Particulate Matter Emissions from Aircraft: Comparison
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Particulate matter emissions from aircraft engines contribute to ambient concentrations of ultrafine particles in and around airports together with other combustion sources including road traffic. The impact of emissions on ambient concentrations from an airport, for which aircraft engine is a main source, differs from airport to airport due to the different relative contributions of other sources such as road traffic, and due to pollutant mix differences, chemical characteristics and size distribution [1,2]. Particulate matter, particularly the ultrafine component made up of small particles with an aerodynamic diameter of less than 0.1 µm, is widely considered a health hazard [3]. Aircraft gas turbine engines result in direct emissions of “non-volatile” (nvPM), also described as black carbon (BC) “soot” emissions. In addition to local air quality impacts, particles emitted from aircraft engines can affect climate and cloudiness in a number of ways [8]. There are several on-going projects such as AVIATOR [9] and ACACIA [10], that are taking measurements, linking these to modelling and assessing the particulate impacts on local air quality and climate.

  • emissions
  • aircraft
  • particulate matter

1. NonvPM-Volatile Particulate Matter (nvPM) Emissions from Regulated Aircraft Engines

1.1. Engine Emission Regulations

Regulated aircraft engines are those required by the ICAO nternational Civil Aviation Organization (ICAO) to be certified for their emissions performance, namely turbofan and turbojet engines of maximum rated thrust at sea level greater than 26.7 kN. These engines are typically used in commercial passenger and freight aircraft as well as in larger business jets. Standards limiting the LTO (landing and take-off) emissions of smoke number, nvPM (as maximum mass concentration), unburnt hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) from turbojet and turbofan aircraft engines are contained in Annex 16 Volume II [12][1] to the Convention on International Civil Aviation, Doc 7300. Before 2016, the only emission standard related to PM emissions was the Smoke Number regulation, which effectively requires the engine emission to be invisible. However, in recognition of the growing health concerns regarding ultrafine PM and especially nvPM, the CAEP/10 nvPM mass concentration certification standard was agreed to at ICAO-CAEP (Committee on Aviation Environmental Protection) in 2016, which also required the LTO nvPM mass and nvPM number emission indices (EIs) to be reported.
Using standardized data collected under the CAEP/10 certification procedures, the CAEP was able to develop new emission standards for nvPM, and at the ICAO-CAEP 11th meeting (in February 2019), new emission standards for both nvPM mass and nvPM number and the corresponding additions and amendments (Chapter 4) to Annex 16 Volume II were agreed to. These standards include new regulatory limits for nvPM mass and nvPM number applying to both in-production and new engine types from 1 January 2023. The new engine emissions standards are indicative for LTO total nvPM mass and nvPM number emissions per kN of rated thrust. As part of this agreement, the nvPM mass concentration standard is now considered to preserve the exhaust plume invisibility and there was also agreement to end the Smoke Number Standard applicability for engines of rated thrust greater than 26.7 kN beginning 1 January 2023.
Certification requirements include the need to report data under ISA (International Standard Atmosphere) conditions (except for the absolute humidity which is set to 0.00364 kg water/kg dry air), with a defined fuel specification and following specified test conditions requirements. Engine manufacturers report the emissions data alongside some engine characteristics to ICAO. These data are stored and made publicly available through the ICAO Aircraft Engine Emissions DataBank (EEDB). The databank, which covers all engine types whose emissions are regulated and the information provided by the engine manufacturers, is hosted by the European Union Aviation Safety Agency (EASA) on behalf of ICAO [13][2].
The certification process involves running the engine on a test bed at each thrust setting defined by the standard ICAO LTO cycle (100% for take-off, 85% for climb, 30% for approach and 7% for taxi/ground idle) [12][1]. The results of the engine emissions certification are the fuel flow (kg/s), the Emissions Index (EI) for NOx, HC and CO (mass of emissions per kg fuel), the measured smoke number, the measured nvPM mass concentration and will now also include the EI for nvPM mass and nvPM number. These values allow for the calculation of emission data for each pollutant such as: emission rate (g/s); values of total LTO emission per rated thrust (total mass per kN and total nvPM number per kN); and the maximum Smoke Number.

1.2. Combustion Technologies

1.2.1. Pollutant Formation in Combustion Chamber

The combustion technology has had to evolve to control these regulated pollutants in addition to the principal imperatives of safety and operability. In the current engine designs that are now in service, the environmental focus of the combustion design has been controlling NOx emissions whilst improving fuel efficiency. However, as the relevance of nvPM emissions has increased, the design of combustors has to consider both NOx and nvPM emissions as well as fuel efficiency and, of course, all within the safety and operability constraints (altitude relight, turbine inlet temperature, combustion efficiency, combustion instabilities, thermal load, etc.) which bound the main design decisions.

Modern engines designed for subsonic aeroplanes generally tend to easily achieve the CO and unburnt HC emissions regulations; these pollutants are now of such low concentrations that they are no longer considered to be of much concern in urban or around airport locations. The focus of the following sections is to examine the most recent design features of combustors that affect the emissions of nvPM and NOx, which are of most current environmental concern. The two main modern combustion technologies are covered: the most widespread, Rich-burn, Quick-quench, Lean-burn (RQL) and Lean Burn (LB) technologies.

1.2.2. Rich-Burn, Quick-Quench, Lean Burn (RQL) Technologies

In conventional RQL combustion chambers, fuel is atomized into a swirling and recirculating flow at the swirl fuel injector and the airflow is set to keep the primary flame zone fuel rich. Flow exiting the primary rich zone is then quickly diluted, or “quenched”, to a uniform lean mixture. To minimize the formation of NOx, the RQL design facilitates a fast passage through the high temperature zone where the fuel air ratio is stochiometric and the NOx formation potential is high [14][3]. Compared to previous generation combustion chambers, RQL technologies have significantly reduced NOx emissions [15][4]. The formation of nvPM mainly occurs in the primary zone, close to the fuel spray, where fuel and air are not well mixed, and the intermediate zone reduces gas temperature by addition of a small amount of air to promote the complete oxidation of CO and soot particles [14][3]. However, there is an inherent trade-off in the RQL combustors in that the oxidation and removal of soot particles (formed in the rich primary zone) occur in the hottest zone, where NOx formation is highest. So, the control of nvPM at the same time as reducing NOx  provides some challenges to the RQL ich-burn, Quick-quench, Lean-burn (RQL) design, which was conceived originally for NOx control. Furthermore, it is technologically difficult to cool the primary zone walls (no direct injection of air; double wall principle is often used in practice). Designers strive to minimize both nvPM and NOx, most recently employing staging to separately optimize low and high-power conditions [15][4].

1.2.3. Lean Burn (LB) Combustors

In a lean-burn combustor, where the fuel-to-air ratio is lower than the RQL type combustor, the peak temperatures are not as high. As a result, NOx emissions are lower provided that the overall outlet temperature is not above about 1800 K [15][4]. At the same time, the excess air leads to a lower nvPM production [15][4]. However, a difficulty with hydrocarbon fuels is that they will not burn if the fuel air ratio is far below stoichiometric value and lean flames are inherently unstable. For the demanding operating conditions of aero combustors, a pilot zone is required for stability particularly during low power operation. Conditions in this pilot zone are like a small rich-burn combustor, producing nvPM, but because of the small size of the pilot zone and the small fuel-flow through it, the amount of pollutants is relatively small. Except during pilot-only operation, downstream lean-burn regions promote burnout of any particles formed by the pilot, so levels of nvPM should be expected to be low [15][4]. Lean burn technology has utilized partial premixing such as the TAPS technology [16][5].

1.3. Fuel Composition and PM Emissions

Sustainable Aviation Fuel (SAF) in this context includes sustainable fuels made from biogenic wastes and residues (biofuels), fuels produced through Power-to-Liquid technologies (synfuels), as well as fossil fuels engineered for improved environmental performance. Sustainable biofuels and synfuels are subject to sustainability criteria such as those used in the European Union Renewable Energy Directive and in ICAO’s Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). Both types of sustainable fuel are lower in aromatic hydrocarbon compounds, including naphthalene and their sulphur content may reach—in pure form—near-zero values. There have been several measurement campaigns that have quantified reductions in emission of particulate matter behind aircraft engines at the ground, especially comparing biogenic SAF with conventional kerosene [6,19,20,21,22][6][7][8][9][10]. An overview of fuel composition effects on nvPM emissions can be summarized as follows: the combustion of ‘pure’ or blends of SAF results in lower nvPM emissions when compared to the same Jet A-1 baseline due to the lower aromatic content in the fuel. The change in the nvPM emissions characteristics varies with engine type and operation [6,19,20,21,22,23,24,25,26,27,28][6][7][8][9][10][11][12][13][14][15][16]; the observed reduction in soot number and mass emissions is greatest at low engine thrust conditions and decreases with increasing thrust. In addition, SAF combustion results in: a reduction in the mean particle size distributions [6,19,20,21,22,23,24,25,26,27,28][6][7][8][9][10][11][12][13][14][15][16]; a shift in chemical composition with a sizeable decrease in the emission of organics and sulphates [4,23,24,25,26,27,28,29,30,31,32,33][11][12][13][14][15][16][17][18][19][20][21][22]; an alteration in the nvPM morphology with an increased prevalence of amorphous outer shell structures [34,35,36][23][24][25]; an increase of hygroscopic growth factor and hygroscopicity parameter with fuel sulphur content and engine thrust and a decrease with dry particle diameter [35][24]; and a shift in optical properties with reduced absorption, scattering and extinction coefficients [36][25]. In terms of vPM emissions, SO2 emissions—a precursor of vPM in the exhaust plume—are directly proportional to the sulphur content of the fuel.

2. nvPM Emissions from Non-Regulated Engines

ICAO requires that turbofan engines of maximum rated thrust at sea level greater than 26.7 kN be certified for their emission performance. There are however other classes of engines that do not fall under this regulated category. These non-regulated engines include the following: (i) small turbofan engines with rated thrust below 26.7 kN and often used on business jets and small private jets; (ii) military turbofan engines; (iii) auxiliary power units (APUs); (iv) turboprop engines; (v) turboshaft engines mainly used on helicopters; and (vi) piston (or reciprocating) engines. For these unregulated engines, there are limited publicly available data, exposing a knowledge gap in aviation environmental impact assessment and mitigation. It should be noted that on a global scale, nvPM emissions from non-regulated engines are significantly lower compared to the regulated ones, and on a local scale this is likely to be true around large airports too. However, non-regulated engines may be significant emission sources at airports that mainly service aircraft listed (i) to (vi) above and therefore, could be a concern for the local population around these airports. Here, we present a summary of available data with respect to the emission profiles and emission performance of unregulated engines. Unlike the regulated engines, available data in this case are often not reported according to the ICAO-prescribed LTO cycle. 

A summary of the literature on emission profiles from non-regulated engines can be found in Table 1. One main theme is the lack of publicly available data. Where data were available, there is no clear and consistent standardized measurement method or power setting across the different studies. Engine properties for which emissions were measured in some cases are unknown, for instance, APUs. A standardized test program including information allowing for example loss corrections would improve the quality and usability of available data for air quality modelling and development of inventories. In addition to missing engine characteristics, particulate emission data are limited for most engine classes. For military turbofan, turboprop and turboshaft engines, these data are primarily from the 1970s and 1980s; methods applied are relatively outdated.

Table 1.
 Summary of Data on Emission Profiles of non-Regulated Engines.
Studies Type of Engine Description of Data Measured/Reported Compounds
Durdina et al., 2019

[37]		Durdina et al., 2019

[26]		 Turbofan < 26.7 kN Measured nvPM emissions from a Dassault 900EX carrying three Honeywell TFE731-60 engines were similar in profile to larger engine measurements. nvPM mass and number, GMD
Klapmeyer and Marr (2012) [47]Klapmeyer and Marr (2012) [27] Turbofan < 26.7 kN Plume measurements, during regular airport operations, of NOx, CO2, and PM from Cessna C560 aircrafts carrying two Pratt & Whitney (PW) JTD15-5 engines during idle/taxi and at take-off. NOx, Particle number, CO2
ICAO EEDB [13]ICAO EEDB [2] Turbofan < 26.7 kN Pratt & Whitney reported emissions from JT15D series (-1, -4, -5, -5A, -5B, -5C) and corrected as prescribed by ICAO. Allied Signal reported emissions from TFE731-2-2B and TF3731-3 engines Reported on ICAO EEDB; HC, CO, NOx, SN
Spicer et al., 2009, 1992, 1989, 1987 [40,44,48,49]Spicer et al., 2009, 1992, 1989, 1987 [28][29][30][31] Military turbofans Military turbofan engines have different power modes than non-military turbofan engines including an afterburn power mode. However, excluding afterburn power mode for which emissions data are very scarce, military turbofan engine emission profiles are like other turbofan engines. Particle emissions measured as smoke numbers showed highest smoke numbers at 75% to intermediate power and lowest at idle to 30% of normal rated power. Measured airplane engines include F110, F101, F100-PE-100, TF41-42, TF30-P103, TF30-P109. (JP-4 fuel; [44]: JP-8 + 100) HC, CO, NO(JP-4 fuel; [29]: JP-8 + 100) HC, CO, NOx, SN
Bulzan et al., 2010;

Crayford and Johnson, 2011;

Khandelwal et al., 2019;

Kinsey et al., 2012

Lobo et al., 2015

[4,23,38,39,50]		Bulzan et al., 2010;

Crayford and Johnson, 2011;

Khandelwal et al., 2019;

Kinsey et al., 2012

Lobo et al., 2015

[11][17][32][33][34]		 APUs Generally, APUs show similar CO and HC emission profiles to larger turbofan engines. Observed NOx emissions were different; while some studies observed no change in NOx emissions, others observed some increase in NOx emission with increasing power [38,39]. Particle mass EIs decreased with increasing power demand for GTCP85 series [4,23,38], whereas a Rolls Royce Artouste Mk113 APU had higher PM mass concentration (mg/m emission with increasing power [32][33]. Particle mass EIs decreased with increasing power demand for GTCP85 series [11][17][32], whereas a Rolls Royce Artouste Mk113 APU had higher PM mass concentration (mg/m3) at full power than at idle [39]. Lobo et al. (2015) observed lowest PM number EIs at highest power. Kinsey et al.’s (2012) study was inconclusive in PM number EIs as different research groups in the same campaign showed different particle number EI profiles; some were u-shaped with maximum at highest power, others showed no variation with power. For the Kinsey et al.’s group (2012), using Fischer Tropsch fuel (FT; synfuel) reduced PM number and mass EI, and had a clear profile of decreasing EIs with increasing exhaust gas temperature. Crayford et al. observed higher smoke number (SN) and PM number concentrations (number/cm) at full power than at idle [33]. Lobo et al. (2015) observed lowest PM number EIs at highest power. Kinsey et al.’s (2012) study was inconclusive in PM number EIs as different research groups in the same campaign showed different particle number EI profiles; some were u-shaped with maximum at highest power, others showed no variation with power. For the Kinsey et al.’s group (2012), using Fischer Tropsch fuel (FT; synfuel) reduced PM number and mass EI, and had a clear profile of decreasing EIs with increasing exhaust gas temperature. Crayford et al. observed higher smoke number (SN) and PM number concentrations (number/cm3) at full power than at idle [39].) at full power than at idle [33]. [38] (JP-8, and FT-2): HC, NO[32] (JP-8, and FT-2): HC, NOx, CO, nvPM mass and number

[39]: HC, CO, NO		, CO, nvPM mass and number

[33]: HC, CO, NO		x, SN

[4] (JP-8, and FT-2): SO		, SN

[17] (JP-8, and FT-2): SO		2, HC, CO, NOx, nvPM mass and number

[23] (Jet A1): nvPM mass and number

[50] (Jet A1): CO and NO		, nvPM mass and number

[11] (Jet A1): nvPM mass and number

[34] (Jet A1): CO and NO		x
Cain et al., 2013; Corporan et al., 2007, 2010, 2004; Drozd et al. 2012; Kinsey et al., 2019

[41,42,

[35]		43,[36]51,[52,53]Cain et al., 2013; Corporan et al., 2007, 2010, 2004; Drozd et al. 2012; Kinsey et al., 201937][38][39][40] Turboshaft engines (primarily used on helicopters) Variable observations were made for particulate emissions, probably due to differences in sampling methods. There was a general agreement in particle number emissions. Particulate number and mass emissions (concentrations and EIs) and geometric mean diameter (GMD) increased with increasing power. General emission profiles of emissions of CO, NOx, and HC are like those of turbofan engines. PM emissions were significantly reduced with FT fuel. [51] (JP-8): CO[38] (JP-8): CO2, CO, PM mass and number, particle size distribution (PSD)

[22,41,43,52,54] (JP-8, FT): GMD, SN, CO, NO		, CO, PM mass and number, particle size distribution (PSD)

[10][35][37][39][41] (JP-8, FT): GMD, SN, CO, NO		x, PM mass and number

[42] (JP-8, FT): PM mass, CO, CO		, PM mass and number

[36] (JP-8, FT): PM mass, CO, CO		2, HC

[53] (JP-8, FT): GMD, CO, CO		, HC

[40] (JP-8, FT): GMD, CO, CO		2, HC
Chan et al., 2013; Cheng et al., 2008; Corporan et al., 2008; Spicer et al., 2009

[44,45,54,55]		Chan et al., 2013; Cheng et al., 2008; Corporan et al., 2008; Spicer et al., 2009

[29][41][42][43]		 Turboprop engines (primarily on military aircraft) Emission measurements were primarily conducted on turboprop engines for military purposes as in the T56 series III engines on C-130 Hercules (C-130H) aircraft. Power in turboprop engines is reported as shaft horsepower (shp). The gaseous emission profiles observed for the T56 series engines are like those of turboshaft engines. Particle number and mass emissions tended to decrease with an increase in power. [55] (JP-8): CO, NO[43] (JP-8): CO, NOx, CO2, SOx

[54] (JP-8): SN, PM number and mass, GMD, CO, NO

[41] (JP-8): SN, PM number and mass, GMD, CO, NO		x, CO2

[44] (JP8): CO, NO

[29] (JP8): CO, NO		x, OC

[45] (F-34, 50-50 F34/Camelina-HEFA blend): PM number and mass, NO		, OC

[42] (F-34, 50-50 F34/Camelina-HEFA blend): PM number and mass, NO		x, CO, HC

3. Volatile Particulate Matter (vPM)

As the exhaust leaves the engine, the hot combustion exhaust gases cool down and liquid droplets and condensation nuclei of mainly sulphates are formed, on the surfaces of which further substances such as water and organics can condense. Shortly after formation, these particles have typical diameters of a few nanometers. Both the number and mass of particles change significantly during transport due to processes such as agglomeration, condensation, and evaporation on a timescale from seconds to several tens of minutes. An effective emission rate can be derived from the number of particles in the cooled exhaust gas. These particles are referred to as volatile or semi-volatile ultrafine particles but here, the simplified term volatile particulate matter (vPM) is used.

Timko et al. [29][18] report on measurements during the AAFEX campaign. Particles (sum of vPM and nvPM) were measured at distances between 30 m and 300 m behind a CFM56 engine for different power settings and types of fuel. At distances with moderate dilution, as compared to the smaller distance (30 m) from the engine exit, more particles were measured as condensation nuclei were formed in the plume. The study also observed that the nucleation of particles accelerates with increasing fuel sulphur content, as these new particles are generated largely from sulphates. About one order of magnitude more particles were observed for low power while the difference was less pronounced for higher powers. The autscholars deduce that the PM evolution strongly depends on the ratio of particle precursors (sulphate and organics) to soot; more nvPM particles are generated at higher powers and lead to a greater interaction between precursors and nvPM with more coating of nvPM with sulphuric acid, etc. and thereby to a smaller total number of newly formed particles as compared to lower powers.

4. Emission Inventories

An emission inventory typically comprises a dataset with emission amounts, in terms of mass, for the most relevant air pollutant and greenhouse gas species, split up by source type, time and location. Different levels of spatial and temporal resolution are possible, depending on the scope and purpose of the inventory. Aircraft emissions inventories usually report only those emissions related to the LTO cycle for the aircraft, which covers activities up to 3000 feet (914 m), which coincides with the typical order of magnitude for the height of the neutrally stratified atmospheric mixing layer. Emissions below this altitude are expected to be the dominant influence for local ground-level air quality parameters. The 3000 feet boundary is also used as the cut-off altitude for reporting national emissions for air pollution under EMEP (Gothenburg Protocol) and the EU National Emission Ceilings Directive (2016/2284/EU), so national emission inventories often pay limited attention to what happens at higher altitudes.

4.1. Airport Emission Inventories

At airport level, the compilation of an emission inventory is typically motivated by the permitting process, which requires an Environmental Impact Report investigating continued or planned activities at the airport in question. Other potential reasons for compiling an emission inventory are to perform benchmarking or to monitor emission trends in the light of emission mitigation plans or actions. Since individual airports are not subject to international regulations covering the regular reporting of emissions by country, these emission inventories do not have a fixed format, scope or methodology. The scope of the inventory will likely be influenced by the local issues that are thought to be most important. When considering only a single airport, it is usually possible to include more detailed information in the analysis and perform a more comprehensive modelling of the emissions. The emission inventory will typically include all significant emission sources at the airport and cover a full calendar year and a range of pollutant species, such as carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), particulate matter, and hydrocarbons (HC) or non-methane volatile organic compounds (NMVOCs).

4.2. Regional Level (Country, District)

EU Member States are subject to emission legislation [62][44] requiring the annual reporting of air pollutant emissions by sector, including the aviation sector. Currently, these national inventories require only the reporting of emission in terms of mass, not in terms of particle numbers and/or particle size distributions. Although the country-level emission inventory will distinguish between the different airports in the countries, it is constructed at a national level and not by combining separate airport level inventories. This is motivated by the requirement that a single consistent and transparent methodology is used for all years and sources. The full requirements on data and methodologies used are specified in the CLRTAP guidelines [63][45] and the EMEP/EEA Emission Inventory Guidebook [64][46]. For the highest level of detail (Tier 3) recommended by the Guidebook, information on the aircraft type and destination of each flight is used in combination with more detailed aircraft/engine- and LTO phase-specific emission factors. The Guidebook notes the availability of emission factors for CO, NOx, HC from the ICAO Engine Emission Databank (EEDB) [13][2] and lists the First Order Approximation v.3 (FOA3) method for deriving PM emission factors from the EEDB reported smoke number.

4.3. Global Level Inventories

The ICAO-CAEP produces full-flight global aviation inventory for greenhouse gases (CO2, NOx) and local air quality emission (NOx) every three years and the international aviation totals are provided in the ICAO Environmental Report (Environmental Trends) [67][47]. The inventory is based on emission results from the US FAA AEDT model. These results usually compare well with the other two models (the European IMPACT and the UK FAST models) used in ICAO-CAEP analyses [68][48]. The 2022 report will include the 2018 annual estimate for full-flight and LTO nvPM mass, in addition to post-COVID forecast years of 2028, 2038 and 2050.

65. Dispersion Modelling

Dispersion models provide a 3-dimensional concentration distribution of pollutants for subsequent time intervals. They are an important tool to supplement measurements that take place at a few, specific locations. Dispersion modelling is also used to investigate future scenarios or to study specific processes of atmospheric dispersion. The range of dispersion models extends from simple analytic solutions based on physical conservation laws to complex numerical algorithms. ICAO provides with the Airport Air Quality Manual Document 9889 [71][49] an extensive description of emission and dispersion calculation methodologies at airports, in particular for aircraft main engines and APU. The 2nd edition includes a calculation methodology for nvPM mass and number emission indices (FOA4) based on work in the CAEP/11 cycle. ICAO Doc 9889 has been continuously improved in the past cycles including CAEP/12 and the current CAEP/13 cycle, with an update expected in 2022. ICAO-CAEP evaluated several dispersion models (ADMS, AEDT/EDMS, ALAQS and LASPORT), which are designed for local air quality (LAQ) studies at and around airports [81][50]. The aim was to investigate which tools are sufficiently robust, transparent, and appropriate for CAEP analyses and to understand potential differences in modelling results. The use of multiple approved models provided CAEP insight into model-dependent sensitivities of the results. Some of these models are routinely applied in scientific projects and airport assessment procedures. The evaluation noted that advanced/sophisticated approaches are resource intensive but provided more realistic results than simple approaches that tend to be conservative in nature but quick to run. The models provided pollutant concentrations that could be compared to ambient air quality standards, with the understanding that uncertainties exist and are dependent on input data quality and model complexity. Further PM emission comparison to account for updated methodology is being evaluated in the current CAEP work programme. In the Project for the Sustainable Development of Heathrow (PSDH), several dispersion models were evaluated and compared against measurements [82][51]. A further summary of aspects relevant for airport dispersion modelling is provided with focus on Los Angeles Airport [83][52]. Finally, an extended guidance for a model-based quantification of the contribution of airport emissions to local air quality can be found in the ACRP Report 71 [84][53]. In summary, dispersion calculations for airports are complex because many different source groups need to be accounted for. For some groups, standardized emission databanks (ICAO Engine Emission Databank, national vehicle emissions) or recommendations (APU emissions according to ICAO Doc 9889) are available, but for other groups (ground support equipment, stationary sources), airport-specific data must be gathered. The resulting concentrations do not only depend on the applied emissions, but also on the applied type of dispersion model, on the applied meteorological data, and, in the near field of aircraft operations, on the assumed dispersion dynamics of the aircraft engine exhaust. Comparisons with measurements are hampered by the fact that non-airport emission sources and background concentrations contribute to the measured concentrations. Although there exist several data sets that have been used for comparisons, up to today, there exists no generally agreed gold-standard for the validation of airport dispersion models.

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