Editorial for the Special Issue “Transport Emissions and Their Environmental Impact”

The transport of passengers and freight around the world has increased dramatically in recent decades. According to Ganapati and Wong (2023) [1], the amount of ton–kilometers has increased eight-fold since the 1960s and even two-fold when corrected for the increase in the world’s gross domestic product.

1. Air Pollutants and Greenhouse Gas Emissions from the Transport Sector

Emissions from transport are a significant source of air pollution. In urban environments, road transport often dominates total NO_x and VOC emissions. Along coastlines and in harbor cities, high concentrations of air pollutants may also result from emissions of NO_x, SO_x, and particulate matter from shipping. Aviation is responsible for high emissions of ultrafine particles (UFP, diameter below 0.1 µm), as well as NO_x, SO_x, and VOCs, contributing to air pollution around airports.

The transport sector accounts for 14–16% of global greenhouse gas (GHG) emissions, with road transport contributing the majority [2]. Shipping is responsible for nearly 3% of global CO₂ emissions [3], and aviation accounts for about 2% [4]. Non-GHG emissions from shipping and aviation have important indirect effects on climate. SO_x emissions from shipping lead to aerosol formation and subsequent modifications of low-level clouds. Aerosols and clouds contribute to cooling [2], but this effect has been reduced since the implementation of the global sulfur cap on ship fuels in 2020. In contrast, aviation has a disproportionate impact on climate change due to non-CO₂ effects such as warming from contrail-induced cirrus clouds and NO_x-induced ozone formation in the upper troposphere [4].

2. Emission Reduction Measures

There have been numerous attempts to reduce transport emissions. Electrification is the most promising measure to tackle both emissions of climate gases and air pollutants, especially in land transport sectors. The expected drop in exhaust emissions from road traffic has shifted the research and regulatory focus to wear particles from road, rail, tire, and brake abrasion.

In sectors where electrification is not feasible, such as high-sea shipping and aviation, and as a complement in other transport sectors to achieve good air quality status, the emission reduction measures comprise improved engine operation, exhaust gas aftertreatment, or the use of alternative fuels. The latter is of high importance when new climate-friendly fuels with lower emissions of greenhouse gases are introduced, which may also have much lower emissions of air pollutants. For instance, the use of regenerative electro-fuels such as green hydrogen and its derivatives, methane, methanol, and ammonia, is being explored in shipping with the aim to reduce both GHG emissions and local air pollutants [5,6,7].

Currently, liquified natural gas (LNG) has emerged as a transitional marine fuel offering immediate air quality benefits. With its low carbon-to-hydrogen ratio, virtually zero sulfur content, and compatibility with lean-burn high-efficiency engine technologies, LNG presents a viable intermediate solution. Research shows that LNG engines can reduce emissions significantly. Compared to high-speed diesel engines, NO_x drops by 86%, SO₂ by 98%, CO₂ by 11%, and PM by 96%; for two-stroke slow-speed engines, reductions reach 90% for NO_x, 100% for SO₂, 20% for CO₂, and 99% for PM [8,9]. However, LNG and even its sustainable alternatives (bio- or electro-methane) are problematic from a climate perspective, due to potentially high emissions of methane during the operational phase and the well-to-tank phase. In a plume measurement campaign, Comer et al. (2024) [10] found that the engine slip and fugitive emissions of methane from four-stroke dual-fuel engines were 6.4% on average.

In aviation, the use of bio- or electro-fuels with low LCA emissions of fossil CO₂ is considered the main pathway for decarbonization in the short and medium term. These fuels can potentially reduce emissions of particles, bringing co-benefits in terms of reduced air pollution and non-CO₂ climate impacts [11,12,13].

3. New Results on Transport Emissions and Their Environmental Impact

This Special Issue presents research articles related to emissions from road traffic, shipping, and aviation. Some of the topics were presented at the joint Conference on Transport and Air Pollution (TAP) and Shipping and Environment (S&E) in Gothenburg, Sweden, between 25 and 28 September 2023. It also includes other research articles on transport emissions and their environmental impact.

Wang et al. (2025) [14] developed an approach for the optimization of the ship speed and bunkering strategy considering ship emissions. This study aimed to provide a solution for shipping operators to benefit from cost-effectiveness while complying with environmental regulations.

Emission monitoring of road traffic on an individual vehicle level is important to obtain correct real-driving emissions and to detect high emitters, which can, even if there are few, significantly contribute to the emission totals. A state-of-the-art technique for monitoring thousands of vehicles on the road every day is remote emission sensing, measuring the ratio of pollutants to CO₂ from vehicle exhaust gases. Imtiaz et al. 2024 [15] present the development of a novel technique enabling direct concentration measurements in vehicle plumes with a gas schlieren imaging sensor, allowing a quantitative analysis of vehicle exhaust and plume dimensions, which would enable the direct sensing of emissions of pollutants on the individual vehicle level.

Kupper et al. (2024) [16] investigated particle emissions from brake and tire wear on a wheel and suspension test bed. They confirmed that non-exhaust emissions from road traffic may be considerable, and the size of the emitted particles may reach the ultrafine range.

Dispersion modeling of pollutants from urban traffic was the focus of the article by Ioannidis et al. (2024) [17]. They point out that high spatial resolution information on air pollutant concentrations is needed in cities to identify pollution hot spots. Computational fluid dynamics (CFD) modeling can help to determine the representativeness of air quality measurement stations in complex urban environments.

The COVID-19 pandemic struck many sectors and caused unprecedented drops in emissions, particularly in aviation. Investigating such a disruptive emission change is important for our understanding of its effect on the environment. Stloukal et al. (2025) [18] calculated landing and takeoff (LTO) emissions for air traffic at Prague Airport and compared them with the pre-pandemic situation, finding a drop of approx. 70% on average, peaking at 90% in April and May 2020.

4. Key Challenges for Future Research

The ongoing efforts to decarbonize the transport sector offer opportunities for simultaneous reductions in air pollutant emissions. Emerging low-carbon fuels—such as ammonia, methanol, biofuels, and e-fuels—require thorough evaluation not only of their greenhouse gas reduction potential but also of their possible impacts on air quality. This is particularly challenging because neither these fuels nor the engines using them are widely used currently. In addition, continuous progress in engine design, particularly in dual-fuel low-emission technologies, and exhaust gas recirculation, is essential to improve the combustion efficiency and reduce unintended emissions such as unburned hydrocarbons or nitrogen-based byproducts. Future research on transport emissions should focus on investigating emissions from synthetic fuels before they are widely applied and identify the advantages and disadvantages of these fuels given their potential effects on ecosystems and health. In addition, particle emissions resulting from abrasion in transport are an important area of future research that has just begun and that needs to include the search for efficient ways to reduce them.