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.