Aviation Background

Aviation Background

The aviation sector is a fast growing sector of Europe’s economy and is associated with a wide range of economics and social benefits. However one of the most important drawbacks of the anticipated rate of growth of air transport, despite new deployed technologies, is the environmental impacts.

European demand for air transport is anticipated to grow continuously until 2050 and beyond. Sustainable mobility is required to satisfy this growth and it is essential that travel remains safe, secure, fast, affordable and environmentally friendly.

Industrial competition is fierce, not only from established world regions but also from new, strong challengers. In this context, there is more to be done in the regulatory field within and outside Europe to ensure a global level playing field in the sector.

ACARE and Flightpath 2050 are part of a comprehensive family of initiatives in aeronautics and air transport including the EC’s Framework Programme research FP6, FP7 and Horizon 2020, as well as the SESAR Joint Undertaking. Clean Sky is the vital part of that wide-ranging set of Eurocentric initiatives with a focus on fostering new technologies that will make tomorrow's aircraft greener and more efficient. Clean Sky 2 is setting its sights for the even more stringent environmental challenges of commercial flight in the 2025 to 2050 timeframe.

What is PM ?

What is PM ?

Particulate Matter (PM) is “a solid, a liquid or a solid phase coated with liquid (…) [and] does not necessarily distinguish between solid and liquid phase”(F.Hemond and J.Fechner, 2015). Its chemical composition depends on its source, and its phase is often unknown (F.Hemond and J.Fechner, 2015). Though particles emitted could have any shape and asperities, they are assimilated to a perfectly round particle which diameter is the aerodynamic diameter. Particulate Matter is a classification based on a physical property, the aerodynamic diameter’s size, rather than a chemical property. They are often named after their aerodynamic diameter size, in µm. PM10 an PM2.5 stand for Particulate Matter with a diameter of 10µm or less and 2.5µm or less, respectively. Some papers also refer to the coarse fraction, which corresponds to PM with an aerodynamic diameter included between 2.5µm and 10µm.

PM is divided into two major categories: non-volatile PM (nvPM) and volatile PM (vPM), the former one being the majority of an engine’s PM direct emission and comprised between 0.015µm and 0.06µm (Rindlisbacher and Jacob, 2016). Pollutants directly emitted in their final form are called primary pollutants, while some of them, called precursors, are being transformed as they mix with ambient air, forming secondary pollutants. The nvPM category is mostly composed of black carbon and ultrafine soot (Rindlisbacher and Jacob, 2016). vPM is formed concurrently as several other gaseous components are exhausted from the nozzle engine and undergo complex transformations in the ambient air. Their composition and phase are more ambiguous to know as they depend on several parameters which are not yet fully understood. vPM may also undergo further transformations as it mixes with ambient air.  Quite the contrary, as nvPM is mixed with the ambient air from the nozzle engine their shape mass and number does not change (Rindlisbacher and Jacob, 2016).

Why PM is important ?

Why PM is important?

Emissions from aircraft have adverse effects on the air quality in and around airports, contributing to public health concerns within neighbouring communities. Therefore, the evaluation of aircraft emissions and dispersion is an important part towards ensuring that local air quality standards in and around airports are not exceeded. Many large airports across Europe are subject to regular assessment of their emissions impact on air quality as a statutory requirement to:

  • meet EU ambient air quality directives;
  • review the impact of planned infrastructural developments;  
  • act as part of a good neighbour-programme.

Particulate Matter (PM) emissions from aircraft engines adversely affect air quality in and around airports, contributing to public health concerns for airport workers and within neighbouring communities.

Scientists and regulators have an increasingly profound understanding of the complex nature of Particulate Matter (PM) in ambient air. PM can be attributed with different properties, dependent of their size and chemical composition from different sources, both natural and anthropogenic. What is less certain, is how PM species and precursors evolve and interact within the atmosphere, and which characteristics of the PM are most harmful to public health. For instance, some particles can form complicated reactions in the atmosphere of chemicals such as sulfur dioxides and nitrogen oxides that are emitted from exhaust engines. Those secondary particles make up the most of fine particles. It is considered that Ultra Fine Particles (UFP), defined as particles of aerodynamic diameter less than 100 nm, may have greater toxicity on an equal mass basis than currently regulated larger particles (PM2.5/PM10 ambient standards) because their vast numbers and small diameters provide a high surface area which is a potentially important toxicological interface. These UFP’s are relevant to civil aviation, and recent studies have shown that aircraft engines emit primary aerosol as non-volatile Particulate Matter (nvPM) as well as secondary aerosol precursor gases such as organic gases, nitrates and sulphates that nucleate within the exhaust plume within this size range. The contribution of UFP from aircraft operations to the ambient concentration in and around airports is therefore largely unknown, and could be significant.

Aircraft particulate emissions, as with emissions of nvPM from other sources, are subject to regulation. Within the current ICAO-CAEP cycle, CAEP/11, a new standard for LTO nvPM mass and number for engine with thrust >26.7kN is being developed, the first standard of its kind. In-production regulatory limit values for nvPM mass and number will be set at a level to allow in production engines to pass and will act as an anti-backsliding standard. However, there remains uncertainty in the methodology surrounding the effects of ambient conditions, fuel composition and sampling system calibration and loss corrections. Furthermore, the regulation does not address the effects on the global atmosphere by excluding cruise conditions and the contribution of nvPM by smaller engines and engines rated on shaft speed. These factors, along with the potential impact of emerging technologies, need to be considered for a future European roadmap for improving current nvPM methodologies and future nvPM measurement technologies and regulation beyond CAEP/11.

Historical review of standards and regulations

Historical review of standards and regulations

At the international scale, the International Civil Aviation Organization (ICAO) decides standards and provides guidelines for engine emissions, including nvPM standards. The World Health Organization (WHO) regularly publishes recommendations for concentrations’ limit values of pollutants for ambient air. Reports published by the WHO provide to policymakers, like the European Union and the different governments, supporting data for improving protection against air pollutants. Accordingly, one of the ICAO roles is to implement these data for the specific case of an aircraft engine and to develop new recommended standards for aircrafts’ emissions. At the European scale, the European Commission votes and decides directives on Ambient Air Quality (AAQ).


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WHO Guidelines

WHO, founded in 1948, first reports its concern about air pollution in 1958 (World Health Organization. Expert Committee on Environmental Sanitation, 1958), where a hundred of species are identified as an outdoor air pollutant and harmful towards human, and in which smoke is already recognized as an indicator of “air contaminant”. The definition of PM may not be clearly yet stated, but WHO recognizes the particularly hazardous aspect of “solid particles (…) [generally] larger than 2µm but may be as small as 0.1µm”. Fluctuations of meteorological data depending on the season and the hour of the day is enhanced, to highlight the importance of accurate meteorological data, as well as accurate data about topology characteristics of the considered place. WHO also recommends in this report air pollution to be measured with “the degree of blackness” of smoke using “Ringelmann chart or a similar device”, though the WHO recognize the limitations of the method. WHO calls policies’ makers for regulation rather than legislation on air quality. No standard is fixed, as it might affect economical balance of some fields without guarantying absence of health impact.

In 1987, WHO publishes the first edition of Air Quality for Europe Guideline (World Health Organization (WHO), 2017). It is the first time WHO provides guidance on PM10 concentration with numerical values. PM10 are referred as “total suspended/total thoracic particles”, measured by gravimetric methods, and are recommended to be assessed alongside sulphur. The recommended concentration is of 125µg/mfor a short term (24-hour average) and 50 µg/m3 for long term (1-year average) WHO recommends not to regard these values as standards below which no health impact occurs, as the contrary had been reported.

A second edition of the Air Quality for Europe Guideline is published in 2000.(WHO Regional Office for Europe, 2000). PM is reported in the “Classical pollutants” category, separately from sulphur oxides, and the difference is made between PM10 and PM2.5. For short term exposure, PM10 low level is defined to be of 100µg/m3 on a 24-hour average or below. However, WHO maintains its warning concerning “[the] current database [which] does not allow the derivation of a threshold below which no effects occur.” It is stated that PM10 is more hazardous than a coarse fraction (between 10µm and 2.5µm) and that PM issue is not significantly affected by the difference rural/urban nature of the localisation. It is the first time that the WHO introduces recommendations on PM2.5, stating that the hazardous effect of PM10 depends on its provenance and composition, especially if a lot of PM2.5 is present. PM2.5 is thus a better indicator of health effects than PM10. While previous reports 13 years ago gingerly recommended PM10 concentrations to be below 125µg/mfor a short term (24-hour average) and 50 µg/m3 for long-term, the report from 2000 recommends lower levels. The short term recommended limit falls to 100 µg/m3, while the annual exposure falls to 30 µg/m3. Furthermore, WHO also suggests a recommended value for PM2.5 long-term exposure of 20 µg/m3. Once again, the WHO advises taking this recommendation with caution and recommend to base policies on the risk assessment graphics.

2006 third edition on air quality extends its recommendation on a global scale (international), and distinguishes for the first time indoor and outdoor air quality (World Health Organization (WHO), 2006). This report recommends a more stringent concentration for both PM2.5 and PM10. For PM10 WHO recommends not to exceed 20µg/m3 on a 1-year averages concentration and 50µg/m3 on a 24-hour average. For PM2.5 WHO recommends not to exceed 10µg/m3 on a 1-year averages concentration and 25µg/m3 on a 24-hour average. The WHO warns that these concentrations are not a guaranty of safety for health for all the population and adds explicitly the standard-setting process needs to aim at achieving the lowest concentrations possible in the context of local constraints, capabilities and public health priorities. The WHO encourages countries to decrease their standardized threshold concentration values for PM progressively. Accordingly, WHO recommends intermediary concentrations to reach before complying with the recommended values.

WHO held its first conference on air quality in October 2018 during which the United Nations Environment Programme (UN Environment) formed a formal agreement on health issues on a scale of 15 years.

ICAO standards

ICAO engine’s emission regulation is based on the ground level emission, occurring during the Landing-Take-off cycle (LTO). In 1981, ICAO adopts its “first smoke, fuel venting, and gaseous emissions standards for turbojet and turbofan”(Rindlisbacher and Jacob, 2016), that is the Smoke Number (SN). SN is dimensionless and measured from “the loss of reflectance of a filter used to trap smoke particle from a prescribed mass of exhaust per unit area of filter” (Rindlisbacher and Jacob, 2016). This is done for each of the four setting of the LTO cycle (taxiing, take-off, idle, landing). SN is calculated as follow[1]:

Where Rw and Rs are the absolute reflectance of the clean filter material and the stained filter respectively. The regulatory limit is a function of the rated thrust (FOO) [1]. SN does not allow the assessment of  PM concentration without assuming certain hypotheses.

In 1983, the Committee on Aviation Environmental Protection (CAEP) is established. Its mission is to develop and maintain the international standards for aircraft noise, aeroplane CO2 emissions, fuel venting and aircraft engine emissions: oxides of nitrogen (NOx), unburned hydrocarbons (HC), carbon monoxide (CO), smoke and non-volatile particulate matter (nvPM). For the 8th meeting in 2008 (CAEP/8) the first proposal to account for PM in ICAO regulation is submitted. The 9th meeting, CAEP/9 endorsed the planification of a working group on a nvPM certification for turbofan engine’s emissions of rated thrust>26.7kN.

Finally it is in 2016, with the 10th meeting CAEP/10, that the nvPM certification is ratified (ICAO, 2017). This new certification has been established after statistical studies on the relationship between the SN certification and nvPM maximum concentration.  Thus, nvPM certification does not introduce more stringency as it merely is an equivalent of the actual SN certification in term of nvPM emissions indices (EIs). At the latest the 1st January 2020, manufacturers must report for new engines or in-production engines this following information:

  • fuel flow at each thrust setting of the LTO cycle
  • nvPM mass and number indices EIs for the four LTO points
  • maximum nvPM EI mass
  • maximum nvPM EI number
  • maximum nvPM mass concentration

Now latest CAEP meeting, CAEP/11 in 2019, announced the end of the SN standard applicability the 1st January 2023 (except for small engines with rated Thrust<27.6kN) and introduces new stringencies for nvPM emissions. The data available from the CAEP/10 standard allowed the definition of an LTO-based nvPM mass and number metric system. The new standard will apply to new type and in-production engines with rated thrust greater than 26.7kN from 1st January 2023 and introduces a new stringency with respect of the previous standard. There are 2 different limits for in-production and new engines types. The limits in this case are not on the concentration but on the Mass & number emissions with respect of the rated thrust [2].


European Commission. (2019). FITNESS CHECK of the Ambient Air Quality Directives. SWD(2019) 427 Final, September.

European Environment Agency. (2017). Air quality standards. In Springer Series in Reliability Engineering. https://doi.org/10.1007/978-1-84882-602-1_4

F.Hemond, & J.Fechner, E. (2015). The Atmosphere. In Elsevier (Ed.), Chemical Fate and Transport in the Environment (third). https://doi.org/10.1016/B978-0-12-398256-8.00004-9

ICAO. (2016). Airport air quality Manual. In Air & Space Europe (Issues 1–2). https://doi.org/10.1016/s1290-0958(01)90012-7

ICAO. (2017). Annex 16 : Environmental Protection: Vol. II (Fourth, Issue January).

Rindlisbacher, T., & Jacob, S. . D. (2016). New Particulate Matter Standard for Aircraft Gas Turbine Engines.

WHO Regional Office for Europe. (2000). Air Quality Guidelines. Air Quality Guidelines for Europe, 22(8), 1–8. https://doi.org/10.1007/BF02986808

Wong, H. W., Jun, M., Peck, J., Waitz, I. A., & Miake-Lye, R. C. (2015). Roles of Organic Emissions in the Formation of Near Field Aircraft-Emitted Volatile Particulate Matter: A Kinetic Microphysical Modeling Study. Journal of Engineering for Gas Turbines and Power, 137(7). https://doi.org/10.1115/1.4029366

World Health Organization. Expert Committee on Environmental Sanitation. (1958). Air Pollution Fifth Report (p. 26 p).

World Health Organization (WHO). (2006). WHO Air quality guidelines for particulate matter, ozone, nitrogen, dioxide and sulfur dioxide (Vol. 51).

World Health Organization (WHO). (2017). Evolution of WHO air quality guidelines: past, present and future. In WHO Regional Office for Europe.

This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research program under grant agreement No 863969