Glossary
REALCHEM is a highly technical project, but understanding it shouldn’t require a scientific background. This glossary explains the key terms, technologies, and policy concepts you will encounter across the website in simple, accessible language, helping everyone better understand how the project is contributing to cleaner air and more sustainable mobility.
Table of Contents
Hard-to-electrify sectors
Hard-to-electrify (HtE) sectors are industries and activities where replacing fossil fuels with electricity is technically difficult, economically challenging, or currently impractical due to technological limitations. These sectors typically require very high energy density, continuous operation, extreme process temperatures, or the ability to transport heavy loads over long distances. Examples include aviation, where current battery technologies are too heavy for long-haul flights; maritime shipping, which relies on energy-dense fuels for long-distance voyages; heavy road transport, where trucks require long driving ranges and rapid refuelling; and energy-intensive industries such as steel, cement, and chemicals, whose production processes depend on extremely high temperatures or fossil-based feedstocks. Certain industrial heating systems with continuous energy demand also fall into this category.
As direct electrification is often not yet a viable solution, decarbonising these sectors requires alternative technologies and energy carriers. These include renewable and low-carbon hydrogen, sustainable aviation fuels (SAF), synthetic e-fuels, advanced biofuels, and carbon capture, utilisation and storage (CCUS) technologies. Due to the complexity of replacing fossil fuels in these applications, reducing emissions from hard-to-electrify sectors is widely recognised as one of the greatest challenges in achieving global climate and net-zero objectives.
Low-carbon fuels
Low-carbon fuels are energy carriers that generate significantly lower greenhouse gas emissions than conventional fossil fuels such as coal, oil, and natural gas. They play a key role in reducing carbon dioxide (CO₂) emissions across sectors including transport, industry, heating, and power generation. Examples of low-carbon fuels include biofuels, such as ethanol, biodiesel, and sustainable aviation fuel (SAF); renewable hydrogen produced through electrolysis using renewable electricity (commonly referred to as “green hydrogen”); biogas generated from organic waste; and synthetic fuels (e-fuels) produced by combining captured carbon dioxide with renewable electricity.
The emissions reduction potential of low-carbon fuels stems from their use of renewable or recycled carbon sources, as well as cleaner production pathways that reduce lifecycle greenhouse gas emissions compared with conventional fossil fuels. They are particularly important for decarbonising hard-to-electrify sectors, including aviation, maritime shipping, heavy road transport, and certain industrial processes where direct electrification remains technically or economically challenging. However, the overall environmental performance of low-carbon fuels depends on their full lifecycle, including feedstock sourcing, production processes, transportation, and end use.
E-fuels
E-fuels (electrofuels or synthetic fuels) are man-made fuels produced using renewable electricity. They are created by combining green hydrogen, generated through water electrolysis powered by renewable energy, with captured carbon dioxide (CO₂) from the air or industrial sources. The result is liquid or gaseous fuels such as synthetic gasoline, diesel, kerosene, or methane that can be used in existing engines and fuel infrastructure.
E-fuels are considered low-carbon because the CO₂ released during combustion can be balanced by the CO₂ captured during production, especially when renewable energy is used throughout the process. They are particularly important for sectors that are difficult to electrify, including aviation, shipping, and heavy transport.
Advantages include compatibility with current engines and easier fuel storage. However, e-fuels are currently expensive, energy-intensive to produce, and less energy-efficient than direct electrification.
Sustainable aviation fuels
Sustainable Aviation Fuel (SAF) is a low-carbon alternative to conventional jet fuel that can significantly reduce greenhouse gas emissions from air transport. It is produced from sustainable feedstocks, including used cooking oil, agricultural and forestry residues, municipal solid waste, algae, biomass, and synthetic fuels (e-fuels) generated using renewable electricity and captured carbon dioxide. One of the main advantages of SAF is that it can typically be blended with conventional jet fuel and used in existing aircraft engines and airport infrastructure without requiring significant technical modifications.
Compared with conventional aviation fuels, SAF offers the potential to substantially reduce lifecycle greenhouse gas emissions, decrease dependence on fossil fuels, and lower emissions of certain air pollutants. As aviation is one of the most difficult sectors to decarbonise through direct electrification due to the high energy density required for long-distance flights, SAF is widely recognised as a critical pathway towards reducing the sector’s climate impact.
Despite its potential, the large-scale deployment of SAF faces several challenges, including high production costs, limited production capacity, and the need to ensure that feedstocks are sourced sustainably without creating adverse environmental or social impacts. To address these barriers and accelerate the transition to more sustainable aviation, airlines, aircraft manufacturers, fuel producers, and policymakers are increasingly investing in the development, production, and adoption of SAF.
Zero Pollution Action Plan
The Zero Pollution Action Plan is a strategic initiative launched by the European Commission to reduce pollution across air, water, and soil to levels that are no longer considered harmful to human health and ecosystems by 2050. As a key component of the European Green Deal, the Action Plan contributes to the European Union’s broader objectives of achieving climate neutrality, protecting biodiversity, and improving environmental quality.
The Action Plan establishes a comprehensive framework for preventing and reducing pollution by improving air and water quality, tackling plastic litter and chemical pollution, reducing harmful noise levels, promoting cleaner industrial processes and transport systems, and strengthening the monitoring and enforcement of environmental legislation. It also sets several ambitious targets for 2030, including reducing premature deaths caused by air pollution, decreasing plastic waste entering marine environments, and limiting nutrient losses from agricultural activities.
To achieve these objectives, the Zero Pollution Action Plan promotes sustainable production and consumption patterns, supports the transition towards a circular economy, and encourages innovation in clean technologies and pollution prevention. Together, these measures aim to create a healthier, more resilient environment while supporting the European Union’s long-term sustainability and climate ambitions.
Convention on long-range transboundary air pollution
The Convention on Long-Range Transboundary Air Pollution (CLRTAP) is an international environmental agreement established in 1979 under the United Nations Economic Commission for Europe (UNECE) to address air pollution that crosses national borders. The Convention was developed in response to growing evidence that pollutants such as sulphur dioxide (SO₂), nitrogen oxides (NOₓ), and particulate matter can travel long distances through the atmosphere, causing adverse impacts on air quality, ecosystems, and human health far beyond their country of origin.
CLRTAP provides a framework for international cooperation to monitor and assess transboundary air pollution, promote scientific research and data sharing, develop coordinated air quality policies, and implement strategies to reduce pollutant emissions. Over time, the Convention has been complemented by several protocols that establish legally binding commitments to limit emissions of key pollutants responsible for acid rain, ground-level ozone, heavy metal contamination, and persistent organic pollutants.
The Convention has played a significant role in improving air quality across Europe and North America by fostering coordinated action and evidence-based policymaking. It continues to serve as a cornerstone of regional air pollution governance and remains an important instrument for addressing the environmental and health impacts of transboundary air pollution.
Emerging air pollutants
Emerging air pollutants are contaminants that have only recently been recognised, or are receiving increasing scientific and regulatory attention, due to their potential risks to human health and the environment. While many of these pollutants are not yet comprehensively regulated or routinely monitored, growing evidence suggests that they may have significant environmental and public health implications. Examples include ultrafine particles and nanoparticles, microplastics, volatile chemical compounds, persistent organic pollutants (POPs), ammonia and secondary aerosols, as well as emissions associated with emerging fuels and technologies, including e-fuels.
These pollutants originate from a wide range of sources, including transportation, industrial activities, agriculture, household products, waste treatment, and advanced manufacturing processes. Their increasing prevalence has raised concerns about their potential contribution to respiratory and cardiovascular diseases, ecosystem degradation, climate change, and declining indoor and outdoor air quality. As a result, ongoing research is focused on improving the understanding of their sources, atmospheric behaviour, exposure pathways, and long-term environmental and health impacts, with the aim of supporting future monitoring efforts and informing evidence-based policy development.
Emission factors
Emission factors are numerical coefficients or values used to estimate the quantity of pollutants or greenhouse gases emitted from a specific activity, fuel, or industrial process. They establish a relationship between emissions and a measurable unit of activity, such as the amount of fuel consumed, the distance travelled, or the quantity of a product manufactured. For example, emission factors may express the kilograms of carbon dioxide (CO₂) emitted per litre of diesel burned, the grams of particulate matter released per kilometre driven, or the methane emissions generated per tonne of waste decomposed.
Emission factors are widely applied in the development of air pollution inventories, carbon footprint assessments, climate and environmental modelling, and regulatory reporting and policy evaluation. They provide a practical means of estimating emissions in situations where direct measurement is technically challenging, time-consuming, or prohibitively expensive. By enabling consistent and comparable emissions calculations, they support governments, industries, and researchers in assessing environmental impacts and monitoring progress towards emissions reduction targets.
The accuracy of emission estimates depends on the quality and applicability of the emission factors used. These factors can vary considerably depending on the technology employed, fuel characteristics, operating conditions, equipment maintenance, and the presence of emission control systems. Consequently, the use of reliable and up-to-date emission factors is essential for producing robust environmental assessments and informing effective air quality and climate mitigation policies.
Non-road-mobile machinery
Non-Road Mobile Machinery (NRMM) refers to mobile machines and equipment powered by internal combustion engines that are not designed for transporting passengers or goods on public roads. NRMM encompasses a wide range of machinery used across sectors such as construction, agriculture, forestry, industry, airports, and rail operations. Examples include excavators, bulldozers, tractors, harvesters, forklifts, generators, compressors, lawn mowers, chainsaws, locomotives, and inland waterway vessels.
Traditionally, most NRMM has been powered by diesel engines, which are significant sources of air pollutants, including nitrogen oxides (NOₓ), particulate matter (PM), and carbon dioxide (CO₂). As a result, emissions from these machines contribute to both air quality degradation and climate change, particularly in urban and industrial environments where their operation is concentrated.
Recognising their environmental impact, many countries and regions, including the European Union, have introduced increasingly stringent emission standards for NRMM engines to reduce pollutant emissions. At the same time, the sector is undergoing a transition towards cleaner propulsion technologies, including battery-electric systems, hydrogen-powered solutions, and the use of low-carbon fuels. These developments are expected to play an important role in reducing emissions from off-road machinery while supporting broader climate and air quality objectives.
Life cycle assessment
Life Cycle Assessment (LCA) is a systematic methodology used to evaluate the environmental impacts associated with a product, process, or service throughout its entire life cycle. This assessment considers every stage of the value chain, from raw material extraction and processing to manufacturing, transportation, use, maintenance, recycling, and final disposal. Often referred to as a “cradle-to-grave” approach, LCA provides a comprehensive understanding of a product’s overall environmental footprint rather than focusing on a single stage of its life cycle.
An LCA can assess a wide range of environmental impacts, including greenhouse gas emissions, energy and water consumption, air and water pollution, and waste generation. By quantifying these impacts across the full life cycle, the methodology helps identify opportunities to improve environmental performance and reduce resource consumption.
Life Cycle Assessments are widely used by industry, researchers, and policymakers to compare products and technologies, support the development of more sustainable solutions, inform environmental regulations, and guide low-carbon and circular economy strategies. For example, LCAs are frequently applied to evaluate the overall environmental performance of technologies such as electric vehicles, biofuels, and sustainable aviation fuels, enabling informed comparisons with conventional alternatives based on their full lifecycle impacts rather than emissions during operation alone.
Technology trajectories
Technology trajectories describe the long-term pathways through which technologies develop, mature, and improve over time. They reflect the evolution of a technology as it responds to advances in research and innovation, changing market demands, regulatory frameworks, and competitive pressures. By examining these patterns of development, technology trajectories provide insight into how technologies are likely to progress in terms of performance, cost, efficiency, and functionality.
Typically, a technology trajectory is characterised by continuous improvements in technical performance, reductions in production and operating costs, increased efficiency, and the emergence of new applications, designs, and business models. For example, battery technologies have followed a trajectory of increasing energy density, declining costs, and faster charging capabilities, which has been instrumental in accelerating the adoption of electric vehicles and other electrified transport solutions.
Understanding technology trajectories enables researchers, industry stakeholders, and policymakers to anticipate future technological developments, identify innovation opportunities, assess potential market transformations, and evaluate their environmental and economic implications. This knowledge supports strategic planning and investment decisions in sectors such as energy, transportation, manufacturing, and digital technologies, where long-term competitiveness and sustainability depend on the successful evolution and deployment of emerging technologies.
Athmospheric chemistry transport models
Atmospheric Chemistry Transport Models (CTMs) are advanced computer models used to simulate the transport, transformation, and removal of air pollutants and other chemical substances within the atmosphere. These models integrate data on pollutant emissions, meteorological conditions, atmospheric chemistry, and physical transport processes to predict air quality and the distribution of chemical species across different spatial and temporal scales.
CTMs simulate a wide range of atmospheric processes, including the transport of pollutants by wind, chemical reactions between atmospheric compounds, the formation of secondary pollutants such as ground-level ozone and secondary aerosols, and the deposition of pollutants onto land and water surfaces. By representing these complex interactions, CTMs provide a comprehensive understanding of how pollutants are emitted, dispersed, transformed, and ultimately removed from the atmosphere.
These models are widely used to investigate air pollution episodes, smog formation, long-range transboundary transport of pollutants, and the atmospheric impacts of emissions from sectors such as transport, industry, agriculture, energy production, and wildfires. Governments, environmental agencies, and researchers rely on CTMs to forecast air quality, assess the health and environmental impacts of air pollution, support the development and evaluation of environmental policies, and analyse the effectiveness of emission reduction strategies. Widely used examples of CTMs include the Community Multiscale Air Quality (CMAQ) model, CHIMERE, and GEOS-Chem.
Oxidation flow reactors
An Oxidation Flow Reactor (OFR) is a laboratory instrument used to rapidly simulate the atmospheric ageing of gases and particulate matter under controlled conditions. By exposing continuously flowing air samples to high concentrations of atmospheric oxidants, such as hydroxyl (OH) radicals, ozone (O₃), or nitrate (NO₃) radicals, OFRs accelerate chemical processes that would normally occur over several days or weeks in the atmosphere, allowing them to be reproduced within minutes or hours.
OFRs are widely used in atmospheric chemistry research to investigate the formation of secondary organic aerosols (SOA), the ageing of air pollutants, the influence of aerosols on climate, and the potential health effects of chemically transformed particles. Most OFRs generate oxidants using ultraviolet (UV) light within a compact continuous-flow chamber, providing a rapid and efficient means of studying atmospheric chemical reactions under controlled laboratory conditions.
Compared with large environmental simulation chambers, OFRs offer several practical advantages, including shorter experimental times, lower operating costs, and a more compact design. However, because they typically rely on oxidant concentrations that are substantially higher than those found in the natural atmosphere, the simulated conditions may not always fully replicate real-world atmospheric processes. Nevertheless, OFRs have become an important research tool for examining the atmospheric evolution of emissions from a wide range of sources, including road transport, industrial activities, wildfires, and natural ecosystems, thereby improving the understanding of pollutant transformation and its implications for air quality, climate, and human health.
Secondary aerosol formation
Secondary aerosol formation is the process by which fine airborne particles are generated in the atmosphere through chemical reactions involving gaseous precursor compounds. Unlike primary aerosols, which are emitted directly into the atmosphere, such as dust, soot, or sea salt, secondary aerosols are formed when gases including volatile organic compounds (VOCs), sulphur dioxide (SO₂), and nitrogen oxides (NOₓ) react with atmospheric oxidants such as ozone (O₃) and hydroxyl (OH) radicals. These reactions produce low-volatility compounds that condense to form new particulate matter or contribute to the growth of existing particles, resulting in the formation of secondary aerosols, including secondary organic aerosols (SOA).
Secondary aerosols constitute a significant proportion of fine particulate matter (PM₂.₅) and are therefore an important contributor to air pollution. They play a key role in reducing visibility and the formation of smog, while also influencing the Earth’s climate by scattering and absorbing solar radiation and affecting cloud formation and properties. In addition, exposure to secondary aerosols has been associated with adverse health effects, particularly respiratory and cardiovascular diseases. The gaseous precursors responsible for secondary aerosol formation originate from a variety of anthropogenic and natural sources, including vehicle exhaust, industrial activities, biomass burning, and emissions of biogenic volatile organic compounds from vegetation. Understanding the mechanisms of secondary aerosol formation is therefore essential for accurately assessing air quality and developing effective emission reduction strategies.