Key messages

In recent decades, policy measures and subsequent technological developments have led to a decrease in emissions for most air pollutants related to transport activity in the EU-27; with variations ranging from -90.4% to -46.1%, depending on the compound.

Reductions in exhaust emissions in the road transport sector account for the greatest share of this progress. This has been made possible by the introduction of progressively tighter emission standards requiring the use of more advanced aftertreatment systems.

Emissions of most pollutants from aviation increased between 1990 and 2019; meanwhile, emissions from international maritime transport increased up to the mid-2000s and then decreased, with the most pronounced reduction for SOx, -72.9%.

Overall PM emissions decreased by 51.6% in the 1990-2022 period; while most sectors contributed to this decrease, non-exhaust emissions from road transport (due to tyre, brake and road abrasion) increased by 61.5%.

Emissions of most air pollutants from transport in the EU-27 have fallen compared to 1990, thanks to significant policy efforts and developments in pollution control technologies. Indeed, Member States are required to meet the limits and target values for concentrations of pollutants in the ambient air set by the Ambient Air Quality Directives (EU, 2004, EU, 2008, EU, 2015 and EC, 2022b). They are also required to achieve the emission reduction commitments on total national emissions for five air pollutants (NOx, SO2, NMVOC, NH3 and PM2.5) that are set by the National Emission reduction Commitments Directive, NECD (EU, 2016a). Notice that there is a generally complex and non-linear relationship between direct emissions of air pollutants, their overall concentration in the atmosphere (which affects air quality) and their impact on human health (Thunis et al., 2019).

Additional information on Europe’s air quality status, how it has changed in recent decades and examples of mitigation strategies can be found, respectively, in the European Environment Agency’s Europe’s air quality status 2024 (EEA, 2024b), Air quality statistics (EEA, 2024a) and Managing air quality in Europe products (EEA, 2024d). In addition, the EEA maintains interactive maps giving data from all the air quality monitoring stations in Europe (EEA, 2019, EEA, 2023g).

In spite of reduced emissions and the consequent improvements in air quality over the last two decades, air pollution remains a major health concern for Europeans. In 2021, 97% of the urban population was exposed to concentrations of fine PM above the health-based guideline level set by the World Health Organization (WHO). Furthermore, all countries reported levels of ozone and nitrogen dioxide, which are linked by photochemistry, above the corresponding WHO guidance levels. In the case of NO2, around half of the stations with values above the WHO guidance levels and 3 out of 4 of the stations with values above the EU annual limit value are traffic stations (EEA, 2024b). Information on the overall health impact of air pollution in Europe can be found in dedicated publications from the the European Environment Agency (EEA, 2023c).

The EU has formulated the Zero Pollution Action Plan (EC, 2021c) with the ambition to reduce the health impacts of air pollution by 55% by 2030, compared to 2005. In this context, in October 2022 the EC also proposed a revision of the Ambient Air Quality Directives (EC, 2022b). The revision aims to align air quality standards more closely with the guidelines of the WHO.

Air pollutant emissions from the transport sector are also regulated by EU emissions standards (such as the Euro emission standards for road vehicles) and fuel quality requirements. For example, for cars, vans and heavy-duty vehicles, the Euro 7 standard has recently been adopted requiring improvements in exhaust emissions for heavy-duty vehicles and non-exhaust emissions of all motor vehicles (EU, 2024d). Likewise, Annex VI to the MARPOL Convention of the International Maritime Organisation regulates the NOx and SOx emissions from international maritime transport (IMO, 2005) and the EU Sulphur Directive (EU, 2016b) sets limits on the sulphur content of a range of fuels, including marine fuels.

In spite of the improvements discussed above and ongoing decarbonisation efforts in the sector, air pollution will remain a significant challenge into the future. For example, non-exhaust emissions in vehicles, including electric ones, and emissions of currently regulated or unregulated compounds from internal combustion engines burning zero carbon or sustainable fuels will remain relevant.

Figure 13. EU-27 air pollutant emissions as reported to the Convention on Long-range Transboundary Air Pollution (LRTAP Convention)

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Evolution of main air pollutant emissions

Figure 13 indicates that, as a result of the discussed developments, between 1990 and 2022 (including COVID-19 pandemic period) across the EU-27, emissions of NOx from transport decreased by 51%, SOx by 83%, non-methane volatile organic compounds (NMVOCs) by 90% and carbon monoxide (CO) by 90%. As already indicated in the section on climate and in Figure 11, methane (CH4) emissions decreased by 75.6% in the same timeframe while N2O emissions, given in Figure 12, increased by 35.0%. These compounds are relevant not only in the climate change context but also due to their significant contribution to air quality deterioration and depletion of the ozone layer (Fang et al., 2013, Ravishankara et al., 2009).

Similarly, between 1990 and 2022, ammonia (NH3) emissions from transport activity increased by 118%. Although, in global terms, NH3 emissions from transport are limited compared to those from agriculture and other sectors, their impact on air quality, especially within cities, is reported to be very high, also due to secondary aerosol formation through interaction with SOx and NOx (Suarez-Bertoa et al., 2014, Fenn et al., 2018, Sun et al., 2017, Osada et al., 2019, Elser et al., 2018, Dabek-Zlotorzynska et al., 2019). Similarly to what was already discussed in the climate section for N2O, this increase in NH3 emissions, especially the one that is occurring in recent years and mostly into the heavy-duty sector, is mainly due to the introduction of SCR aftertreatment systems (Selleri et al., 2021), where NH3 is used as reducing agent to convert NOx into harmless nitrogen (N2) and water (H2O). In such systems NOx, NH3 and N2O emissions are highly interdependent and they should be regulated at the same time to avoid unwanted increases in any of the three compounds.

In addition, it is worth noting that recent literature studies indicate that NH3 emissions can also be significant for modern light-duty vehicles. This due not only to the mechanisms described above for heavy-duty vehicles but also to the use of enriched air-fuel mixtures, especially at higher loads in petrol vehicles. This is once more done to facilitate the control of NOx which, at variance with NH3, is a regulated pollutant for these vehicles (Selleri et al., 2022b).

In the same period, EU-27 transport emissions of particulate matter (including non-exhaust emissions) with a particle diameter of 10 µm and 2.5 µm or less (PM10/PM2.5) decreased by 46% and 58% respectively, while black carbon (BC) emissions decreased by 70%. Transport is also a large emitter of ultrafine particles, hardly accounted for by PM measurements because of their small mass but critical in terms of health effects, as they constitute the largest part of the inhalable fraction of particulate matter.

For road emissions standards, particle emissions are regulated through solid particle number (SPN) limits and measurements (Giechaskiel et al., 2022b), not only at type-approval but also in real operation. Such measurements are not required in other transport sectors. Limits on non-exhaust emissions from tyres and brake wear have been included in the recently adopted Euro 7 regulation for road vehicles (EU, 2024d). Both categories are responsible for airborne PM, with studies estimating that up to 40% of emissions from brake wear is airborne (Giechaskiel et al., 2024b).

In the case of tyres, between 0.2% and 22% of emissions are airborne, with an average of 5%. Importantly, tyre wear is considered to be the most significant contributor to microplastic emissions in the environment and a source of emissions of heavy metal particles emissions as well (Giechaskiel et al., 2024a). Despite the overall decrease in PM emissions, the non-exhaust fraction of PM emissions has been increasing in recent years, with a variation of 61.5% in the 1990-2022 period.

There are still uncertainties associated with the quantification of such emissions, with virtually no data available in case of tyre wear from motorcycles, light commercial vehicles, and heavy-duty, or lack of high-quality and recent data on emission factors just to mention few examples. For brakes emissions a measurement procedure has been standardised only recently (UNECE GTR 24). Two recent reviews summarise the current understanding on the topic as well as the variability in the emission factors reported in the available literature (Giechaskiel et al., 2024a, Giechaskiel et al., 2024b).

Figure 13 also shows BC trends across transport modes. BC is a form of ultrafine particulate matter; it is both a climate forcer and an air pollutant which can pose a significant threat to human health, as also recognized by the WHO, in its 2021 air quality guidelines (EEA, 2022a, WHO, 2021). For certain transport sectors, the impact of BC as a climate forcer has been evaluated. In the maritime sector, it is estimated that BC emissions were responsible for 6.85% of the global warming contribution of the sector in 2018 (IMO, 2021). BC emissions arise from the combustion of carbon fuels and will therefore remain relevant also for carbon-neutral fuels containing carbon.

The onset of the COVID-19 pandemic had an influence on the figures discussed so far, due to the significant contraction in transport volumes during 2020 and 2021, as discussed in the sections on passenger transport activity and freight transport activity and illustrated in Figure 1 and Figure 3. Indeed, when calculated between 1990 and 2019 the variations in air pollutants emissions discussed above were different for some pollutants: -42% for NOx, -60% for SOx, 134% for NH3, -89% for NMVOC, -88% for CO, -74.2% for CH4, 39.7% for N2O, -39% for PM10, -50% for PM2.5, -62% for BC. Additional information on how the lockdown measures put in place during the pandemic impacted air quality in Europe can be found in Chapter 2 of Air quality in Europe 2020 report (EEA, 2020).

Contribution of the transport sector to air pollution

In the EU-27, transport was the largest emitter of NOx in 2022, with a share of 56.5%. Transport was also responsible for 29.3% of PM10 and PM2.5 in the EU-27 in 2022. Additionally, 26.3% of BC emissions, 19.2% of SOx emissions, 18.8% of CO emissions, 8.1% of the emissions of NMVOC, 5.75% of N2O emissions and 1.4% of NH3 emissions in the EU-27 were due to transport in 2022. Additional information can be found in the EEA interactive data viewer (EEA, 2023e), based on the inventories reported to the EEA by EU Member States under the NECD (EU, 2016a). Importantly, these figures must not be taken as a direct indication of the relative importance of the impact of the transport system on air quality and human health, due to their complex and non-linear relationship, as discussed in the section above. Moreover, the impact of transport, especially in urban areas, is high, because emissions take place close to the ground and the dilution effect is limited. In addition, as population density is higher in urban areas than in rural ones, the health impacts of transport-related air pollution in urban areas are greater.

Aviation

While aviation (both domestic and international) contributes to all pollutants considered so far, the sector is an important emitter of NOx. In 2022 the share of aviation in transport emissions was equal to 14% with 553 Gg emitted. In 2019, the same figures were respectively 11% and 674 Gg. The NOx emissions of aviation showed a strong increasing trend between 1990 and 2019. This is also the case for the emissions of most of the other pollutants.

In the aviation sector most of the reduction of air pollutant emissions are achieved at the engine design level. Indeed, the application of advanced aftertreatment systems similar to those deployed in road vehicles and that have allowed the significant reductions already discussed is technologically very challenging. This is mostly related to the high flow rates and the small contact areas available downstream a conventional gas turbine.

Research is investigating alternatives such as the possibility of including catalytic converters at the combustor level (Gutakovskis, 2019). Another option would be to use hybrid electric propulsion whereby the aircraft is propelled by an electrical engine powered with the electricity generated by a conventional gas turbine; in this kind of system, emissions could be catalytically controlled since the contact area can be increased without the loss of performance characteristic of a conventional design (Prashanth et al., 2021).

Waterborne transport

International maritime transport is the main transport source of SOx emissions, and an important transport source of PM, BC and NOx emissions. From 1990, emissions of these pollutants increased until the mid-2000s and then decreased; SOx emissions decreased the most with a 73% drop. From 2020, SOx emissions were also lower because of the entering into force of stricter regulation on the sulphur content of marine fuels. For example, in 2020 the International Maritime Organisation (IMO) established the ‘global sulphur cap’, resolution MEPC.305(73) (IMO, 2018). This resolution limits the content of sulphur in marine fuel to a maximum of 0.50% m/m for ships sailing in sea areas outside Emission Control Areas (ECAs), which have a limit of 0.10% m/m. It also prohibits carrying onboard marine fuels for combustion purposes, i.e. propulsion or operations on board a ship, that are non-compliant with the regulation. ECAs, as defined under MARPOL Annex VI (IMO, 2005), further restrict SOx and NOx emissions in specific sea regions. There are currently two such areas in EU seas, the Baltic Sea and the North Sea (including the English Channel). From 1 May 2025, the Mediterranean Sea will also become an ECA for SOx, resolution MEPC.361(79) (IMO, 2022).

Maritime transport is also a significant emitter of BC; it is the second largest transport emitter after the road sector as shown in Figure 13. Total BC emissions from the sector (i.e. including domestic, international and international inland waterways navigation) oscillated during the 1990-2022 period, peaking at 16.4 Gg in 2004 (+46.8% compared to 1990) but with an overall decrease of 27.6% in 2022 compared to 1990. Within this sector, international navigation was the most significant contributor among those considered in Figure 13. However, it is interesting that the reduction in domestic emissions have been very limited in the period considered; they reached 2.6 Gg in 2022, representing only a 2.9% decrease compared to 1990.