Use of high-time resolution roadside measurements to inform NOx emission ratios 

As air quality targets become more stringent, a better understanding of urban air pollution sources continues to be important for the development of targeted and effective policy.  Ambient measurements of the enhancement of NOx and CO2 concentrations above background can be linked to direct emissions via the calculation of emission ratios (ΔNOx/ΔCO2), which in turn provide insight into emission sources. In this work, we use a regression analysis method to quantify emission ratios from high time resolution (1 second) roadside measurements of NOx and CO2 taken in two major UK cities, London and Manchester. Calculated emission ratios allow us to gain a greater understanding of the effect of factors such as fleet composition and vehicle operating conditions on NOx emissions, along the roads measured. Additionally, a comparison of emission ratios across the two cities allows for an interesting discussion on the differences in traffic behaviour and effectiveness of local traffic-related policy (e.g. low emission zones). Analysis of long-term measurements of NOx and CO2 at the Marylebone Road site demonstrate the effect of changes in vehicle technology and the effect of the ULEZ. This work also aims to highlight the benefit of long-term high time resolution measurements on a local level to develop an advanced understanding of local traffic-related NOx emission characteristics.