Measurements of urban emissions of NOx

This Project has been filled


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 2.5µm in diameter, measured by their mass (PM2.5), alongside ozone (O3) 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.

Use of models to forecast the effects of changes in emissions relies on accurate and complete inventories and a complete description of physical and chemical atmospheric processes. Emissions inventories have frequently been found lacking, most recently exemplified by the under-representation of NO2 emissions from passenger diesel vehicles. Estimates of emissions in the National Atmospheric Emissions Inventory (NAEI) are derived annually based on a paper exercise, with only occasional quantification using observations – largely as the latter are not available. It is known that this method can contain large uncertainties, with errors propagating through into errors in models. It has been shown by direct measurements of fluxes from both tower (Lee et al., Env. Sci. Tech., 2015) and airborne (Vaughan et al., Farad. Discuss., 2016) platforms that emissions of NOx were significantly higher than those estimated in the inventory, which was postulated to be due to errors in the estimated traffic source. This hypothesis is driven in part by the results of direct measurements of nitrogen dioxide (NO2) in the UK using a vehicle emission remote sensing technique (Carslaw et al, Atmos. Env., 2013), showed that only petrol fuelled vehicles showed an appreciable reduction in total NOx emissions over the previous 15-20 years (up to 2011) and emissions of NOx from diesel vehicles, including those with after-treatment systems designed to reduce emissions of NOx, had not reduced as expected over the same period of time. VOCs play a key role in O3 formation and it is vital that VOC emissions and loading in a city is understood. Emissions of VOCs in cities such as London have been well studied, with vehicles recognised as a significant source, either via emission in the exhaust gas or by the evaporation of unburnt or partially burnt fuel. Quantifying the emission rates of individual VOCs is a prerequisite to their cost-effective and successful control and this is routinely attempted by the construction of bottom-up emission inventories such as the (NAEI).

Currently, 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) (Carslaw et al, 2019). Previous standards have failed to deliver the expected reductions of NOx 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.


Project Aims

This project will, for the first time, link total measured emissions with direct vehicle exhaust emission measurements made within the flux footprint, thus allowing a detailed assessment of the performance of emission inventories and improved inventories to be developed. Emissions are measured via eddy covariance which utilises high time resolution measurements of NO, NO2, speciated VOCs and micrometeorological data from a tall tower site. Eddy covariance has been widely used to assess greenhouse gas budgets in rural/agricultural areas for many years but is more recently being used for air quality pollutants and in urban environments. NOx emissions measurements in London will be made over the period of the PhD at the BT tower in London. In addition, during two intensive filed campaigns in winter and summer 2021, the student will deploy a Selected Ion Flow Tube Mass Spectrometer (SIFT-MS, Voice 200 ultra, Syft Technologies) combined with Automated Gas Calibration Unit (AGCU) to determine the emissions of Benzene, Toluene, Isoprene, ethanol, methanol, C2-alklyl benzenes and C3-alkyl-benzenes using virtually disjunct eddy covariance (v-DEC). Emission will be calculated via classical and continuous wavelet transform eddy covariance techniques, the latter giving improved temporal information about the emissions.


Figure 1. Average weekday diurnal profiles of measured NOx emission from the BT Tower in Central London during 2017 (black, dashed). Shading shows average total error (random + systematic) in flux measurements. This is compared with the NAEI, calculated using footprint modeling to select grid squares which are then scaled by activity profiles for the hour of day, day of week and month of year (red, solid).


The source of the discrepancy is potentially from underestimations of vehicle emissions in central London. Therefore, in addition, intensive campaigns will be carried out to make direct vehicle exhaust emission measurements within typical flux footprints of the tower, using an Opus AccuScanTM Remote Sensing Device (RSD) 5000 system which has recently been purchased by the University of York. Briefly, the system uses two beams of IR and UV light at tailpipe level, which traverse each passing vehicle’s exhaust plume, hitting a reflecting mirror at the opposite side of the road. The two beams are reflected back through the vehicle’s exhaust plume and are analysed spectrophotometrically to derive five gaseous pollutant concentrations (CO, total HCs, NO, NO2 and PM) and their ratio to CO2. These are combined with vehicle speed, acceleration and other information retrieved from the DVLA (make, model, fuel type, Euro standard, model year, etc.) to give vehicle specific emissions data. Comparisons can then be made between the real-world emissions and those set by the Euro standard of the car in question. This type of on road measurement is crucial to understanding vehicle emissions under real world conditions. It has long been recognised that laboratory emission measurements on rolling roads do not represent real-world emissions. For this reason, the measurement of in-use vehicle emissions is highly valuable.


Figure 2. Vehicle emissions remote sensing uses UV and Infrared light to quantify the emissions in individual vehicle plumes.


The student will compare the flux measurements to emissions estimates from the NAEI, which provides annual estimates of NOx and VOC emission at 1km2 resolution for the whole of the UK. Scaling factors will be used for each source sector individually to take into account the temporal variation in emissions for any given month, day and hour, thus allowing for a realistic comparison with our long term (2 years) flux data series. In addition, the flux measurements will be compared to the London Atmospheric Emissions Inventory (LAEI), which provides emission estimates for NOx at 50 m2 resolution. The inventory reflects the geography of the roads in London, enabling an accurate assessment of population exposure and health impacts. The LAEI uses a ‘bottom-up’ road traffic inventory taking vehicle flow and speed on each road and combining these with national and London-specific vehicle stock data (including buses and taxis) to calculate emissions for each of the 11 vehicle types. Previous results (Vaughan et al., 2016) showed improved agreement between flux measurements and inventories when using the traffic source estimates from the LAEI, although still with an underestimate in the inventory of ~50% in central London. The LAEI estimates used in this previous work benefited from roadside emissions measurements, obtained using the University of Denver Fuel Efficiency Automobile Test (FEAT) system which took measurements from 70,000 vehicles at four locations across London. The work carried out using the very similar OPUS system in this project will provide updated data from the vehicle fleet in London, enabling a more up to date version of the LAEI traffic source to be produced.

Policy makers rely on modelling studies and the emissions inventories that inform them to look at how changing vehicle fleets will affect concentrations. They are also interested in how future policies, like the introductions of Clean Air Zones in cities will impact measured concentrations and therefore achievement of the legal objectives for air quality in different areas of the UK. More accurate information on real world emissions, and how they affect emissions and concentrations at the roadside, at the street scale and at the city scale is key to understanding the drivers of changing pollutant concentrations in our cities. The work carried out in this PhD will be of direct relevance to current considerations of the Joint Air Quality Unit that sits between the Government Department for Environment, Food and Rural Affairs and Department for Transport around the effective reduction in nitrogen dioxide concentrations in urban areas.



The student will work under the supervision of Prof. James Lee, Dr. Sarah Moller, Dr Marvin. Shaw and Dr. David Carslaw (University of York) and will be based at the Wolfson Atmospheric Chemistry Laboratories, part of the Department of Chemistry, University of York. The student will receive training on the instruments for measurement of NOx/VOC flux and the vehicle exhaust emissions, techniques that are widely used in air quality measurements worldwide. Training will be given into the use of existing code and the fundamental principles of the flux and footprint calculations, which are widely used in environmental science. Through the collaboration with Ricardo, the student will learn how to develop emission inventories and will benefit from being involved with applied research that has direct policy relevance, e.g. the development of the NAEI/LAEI.

The University of York and the wider NERC PANORAMA DTP provide comprehensive training programmes for PhD students with a range of courses on both hard and soft skills. Prof. Lee, Dr Moller and Dr Marvin Shaw work for the National Centre for Atmospheric Science (NCAS), and thus the student will have access to the wider resources that NCAS provides. The student will also have access to training provided by NCAS such as the Introduction to Atmospheric Science course and Atmospheric Measurement Summer School on the Isle of Arran, and future developments in computations and data analysis. In addition, Dr Moller works closely with the Government Department for Environment, Food and Rural Affairs (Defra) as an embedded academic, hence the student will be encouraged to develop an awareness of the role of science in policy making, and where relevant opportunities can be sought to discuss findings with those directly involved in designing and assessing policy options. Dr Carslaw has a joint position with the University of York and Ricardo Energy & Environment. He is principally interested in the quantification of road vehicle emissions through vehicle emission remote sensing and the development of improved statistical methods to better-understand the contribution of urban emissions to ambient measurements of pollutant concentrations. Dr Marvin Shaw has research interests in urban air quality and the identification of primary emission sources of gas phase air pollutants (such as Volatile organic compounds – VOCs) in urban areas, with the aim of understanding how and where these pollutants are emitted from and how this relates to estimates from national and regional emission inventories. Expert in the direct determination of VOC emissions using real-time chemical ionisation mass spectrometry (PTR-MS, SIFT-MS) coupled with meteorological measurements through eddy covariance.

The student will work in the Wolfson Atmospheric Chemistry Laboratories, part of the department of Chemistry, University of York. These were established in 2013 and comprise a state of the art 800 m2 dedicated research building, the first of its kind in the UK. Supported by a large award from the Wolfson Foundation and a private donor, the Laboratories enable experimental and theoretical studies relating to the science of local and global air pollution, stratospheric ozone depletion and climate change. The Laboratories are operated as collaborative venture between the University of York and the National Centre for Atmospheric Science (NCAS), co-locating around 40 researchers from seven academic groups and from NCAS. The Laboratories are also home to independent research fellows, postdoctoral researchers, PhD students and final year undergraduate research projects.

Prior to deployment of the SIFT-MS in 2021, the student will be required to develop the instrument’s high sample throughput pressure controlled inlet system using WACL facilities. The student will also be required to further develop SIFT-MS data interpretation software using software such as python and R. The student will have a funded placement at Syft Technologies in Christchurch, New Zealand, for a period of 3-6 months. The training will include hands-on SIFT-MS instrument operation, as well as theoretical aspects such as SIFT-MS ion chemistry and method development. This experience at an instrument manufacturer will not only provide the student with exposure to an industry environment, but also provide the necessary training needed to properly operate and measure compounds using SIFT-MS, as part of the PhD project.

Figure 3. VOC emissions using v-DEC uses SIFT-MS coupled with an automated sampling system and AGCU.


The student will have the opportunity to present their work to the scientific community at national and international meetings and conferences. They will also be encouraged to take part in outreach events organised by both WACL and NCAS in order to disseminate the research beyond the immediate scientific community (e.g. to policymakers and the general public).

We appreciate that this PhD project encompasses several different science and technology areas, and we don’t expect applicants to have experience in many of these fields. The project is very well supported with experienced scientists and training in these new techniques and disciplines is all part of the PhD.



D. Carslaw et al., (2013), New insights from comprehensive on-road measurements of NOx, NO2 and NH3 from vehicle emission remote sensing in London, UK, Atmos. Env., 81, 339-347.

D. Carslaw et al., (2019), The diminishing importance of nitrogen dioxide emissions from road vehicle exhaust, UK, Atmos. Env. X, 1, 100002.

J. Lee et al., (2015), Measurement of NOx Fluxes from a Tall Tower in Central London, UK
and Comparison with Emissions Inventories, Env. Sci. Tech. DOI: 10.1021/es5049072.

A. Vaughan et al., (2016), Spatially resolved flux measurements of NOx from London suggest significantly higher emissions than predicted by inventories, Farad. Discuss., 189, 455-461.

A. Vaughan et al., (2017), VOC emission rates over London and South East England obtained by airborne eddy covariance, Farad. Discuss., 200.

A. Valach et al., (2017), Seasonal and diurnal trends in concentrations and fluxes of volatile organic compounds in central London, Atmos. Chem. Phys., 15, 7777–7796, 2015.