Detecting and quantifying aircraft and other on-airport contributions to ambient nitrogen oxides in the vicinity of a large international airport

Plans to build a third runway at London Heathrow (LHR) airport have been held back because of concerns that the development would lead to annual mean concentrations of nitrogen dioxide (NO2) in excess of the EU Directive limit value, which must be met by 2010. The dominant effect of other sources of NOx close to the airport, primarily from road traffic, makes it difficult to detect and quantify the contribution made by the airport to local NOx and NO2 concentrations. This work presents approaches that aim to detect and quantify the airport contribution to NOx concentrations for a network of seven measurement sites close to the airport. Two principal approaches are used. First, a graphical technique using bivariate polar plots that develops the idea of a pollution rose is used to help discriminate between different source types. The sampling uncertainties associated with the technique have been calculated through a randomised re-sampling approach. Second, the unique pattern of aircraft activity at LHR enables data filtering techniques to be used to statistically verify the presence of aircraft sources. It is shown that aircraft NOx sources can be detected to at least 2.6 km from the airport, even though the airport contribution at that distance is very small. Using these approaches, estimates have been made of the airport contribution to long-term mean concentrations of NOx and NO2. At the airfield boundary we estimate that approximately 27% of the annual mean NOx and NO2 is due to airport operations. At background locations 2–3 km downwind of the airport we estimate that the upper limit of the airport contribution to be less than 15% (<10 μg m−3). This work also provides approaches that would help validate and refine dispersion modelling studies used for airport assessments.