Air Quality Assessment: Harlesden town centre and A406 junction with Brentfield Road

The London Borough of Brent commissioned the Transport Research Laboratory (TRL) to undertake air quality assessments in three parts of the borough: Wembley town centre, Harlesden town centre and the A406 junction with Brentfield Road. A previous air quality assessment using average-speed emissions functions had concluded that the model approach underestimated concentrations of nitrogen dioxide (NO2) and particulate matter (PM10) at roadside monitoring sites when compared to measured data. Reasons for the underestimation were considered to be a combination of site-specific and generic reasons, for example, limitations of emissions factors to represent traffic queuing (or stop start traffic activity) in the model setup. This report constitutes a reassessment of air quality concentrations at two of the three original sites: Harlesden and the A406. The objective was to determine whether emission rates derived from recording transient vehicle operations are more representative than emission rates derived from average speed functions. The results showed that the model under-predicted the road contribution to NOx and PM10 at both sites using the average-speed emissions approach. At Harlesden, modelling with the instantaneous emissions approach over-predicted the road contribution of NOx and slightly over-predicted the road contribution of PM10. At the A406, modelling with instantaneous emissions has slightly under-predicted the road contribution of NOx and under-predicted the road PM10 contribution. Further work is required to understand the robustness of background contributions. Overall, the instantaneous emission approach, to assessing air quality, has shown itself to be appropriate for air quality assessment and, theoretically, should offer an improvement over average-speed emissions calculations.