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 from 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: Small gasoline non road working machinery in the commercial and institutional sector
• Other: Military activities (military flights and land based activities), recreational craft

The emission inventory basis for mobile sources is fuel consumption information from the Danish energy statistics. In addition, background data for road transport (fleet and mileage), 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 to make the emission estimates sufficiently detailed. Emission factor data mainly comes from the EMEP/EEA Air Pollutant Emission Inventory Guidebook or from specific Danish research studies (e.g. Winther, 2020; Winther and Nielsen, 2006).

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 2020 the emission percentages were 25, 36, 26, 19, 10, 9, 5 and 3 %, respectively. The emissions of NH₃, N₂O, CH₄ and SO₂ have marginal shares of 1.0, 2.1, 0.1 and 0.8 %, respectively.

From 1990 to 2020 the calculated fuel consumption and emission changes for CO₂, CH₄ and N₂O are 26, 19, -90 and 42 %. The calculated 1985-2020 fuel consumption and emission changes for NO_x, NMVOC, CO, particulates (exhaust only: Size is below PM2.5) and BC are 43, -76, -92, -91, -90 and -88 %.

The most significant emission changes from 1985 to 2020 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 1068 % (due to the introduction of catalyst cars).

Figure 1: Emissions from road transport, 2020. 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 2020, the road transport fuel consumption share of the CO₂ neutral bio fuels (ethanol, biodiesel, bio natural gas), has increased to 5.4 %. 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 II, III, IV and V 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 diesel passenger cars and light duty vehicles which entered into the fleet in 2018 and 2019, 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 is going to reduce the emissions for diesel passenger cars and light duty vehicles in the years to come. Relatively large NOx emission reductions is 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 2020 the emission share of non exhaust PM_2.5 was 76 % 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, 1990-2020. Click the figures to enlarge. 

Other mobile sources

For other mobile sources the emissions of NO_x, CO, BC and CO₂ have the largest shares of the national totals in 2020. The shares are 25, 29, 21 and 9 %, respectively. The 2020 SO₂, NMVOC, TSP, PM₁₀ and PM_2.5 emission shares are 6, 4, 1, 4 and 8 %, respectively.

From 1990 to 2020 the total fuel consumption and the emissions of CO₂, CH₄, and N₂O have changed by -19, -20, -64 and -5 %, respectively. From 1985 to 2020, the calculated emission changes for SO₂, NO_X, NMVOC, CO, PM (all size fractions) BC are -96, -49, -66, -37, -79 and -80 %, respectively.

Figure 3: SO₂, NO_X, NMVOC, CO, CO₂, PM and BC emission shares per source category for other mobile sources in 2020. Click the figures to enlarge.

For other mobile sources, the highest CO₂ emissions in 2020 come from Agriculture/forestry/fisheries (1A4c), Industry-other (1A2g) and Navigation (1A3d), with shares of 39 %, 19 %, 16, respectively. The 1990-2020 emission trend is directly related to the fuel consumption development in the same time-period. Minor CO₂ emission contributors are sectors such as Commercial/Institutional (1A4a), Residential (1A4b), Railways (1A3c), Civil Aviation (1A3a) and Other (1A5).

For SO₂ the trends in the Navigation (1A3d) emissions mainly follow the development of the heavy fuel oil consumption and the sulphur content of the fuel. The SO₂ emissions for Fisheries (1A4c) correspond with the development in the consumption of marine gas oil. The main explanation for the development of the SO₂ emission curves for Railways (1A3c) and non-road machinery in Agriculture/forestry (1A4c) and Industry (1A2f), are the stepwise sulphur content reductions for diesel used by machinery in these sectors.

In general, the emissions of NO_X, NMVOC, CO, PM_2.5 and BC from diesel-fuelled working equipment and machinery in agriculture, forestry, industry and commercial/institutional have decreased slightly since the end of the 1990s mainly due to gradually strengthened emission standards given by the EU emission legislation directives. For industry, the emission impact from the global financial crisis becomes very visible for 2009.

The opening of the Great Belt Bridge in 1997 has caused significant reductions of the NO_x and particulate emissions from navigation, due to the closing of several domestic ferry routes. For railways, the gradual shift towards electrification explains the declining trend in diesel fuel consumption and NO_X emissions for this transport sector until 2001. The 1990-2020 PM_2.5 emissions for navigation and fisheries are determined by the fuel use fluctuations in these years, and the development of the emission factors, which to a major extent is a function of the fuel sulphur content.

Apart from marine engines, BC is calculated as shares of TSP for each engine emission technology class and in broad terms the development in BC emissions follows the TSP emission trend. For marine engines (used in navigation and fisheries) fuel type and engine type specific BC emission factors are used in the emission calculations, and hence the BC emissions rely on the fuel consumption development per fuel type and engine type in the inventory period.

For NMVOC and CO, the significant emission increases for the commercial/institutional and residential sectors after 2000 are due to the increased number of gasoline working machines. Improved NMVOC emission factors for diesel machinery in agriculture and gasoline equipment in forestry (chain saws) are the most important explanations for the NMVOC emission decline in the Agriculture/forestry/fisheries sector. This explanation also applies for the industrial sector, which is dominated by diesel-fuelled machinery. From 1997 onwards, the NMVOC emissions from Other (1A5) decrease due to the gradually phase-out of the 2-stroke engine technology for recreational craft. The main reason for the significant 1985-2006 CO emission decrease for Agriculture/forestry-/fisheries is the phasing out of gasoline tractors.

Figure 4: Time-series for emissions from other mobile sources, 1990-2020. Click the figures to enlarge. 

Emission factors for mobile sources are available on the emission inventory home-page.

For a more detailed description of the data and methodology for the inventory of mobile sources see the report Danish emission inventories for road transport and other mobile sources and the annexes for the report.