The emission inventories for mobile sources comprises the emissions from the following transport sectors and non road machinery types:

• Road transport: Cars, vans, trucks, buses, mopeds and motorcycles
• Railways: Rail cars and locomotives
• Domestic navigation: Domestic ferries, island and short cut ferries, other sea transport between Danish ports, and sea transport from Denmark to Greenland/Faroe Islands
• International navigation: Fuel consumption (statistical sales) by vessels with a foreign destination
• Domestic civil aviation: Flights between Danish airports, and flights between Denmark and Greenland/Faroe Islands
• International civil aviation: International flights from Denmark
• Agriculture/forestry/fisheries: Agricultural and forestry non road working machinery and fishing vessels
• Industry: Non-road working machinery, mainly building and construction
• Residential: Small gasoline non-road working machinery in the residential sector
• Commercial and institutional: Handling machinery used in ports and airports, tractors, forklifts, telescopic loaders, small gasoline non road working machinery in the commercial and institutional sector
• Other: Military activities (military flights and land based activities) and recreational craft

The emission inventory basis for mobile sources is fuel consumption information from the Danish energy statistics. In addition, detailed background data for road transport (fleet and mileage), railways (train litra km, train tonnes km), air traffic (aircraft type, flight numbers, origin and destination airports), navigation (ferry technical data, no. of round trips and sailing time per ferry route), fishing vessels (vessel registration number, build year, type, overall length, brutto tonnes, total installed engine power and hours at sea) and non-road machinery (engine no., engine size, engine load factor and annual working hours) are used as input for the emission inventories. Emission factor data mainly comes from the EMEP/EEA Air Pollutant Emission Inventory Guidebook or from specific Danish research studies (e.g. Winther, 2022; Winther, 2023; Nielsen et al., 2024).

Road transport

Set in relation to the Danish national emission totals, the largest emission shares for road transport are noted for NO_x, CO₂, CO, BC, PM_2.5, PM₁₀, NMVOC and TSP. In 2022 the emission percentages were 24, 38, 24, 17, 10, 10, 4 and 4 %, respectively. The emissions of NH₃, N₂O, CH₄ and SO₂ have marginal shares of 0.9, 2.2, 0.1 and 0.9 %, respectively.

From 1990 to 2022 the calculated fuel consumption and emission changes for CO₂, CH₄ and N₂O are 26, 19, -91 and 44 %. The calculated 1985-2022 fuel consumption and emission changes for NO_x, NMVOC, CO, particulates (exhaust only: Size is below PM2.5) and BC are 43, -78, -93, -92, -94 and -93 %.

The most significant emission changes from 1985 to 2022 occur for SO₂ and NH₃. For SO₂ the emission drop is 99 % (due to reduced sulphur content in the diesel fuel), whereas the NH₃ emissions increase by 1001 % (due to the introduction of catalyst cars).

Figure 1: Emissions from road transport, 2022. Click on the figures to enlarge.

CO₂ emissions are directly fuel consumption dependent and, in this way, the development in the emission reflects the trend in fuel consumption. In 2022, the road transport fuel consumption share of the CO₂ neutral bio fuels (ethanol, biodiesel, bio natural gas), has increased to 6.0 %. The most important CO₂ emission source for road transport is passenger cars, followed by heavy-duty vehicles, light-duty vehicles and 2-wheelers in decreasing order.

Historically, the emission totals of NMVOC and CO have been very dominated by the contributions coming from private cars. However, the NMVOC and CO (and NO_x) emissions from this vehicle type have shown a steady decreasing tendency since the introduction of private catalyst cars in 1990 (EURO I) and the introduction of even more emission-efficient EURO 2, 3, 4 and 5 private cars (introduced in 1997, 2001, 2006 and 2011, respectively).

For NO_x the emission decrease for passenger cars is composed of a significant drop in emissions from gasoline cars driven by technology improvements, and an increase in emissions from diesel cars due to the dieselization of the Danish vehicle fleet, and almost unchanged emission factors for diesel passenger cars until Euro 6, regardless of EU emission legislation demands. For light duty vehicles, the NO_x emission trend is also the result of a technology driven emission reduction for gasoline vehicles, and a traffic induced emission increase for diesel vehicles; the emission factors for the latter vehicle category have been relatively constant until Euro 6 just as for diesel cars.

The most modern Euro 6d-TEMP and 6d diesel passenger cars and Euro 6d-TEMP and 6d light duty vehicles which entered into the fleet in 2018 and 2021, and 2019 and 2022, respectively, however, have significantly lower NO_x emission factors compared to earlier Euro standards. Hence the gradual growth in the numbers of Euro 6d-TEMP and 6d vehicles is going to reduce the emissions for diesel passenger cars and light duty vehicles in the years to come. Relatively large NO_x emission reductions are noted for passenger cars in 2020 related to COVID-19.

For heavy duty vehicles until Euro III, the real traffic NO_x emissions are not reduced in the order as intended by the EU emission legislation. Most markedly for Euro II engines, the emission factors are even higher than for Euro I due to the so-called engine cycle-beating effect. Outside the legislative test cycle stationary measurement points, the electronic engine control for heavy duty Euro II and III engines switches to a fuel efficient engine running mode, thus leading to increasing NO_x emissions (Figure 6.23). However, the reduction in transport activities due to the global financial crisis in 2008 and 2009 and improved emission factors for Euro IV onwards causes the NO_x emissions for heavy duty vehicles to decrease significantly from 2008.

Exhaust particulate emissions from road transportation vehicles are well below PM_2.5. The emissions from light- and heavy-duty vehicles have significantly decreased since the mid-1990s due to gradually stricter EURO emission standards. In recent years until 2008 the environmental benefit of introducing gradually cleaner diesel private cars has been somewhat outbalanced by an increase in sales of new vehicles. After 2008, the PM emissions gradually become lower due to the increasing number of Euro V cars equipped with particulate filter sold in Denmark from 2006 onwards.

BC - commonly understood as the solid part of the particulate emissions - is calculated as shares of TSP for each Euro engine technology class. In broad terms, the development in BC emissions follows the TSP emission trend, but deviates in some cases, most markedly for diesel cars and vans. For these vehicle types the BC share of TSP increases in moderate steps from conventional engine technologies to Euro IV. As a result, the BC emission development becomes environmentally less positive than for TSP, until the introduction of Euro V vehicles, for which the installed particulate filters have very high removal rates of BC.

The trend in non-exhaust particulate matter (brake and tyre wear and road abrasion) follows the traffic growth in general. In 2022 the emission share of non exhaust PM_2.5 was 84 % of total road PM_2.5. The non-exhaust particulate emissions are gaining more relative importance, in pace with the year by year reductions of exhaust particulate emissions.

Figure 2: Time-series for emissions from road transport, 1990-2022. Click the figures to enlarge. 

Non-exhaust emissions of TSP, PM₁₀, PM_2.5 and BC for road transport

Apart from the exhaust emission estimates of particulate matter (PM), the Danish emission inventories also comprise the non-exhaust PM emissions coming from road transport brake and tyre wear, and road abrasion.

The respective source category distributions for TSP, PM₁₀ and PM_2.5 emissions are identical for each of the non-exhaust emission types brake wear, tyre wear and road abrasion, the PM₁₀ distributions are shown in Figure 3. For Brake wear, passenger cars caused the highest emissions in 2022, followed by light-duty vehicles, trucks, buses and 2-wheelers. In the case of tyre wear and road abrasion, passenger cars caused the highest emissions in 2022, followed by trucks, light-duty vehicles, buses and 2-wheelers.

Figure 3   Brake and tyre wear and road abrasion PM₁₀ emission shares and PM and BC exhaust/non-exhaust distributions for road traffic in 2022.