London’s Ultra Low Emission Zone: Not So Low Emission?

11.07.18 | Blog | By:

This post republished from Hug Your Engine.

The Mayor of London, Sadiq Khan, and his agency Transport for London (TfL) has announced a so-called Ultra Low Emission Zone (ULEZ) in central London from Apr. 8, 2019. The stated aim of this ULEZ is to help improve air quality. A number of cities around the world are planning similar measures, but London’s is one of the first with such stringent requirements.

The London ULEZ is being implemented in two phases. The first, taking effect Apr. 8, 2019, covers the area where the London Congestion Charge is currently implemented, a small part of central London with relatively few permanent residents. The second phase, effective Oct. 25, 2021, will be implemented across a much larger area – that enclosed by the North and South circular roads – effectively ring roads encircling large parts of suburban London. The total area enclosed by the 2021 zone will be 123 sq. miles and over 3,250,000 people live in this area – these two areas are shown in Figure 1. If a vehicle does not meet a certain emissions standard, it will need to pay a daily charge to enter the zone, currently £12.50 for vans, cars, and motorcycles and £100 for lorries/trucks over 5 tons.

Figure 1: Areas enclosed by the ULEZ from April 2019 (orange) and October 2021 (yellow)

The London ULEZ attracted my attention not so much for what it does regulate, but for what it does not. Peculiarly it specifies the following emissions standards for vehicles:

  • Euro 3 (2007) for motorcycles
  • Euro 4 (2005) for petrol (gasoline) vehicles
  • Euro 6 (2014) for diesel vehicles
  • Euro VI (2013) for lorries (trucks) & buses and other heavy duty (on-road) vehicles

Are these the appropriate technology levels? These emissions standards mean that the emissions from vehicles in the London ULEZ, shown in Figure 2, will be legal.

Figure 2: CO, NOx and Particulate Matter Emissions Permissible from London ULEZ Vehicles*

*The emissions standards for heavy duty vehicles such as buses and lorries/trucks are not directly comparable, so I have made some assumptions on energy use per km to make the comparison.


The result is that motorbikes and petrol (gasoline) vehicles will legally be able to emit unlimited levels of particulate matter (PM) emissions inside the London ULEZ as these are unregulated in Euro 3 and 4 respectively.

Since the VW Dieselgate scandal, the media discourse in the UK has rightly focused heavily on air pollution. However, discussions of air pollution in the media often use NOx alone as a surrogate. NOx is hardly the only pollutant from vehicles. CO, unburned hydrocarbons, ammonia, and particulates (soot) are all emitted as well.

CO2 is also emitted, of course. Typically, a petrol/gasoline car emits around 15% more CO2 than its diesel equivalent. However, CO2, while an important GHG contributing to climate change, is not typically regarded as a pollutant in an air quality sense. This is because it is not currently thought to have a significant short-term impact on human health and so for that reason is not considered in urban air quality legislation.

I am often told that air pollution (NOx) kills 40,000 people each year in the UK. The University of Cambridge’s Winton Centre for Risk and Evidence Communication digs into the data and tells us something quite different. Even accepting that 40,000 figure, it breaks down to 23,500 deaths attributable to NOx and 29,000 to PM2.5 (a common measure of particulate emissions). Therefore the data tells us that particulate emissions are more harmful to human health than NOx, but the London ULEZ will allow unlimited levels of particulate emissions from petrol cars and motorbikes. This does not sound right to me.

Could this really happen?

It is important to understand whether this is a purely hypothetical problem, or one that is likely to occur in practice. To investigate this, I found data from 62 vehicles, all of which will be legal in the ULEZ (30 petrol, 19 diesel, and 13 motorcycle) and looked at their PM emissions (my sources are all identified at the end of this article). In particular, I focused on a type of petrol engine know as Gasoline Direct Injection (GDI) which is now the dominant gasoline technology in the market as it lowers CO2 emissions, but is known to have higher levels of PM emissions. This is shown in Figure 3 which shows the measured PM emissions from these 62 vehicles, presented as the mean (average) alongside the maximum and minimum values observed.

Figure 3: Measured Particulate Emissions from Vehicles Permitted in the London ULEZ

Figure 3 clearly shows that GDI vehicles and motorcycles will emit high levels of PM emissions, with diesel vehicles showing near-zero emissions. The reality is not quite like this, and a rescaling of Figure 3 with a logarithmic ordinate (y-axis) is shown in Figure 4.

Figure 4: Rescaled Figure 3 Showing Measured Particulate Emissions from Vehicles Permitted in the London ULEZ (note the logarithmic scale on the ordinate)

Figure 4 shows that the diesel PM emissions are not zero, but are about 100 times smaller than the GDI and motorcycles on average. This means that in the ULEZ on average one GDI vehicle will emit the same PM emissions as 100 diesels. This is not surprising since PM emissions from diesels have been regulated since Euro 5 (2011) and almost all diesel vehicles are fitted with Diesel Particulate Filters (DPFs) to eliminate PM emissions. Clearly these DPFs are very effective.

I have repeated this exercise for CO emissions. CO is toxic (in high concentrations it has been known to kill people directly, e.g., as a result of faulty domestic boilers) and although dilute in the atmosphere, local emissions can still have an impact. This is shown in Figure 5. In this case, the CO emissions from diesel really are near-zero with the average petrol vehicle emitting around 40 times more CO than a diesel, and motorbikes around 50 times more.

Figure 5: Measured CO Emissions from Vehicles Permitted in the London ULEZ

TfL themselves in their literature encourage people to replace a Euro 5 diesel vehicle (which will be fitted with a DPF) with a Euro 4 or 5 petrol vehicle in order to comply with the zone and reduce emissions. Such an approach will reduce NOx emissions, but may well increase PM  emissions. TfL admit that the ULEZ will do little to PM emissions, stating:

“In Greater London in 2020 and 2025, cars are predicted to make up over 60 per cent of road transport related PM2.5 emissions, with cars and vans still estimated to be contributing about 75 per cent of road transport emissions.”

It is also worth noting that TfL as a result of the ULEZ expect:

“CO2 emissions from larger vehicles such as buses and HGVs are predicted to increase.”

This is insane, a heavily marketed ULEZ leading to an increase in PM and CO2 emissions is not a low emissions zone. It is a low NOx zone, which will reduce NOx levels inside the zone, which is obviously a benefit. However, the complete ignoring of particulates is crazy, given that PM emissions are more harmful to human health. It is in effect an HPZ – a High Politics Zone!

I would suggest all vehicles be subject to Euro 6, in which case the emissions would be as in Figure 6:

Figure 6: Emissions if Euro 6/VI Were Applied to All Vehicles

It should not pass unmentioned that the Euro standards on which the ULEZ is based are not necessarily reflective of real-driving emissions and have been subject to so-called cycle beating (at its worst, illegally by VW during Dieselgate). Such a discussion is outside the scope of this article, but for a good analysis of real emissions look to the Emissions Analytics EQUA index.

To summarize, the TfL ULEZ will reduce NOx emissions, but may well increase PM and CO2 emissions. It is designed to penalize drivers of diesel vehicles. As a result it is not a ULEZ, but an HPZ. The continued demonization of diesel as typified by the ULEZ is not helpful. Rather, it is likely to either increase non-NOx emissions or at best reduce the rate of emissions reduction.

Data from:

doi:10.4271/2010-01-0786; doi:10.4271/2011-01-1219; doi:10.4271/2012-01-0144; doi:10.4271/2011-01-1212; doi:10.4271/2011-01-1224; doi:10.4271/2009-01-1841; doi:10.4271/2003-01-1888; doi:10.1007/s12239−014−0109−4;

Dr. Felix Leach is a post-doctoral researcher in the Centre-of-Excellence for Compression Ignition Engine Combustion Research, a high profile collaboration between Oxford and Jaguar Land Rover.