Road Transport and NOx Emissions

Road transport is a major source of NOx emissions in the UK and in 2013 road vehicles were estimated to contribute 32% to total UK NOx emissions. Figure 3.1 shows the estimated emissions by vehicle class from 1990 to 2013, illustrating the trends in emissions and the relative significance of NOx emissions from the different vehicle classes: cars, LGVs, HGVs/buses and mopeds/motorcycles.

Figure 3.1: Emissions of NOx from road transport in the UK, 1990-2013 [4]

According to the trends shown by the inventory, emissions of NOx have steadily declined since 1990, across all of the major vehicle classes, with passenger cars showing the most substantial reduction.

Figure 3.2 below shows the estimated average NOx emissions per vehicle kilometre for different vehicle types between 1990 and 2013. Various emission regulations on new petrol cars, which have come into effect in stages since 1976, have led to the gradual reduction in emissions per vehicle kilometre from petrol cars. In particular, the decline in emissions from 1992 is due to the increasing number of cars fitted with three-way catalysts. Limits on emissions from diesel cars and light goods vehicles (LGVs) did not come into effect until 1993/4 and since then there has been a significant growth in diesel vehicle activity in the UK.

Limits on emissions from HGVs first came into effect in 1988 leading to a gradual reduction in emission rates as new HGVs penetrated the fleet. Recent vehicle emissions standards for diesel vehicles, including cars and vans, have not delivered the emission reductions originally expected under real world driving conditions. However, with the introduction of Euro 6/VI standards, emissions are expected to continue to decline.

Between 2008 and 2009 there was a noticeable reduction in NOx emissions from the road transport sector. This was primarily due to improvements to catalyst repair rates for cars and LGVs from mid-2009 onwards, which reduces NOx emissions per vehicle kilometre. [5] 

Figure 3.2: Average NOx emissions per vehicles kilometre, by vehicle type 1990-2013[6]

Figure 3.3 below shows emissions of NOx per passenger kilometre for buses since 1990. Emission rates have declined over the period due to the penetration of buses meeting tighter emission standards.

Note that care needs to be taken when interpreting these data as the emission rates presented are significantly influenced by the assumptions of bus passenger loading. The data provided here is based on estimated average Great Britain occupancy rates, but this may well vary substantially across the country.

Figure 3.3: NOx emissions per passenger kilometre for buses, 1990-2013[7]

Emissions from road transport are a significant source of NOx emissions and are particularly problematic in urban areas where high density traffic volumes often contribute to exceedences of local air quality targets. Local Authorities around the UK have responded by declaring almost 600 Air Quality Management Areas (AQMAs) in this regard[8]. Where an AQMA has been declared the Local Authority must produce an Air Quality Action Plan that sets out measures to improve air quality.

[4] Note: the contribution from mopeds and motorcycles is negligible and therefore difficult to see in Figure 3.1.

[5] According to the Department for Transport, there is evidence that (prior to 2009) a high proportion of replacement catalysts were not Type Approved and did not restore the emission performance of the vehicle to its original level. This is being addressed through the Regulations Controlling Sale and Installation of Replacement Catalytic Converters and Particle Filters for Light Vehicles for Euro 3 (or above) after June 2009. Therefore a change in the repair rate is taken into account for Euro 3 and above petrol LDVs from mid-2009 assuming all failed vehicles are rectified properly.

[6] Data are derived from total NOx emissions calculated in the NAEI and the total vehicle km travelled by each vehicle type, taking account of vehicle emissions for a range of driving conditions (e.g. urban, motorway, rural).

[7] DfT National Travel Survey. Published in Transport Statistics Great Britain. Table TSGB0101

[8] Defra UK-AIR AQMA webpages