Direct measurements of NOx emissions in central London: How are they changing and how can they be used to inform government policy?

Air pollution is currently the largest environmental health stressor on the UK population. At present the main pollutants of concern in urban centres are nitrogen dioxide (NO2) and particles less than 2.5 micro meters in diameter, measured by their mass (PM2.5), alongside ozone in suburban and rural environments. It is estimated that air pollution has an effect equivalent to 40,000 premature deaths/year in England and costs the UK economy between £10 billion and £20 billion/year. However, since our transport systems, the way we heat our homes, our energy supply, our use of solvents and our agricultural systems are all changing, we know that profound changes in pollutant emissions are likely in the coming years and indeed are already taking place. It is a critical time with respect to expected reductions in emissions due to new vehicle emission technologies introduced from 2016 onwards (Euro 6/VI standards) and other policy interventions such as the extended London ultra-low emission zone (ULEZ). Previous standards have failed to deliver the expected reductions in emissions under 'real world' driving conditions and the unique combination of direct emission measurements from both individual vehicle exhaust and the total emitted from a wider area will help assess the effectiveness of the new technologies and provide better estimates of the NOx traffic source in the inventories. As a result, there is a critical research need for improved direct assessments of urban NOx emissions to test whether these are responding as predicted to vehicle fleet changes and implementation of new policies in the UK.

This project will use measurements of NOx made at the BT tower in central London to calculate emissions using the eddy covariance technique. A footprint model will then be used to link these local fluxes to the surface allowing them to be compared with emission inventories and to aid in source apportionment. Previous measurements of NOx emissions were made for a month each in spring 2013 and 2017 and then have been made continuously since autumn 2020. Data from these periods showed that the UK National Atmospheric Emissions Inventory (NAEI), underestimated measured emissions during the daytime by a factor of ~1.5, although they agreed well overnight) (Drysdale et al., 2022). In addition, in 2020 during COVID-19 lockdowns, fluxes were shown to be 75% less than those measured in 2017, prior to the pandemic restrictions and the introduction of the Ultra-Low Emissions Zone (ULEZ). This was seemingly at odds with only a 20% reduction in CO2 emissions and 32% reduction in traffic load, with the apparent anomaly attributed in part to the success of air quality policy in central London, but crucially due to the substantial reduction in congestion that resulted from pandemic reduced mobility. Spatial mapping of the fluxes in 2020 suggested that central London was dominated by point source heat and power generation emissions during the period of reduced mobility.

Measurements taken during this project, which will continue for its duration, will be used to further assess the mix of sources of NOx as the vehicle fleet in London becomes increasingly electrified and thus having low or zero exhaust emissions. Such data is crucial for investigating which sources of NOx (e.g. from domestic / commercial combustion or industry) will dominate in future and therefore inform how air pollution needs to respond to those changing sources.