Aviation emission indices and calculation of emission species specific datasets

Monthly resolved aviation emissions datasets are created using monthly resolved aviation fuelburn data (fuelburn_i,j,k) in conjunction with emissions indices for the aviation-borne species of interest (Equation 4.9). Emissions indexes (EI_x) represent the amount of specie of interest (x) emitted per kg of aviation fuel combusted.

emissions_xi,j,k = fuelburn_i,j,k ∙ EI_x 1000

Equation 4.9 Where i,j,k = denote grid position in the lon, lat and vertical position of array

EI_x = emissions index for species of interest (𝑥) in g kg(fuel)^-1 fuelburn = grid resolved fuelburn

For species which are typically not linearly-scalable with fuelburn such as CO, HCs, BC particle number, and OC mass without additional information on ambient conditions, sea-level static conditions, and flame temperature (i.e. temperature of the flame within the combustor) in tandem with assumptions on the make-up of the global aviation fleet these species are treated as linearly scalable (Wilkerson et al., 2010; Eyers et al., 2004; Bond et al., 2004; Hopke, 1985).

For speciated hydrocarbons emissions indices a combination of published emissions indices (in the cases of formaldehyde, methanol, acetone and acetaldehyde) (Knighton et al., 2007; Spicer et al., 1994) and published experimental data (in the cases of ethane and propane) (Anderson et al., 2006) were used. These were used in conjunction with parameters for engine power settings (percentage) obtained from the Airbus Flight Crew Training Manual (Airbus FCTM) for the A318/A319/A320/A321 (Airbus, 2008). The Airbus FCTM for the Airbus A318/A319/A320/A321 range helps highlight a minimum engine power settings for phases of flight above idle of 40%. This assumption is consistent with engine power requirements for the cruise phase of flight (Airbus, 2008) and assumes that majority of emissions occur during cruise.

Anderson et al. (2006)’s experimental work on the EXCAVATE study reported measurements for 33 HCs, taken at four differing power settings (4-7%, 26%, 47% and 61%) relating to four phases of engine operation and flight (idle, approach, low cruise and high cruise), along with three background samples taken throughout the course of the experiment. As such when

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deriving emissions indices using experimental data from Anderson et al. (2006) emissions data for power settings above 40% are used.

For a normal global aviation fleet operating on 100% kerosene an emissions index for carbon monoxide (EI_CO) of 3.61 g kg(fuel)^-1 is employed (Table 4.1), as used by the FAA’s AEDT 2006 (Wilkerson et al., 2010).

An EI_CO of 3.61 g kg(fuel)^-1 is substituted in to Equation 4.9 for EI_x in order to calculate the CO emissions dataset. The EI_CO used here is at the higher end of emissions indices from current literature [2–3.61 g kg(fuel)^-1], but from one for the more recent sources (Lee et al., 2010;

Eyers et al., 2004; Wilkerson et al., 2010).

Emissions indices for speciated hydrocarbons were taken from a range of sources. An emissions index for formaldehyde (EIHCHO) of 1.24 g kg(fuel)^-1 was taken (Table 4.1) from work by Spicer et al. (1994). This value falls in agreement with emissions indices derived by Knighton et al. (2007) during work on the NASA sponsored APEX project (Aircraft Particle Emission Experiment) conducted at NASA’s Dryden Flight Research Centre over April 2004. This project was conducted using NASA’s DC-8 with CFM56-2-C1 engines. This emissions index was obtained at a power setting of 4%, relating to “ground idle” conditions (Knighton et al., 2007).

Using Equation 4.9, substituting EI_HCHO (where EI_HCHO = 1.24 g kg(fuel)^-1) for EI_x, the emissions dataset for HCHO is calculated.

Emissions indices for methanol (EICH₃OH = 0.22 g kg(fuel)^-1), acetone (EI(CH₃)₂CO = 0.18 g kg(fuel)

-1) and acetaldehyde (EI_CH3CHO = 0.33 g kg(fuel)^-1) are taken from the NASA APEX study based on the relationship between formaldehyde and these species for engine power settings of 4% and 7% (Knighton et al., 2007) (Table 4.1). These emissions indices fed in to Equation 4.9 in order to create aviation emissions datasets for methanol, acetone and acetaldehyde; through substitution of for EICH₃OH, EI(CH₃)₂CO and EICH₃CHO (in turn) for EIx.

Emissions indices for ethane (EI_C2H6) and propane (EI_C3H8) were calculated using experimental data from the NASA EXCAVATE study (EXperiment to Characterize Aircraft Volatile Aerosol and Trace-species Emissions), conducted at NASA’s Langley Research Centre over January 2002 using a B757 with Rolls-Royce RB211-535E4 engines (Anderson et al., 2006).

Emission indices for ethane (EIC₂H₆) and propane (EIC₃H₈) are calculated using published experimental data from Anderson et al., (2006) in conjunction with a general HC emissions index from the AEDT emissions inventory (EI_HC = 0.52 g kg(fuel)^-1) (Wilkerson et al., 2010),

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assuming engine power settings above 40% in line with FCTM for the Airbus A318/A319/A320/A321 range (Airbus, 2008). These emissions indices are calculated using the following process: conversion of experimental data (from literature) from ppbv to mg m^-3; conversion of background concentrations from ppbv to mg m^-3; calculation of concentration changes due to aviation; calculation of mass fraction of each aviation-borne NMHC specie, and finally; derivation of ethane and propane emissions indices.

Firstly, measurements [X_xpt] in ppbv for each NMHC of the aviation emitted species measureable via the EXCAVATE project are converted to mg.m^-3 as per Equation 4.10:

[X_xpt]

mg m^-3 = measured species concentration in mg m^-3 [X_xpt]

ppbv = measured species concentration in ppbv

expts = number of experiments providing measurements MW_X = molecular weight of species X (g mol^-1)

24.45 = volume of air at standard atmospheric pressure (m³) 1000 = factor to convert from micrograms to milligrams

Next the same process is used for the background readings (Equation 4.11). Background readings are taken to account for ambient concentrations of measured species, thus allowing for the calculation of increases in species concentrations only attributable to aircraft engine operations.

[X_bk]_ppbv = background species concentration in ppbv samples = number of background samples

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Once both the mass based concentrations for engine measurements and background measurements are calculated the change in concentration due to engine operation are ([X_avi]_{mg m}-3) estimated. This is calculated as follows (Equation 4.12):

[X_avi]_{mg m}-3 = [X_xpt]

mg m^-3 - [X_bk]_{mg m}-3

Equation 4.12 This then allows for the mass fraction of each aviation-borne NHMC (X_{mass_frac}) to be calculated from the mass concentration of the emitted HC species of interest and the total mass concentration of all emitted NMHC species (Equation 4.13):

X_{mass_frac} = ([X_avi]_{mg m}-3 ∑ [X_avi]_{mg m}-3

NMHC_species

n=1

⁄ )

Equation 4.13 Finally emissions indices for ethane (EI_C2H6) and propane (EI_C3H8) are derived using their mass fractions and general emissions index for HCs from the FAA’s AEDT emissions inventory of 0.52 g kg^-1 (EI_{HC_AEDT}) as per Equation 4.14 and Equation 4.15 (presented in Table 4.1):

EI_C2H6 = C₂H_{6mass_frac} ∙ EI_{HC_AEDT}

Equation 4.14

EI_C3H8 = C₃H_{8mass_frac} ∙ EI_{HC_AEDT}

Equation 4.15 Following the process outlined above the emissions indices for ethane (EI_C2H6) and propane (EIC₃H₈) were calculated as 0.0394 g kg(fuel)^-1 and 0.03 g kg(fuel)^-1, respectively. Using the ethane and propane emissions indices derived here aviation emissions datasets for ethane and propane are calculated using Equation 4.9, substituting EIx for EIC₂H₆ (ethane) and EIC₃H₈

(propane) respectively. For sulfur dioxide an emission index (EI_SO2) of 1.176 g kg(fuel)^-1 is used (Wilkerson et al., 2010) (Table 4.1), which makes the assumption that global civil aviation jet fuel has an average sulfur content of 600 ppm (Barrett et al., 2012). Again, as with the case of carbon monoxide this emissions index is at the higher end of emissions indices from previous

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studies (Lee et al., 2010). The sulfur dioxide emissions dataset is calculated using Equation 4.9, while substituting EI_SO2 for EI_x.

As with the BC mass emissions index (EI_{BC_mass}), the emissions index for BC particle number (EIBC_particle) of 2.58x10¹⁴ particles kg(fuel)^-1 is taken from Eyers et al. (2004). Feeding EIBC_particle

in to Equation 4.9 allows for calculation of an emissions dataset for aviation-borne particle emissions. For OC a relationship between BC and OC of 4:1 is taken from Bond et al. (2004) and Hopke (1985), an emissions index (EI_OC) of 0.00625 g kg(fuel)^-1 is derived (Table 4.1). As with all previous aviation-borne emissions species EI_OC is used within Equation 4.9 to calculate the aviation emissions dataset for OC.

From the emissions indices derived or obtained from literature (Table 4.1) a monthly resolved emissions inventory covering a 3-D domain is created. Table 4.1 refers to the studies which have directly provided emissions indices or published experimental work which provided the data processed to calculate emissions indices.

Table 4.1: Emissions indices used to derive aviation emissions inventory for year 2000.

Emissions species Power setting

Nitrogen oxides NOX Cycle average 13.2 Eyers et al. (2004) Carbon monoxide CO Cycle Average 3.61 Wilkerson et al. (2010)

Formaldehyde HCHO LTO cycle 1.24 Knighton et al. (2007);

Spicer et al. (1994) Black carbon part n/a Cycle average 2.58x10¹⁴ Eyers et al. (2004) Organic carbon C Cycle average 0.00625 Bond et al. (2004);

Hopke (1985)

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