previous year (see table ). AEA says tentatively that the figures "may indicate the beginning of the expected [downward] trend in NO2 concentrations" - though in previous years it has written off deteriorations of a similar magnitude as statistically insignificant.
The DETR's automatic urban network sites tell a similar story. Average levels rose from 24ppb in 1993 to 26ppb in 1995, and fell back to 25ppb in 1996.
According to DETR estimates, emissions of nitrogen oxides (NOx) declined by 14% from 1993-96 - with emissions from road transport falling by 17%. Cars with catalysts accounted for just 12% of distance travelled in 1993, rising to 36% in 1996.
The continuing failure of NO2 levels to drop in line with emissions may reflect the secondary nature of the pollutant, AEA believes. In urban areas, NO2 levels are governed largely by the amount of ozone available to oxidise nitric oxide. This may delay the impact of NOx reduction measures.
Nevertheless, the latest survey will add to local authorities' fears that statutory air quality objectives may prove impossible to meet by local traffic management alone. For NO2 the objective is that by 2005 annual average concentrations should not exceed 21ppb at background sites or kerbside locations "where people might reasonably be exposed."
In 1996, the 21ppb level was exceeded at 57% of kerbside sites - with a further 23% being at risk of exceedance. In addition, 17% of intermediate and 5% of urban background sites exceeded the figure - with another 34% and 5%, respectively, being at risk of doing so.
The survey confirms continued breaches of the existing EC standard for NO2. Seven sites had annual average levels of over 40ppb - representing probable breaches of the EC limit value - and 57 sites were "at risk". The number of breaches fell slightly from 1995, but was higher than in both 1993 and 1994.