8 Conclusions

Annual mean urban background NO2 concentrations in 2005 are likely to be below 21 ppb in all areas except for inner London, where current national policies are expected to lead to concentrations in the range 21 - 29 ppb.

Roadside NO2 concentrations in urban areas in 2005 are expected to be significantly higher, with a total of 761 road links likely to have concentrations higher than the annual mean objective of 21 ppb. This represents about 10% of the total number of urban major road links with a total length 670 km. The majority of these links (495, representing 387 km) are in the Greater London area. The remainder are generally confined to the most heavily trafficked roads in other big cities.

Annual mean roadside NO2 concentrations have also been predicted for 2009, for comparison with the European Union 'Daughter' Directive limit value, which is also 21 ppb. The continuing downward trend in traffic emissions is likely to further reduce the number of links exceeding this value by 2009, with about 2% of urban major road links predicted to have concentrations higher than 21 ppb

Roadside NO2 concentrations in 2005 and 2009 have also been estimated for several alternative emission reduction scenarios. Many fewer road links are expected to have concentrations greater than 21 ppb for additional NOx emission reductions of the order of 30% compared with business as usual.

The objectives for 2005 within the NAQS (DoE, 1997) apply to non-occupational near-ground level outdoor locations where a person might reasonably be expected to be exposed over the relevant averaging period. The annual mean NO2 objective of 21 ppb therefore only applies at the facade of buildings where people actually live. Many of the road links in cities other than London with estimated concentrations greater than 21 ppb are unlikely to have people living close to the road, although there are a number of roads with houses present.