Aviation emissions are a product of fuelburn and the emissions indices of each species specific emissions index (EIx). Emissions indices (EIx) represent the amount of a species of interest (x) emitted per kg of aviation fuel combusted; represented as EIx g kg(fuel)-1 (Eyers et al., 2004;
Olsen et al., 2013b). As the CMIP5 emissions inventory does not provide aviation fuelburn this needs to be calculated first in order to derive the emissions datasets for each of the additional emission species required to extend the aviation CMIP5 emissions inventory. In order to calculate fuelburn an understanding of the relationship between aviation fuelburn and emissions indices for aviation-borne species is required.
Past studies have shown that emissions of CO2 (Lee et al., 2009; Eyers et al., 2004; Kim et al., 2007; Wilkerson et al., 2010; Owen et al., 2010), water vapour (H2O) (Eyers et al., 2004; Kim et al., 2007; Wilkerson et al., 2010) and SO2 (Kim et al., 2007; Wilkerson et al., 2010) can be linearly scaled from fuelburn since they are based on total fuel composition (Wilkerson et al., 2010; Hadaller and Momenthy, 1993; Lee et al., 2010). Whereas emissions of NOX, CO and HCs are not typically linearly scalable, as these emissions are a function of a multitude of variables:
fuelburn, referenced emissions indices at sea level conditions, ambient pressure and temperature, and additionally for NOX specific humidity (Wilkerson et al., 2010; Lee et al., 2010; DuBois and Paynter, 2006; Baughcum et al., 1996). This relationship is given by the Boeing Fuel Flow Method 2 (BFFM2) which considers variations in combustor efficiency with flight conditions (DuBois and Paynter, 2006; Baughcum et al., 1996; Owen et al., 2010); as
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Where θamb is a function of ambient temperature in degrees Rankine (R) (Equation 4.4), and δamb a function of ambient pressure in pressure per square inch absolute (Equation 4.5).
Rankine (or degrees Rankine) is a linear thermodynamic scale where 0 °R is the same as 0 K, and where 459.67 °R is exactly equal to 0 °F (equivalent to 255.37 K and −17.78 °C).
θamb = Tamb⁄518.67 R
Equation 4.4 δamb = Pamb⁄14.696 psia
Equation 4.5 The factor H used in order to calculate the idealised emissions index for NOX as a function of the specific humidity (SH) of air at altitude; given by Equation 4.6 (DuBois and Paynter, 2006;
Baughcum et al., 1996).
H = (-19 × (SH-0.00634))
Equation 4.6 In past studies both linear (Wilkerson et al., 2010; Lamarque et al., 2010b) and non-linear (Wayson et al., 2009; Eyers et al., 2004) dependencies have been used when investigating the relationship between fuelburn and BC emissions.
Wayson et al. (2009) discuss that BC mass emissions are related to smoke number as per first order approximate (FOA) 3.0; where smoke number acts as a surrogate for plume opacity, which defined by FOA 1.0 acts as a mechanism to estimate non-volatile PM (particulate matter) emissions from aircraft (ICAO, Undated-b; Wayson et al., 2009). This relationship is shown to be non-linear (Wayson et al., 2009), and variable due to variations in engine behaviour with variations in power setting and engine type (Eyers et al., 2004).
NOX and BC emission datasets within the CMIP5, QUANTIFY Integrated Project and AERO2k emissions inventories were created by Eyers et al. (2004) using the FAST (Future Aviation Scenario Toolkit) model in conjunction with the PIANO (Project Interactive Analysis and Optimisation) aircraft performance model (Eyers et al., 2004; Olsen et al., 2013b; Lamarque et al., 2010b).
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The FAST model employs a two-step process to calculate aviation-emitted soot concentrations.
The first step assesses emitted soot concentrations at sea level static conditions (SLS) (Eyers et al., 2004), by considering the turbine inlet temperature (T3) based on the Brayton/Joule cycle (Eyers et al., 2004; Eastop and McConkey, 1993). The Brayton/Joule cycle is thermodynamic cycle that ideally considers a constant pressure open loop system, e.g. that within a jet engine or gas turbine (Eastop and McConkey, 1993). The next step uses the BC emissions index for SLS conditions as a base to evaluate BC emissions indices for other conditions (as shown in Where P3 = Combustor inlet pressure
T3 = Flame temperature
Φ = Equivalence ratio.
Due to the complicated relationship between combustion, ambient pressures and temperatures in relation to the BC production at sea level static conditions some recent aviation emissions inventories have produced BC emissions dataset assuming a linear relationship between aviation fuelburn and BC emissions indices (Wilkerson et al., 2010;
Lamarque et al., 2010b).
In the 2004 version of the Aviation Environmental Design Tool (AEDT) developed by the Federal Aviation Administration (FAA) with the support of the Volpe National Transportation System Centre used a BC emissions index of 0.2 g kg(fuel)-1, representing the take-off and climb phases of the flight cycle. This was updated to an emissions index of 0.035 g kg(fuel)-1 for their 2006 version of the AEDT emissions inventory in order to better represent the cruise phase of flight, and associated emissions (Wilkerson et al., 2010).
Lamarque et al. (2010b) discuss the production of CMIP5’s aviation BC emissions datasets from fuelburn estimated from FAST and PIANO; where fuelburn is assigned to routes using great circle assumptions and a BC emissions index of 0.025 g kg(fuel)-1 from Eyers et al. (2004).
Akin to BC mass emissions (and associated emissions indices), BC particle number emissions are dependent on engine power settings in addition to sampling location (Wey et al., 2007).
Despite this AERO2k have assessed an aviation-borne particle number emissions index of
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2.58x1014 particles kg(fuel)-1 (Eyers et al., 2004). In tandem with BC particle number emission indices, assessment of the associated geometric mean diameter (GMD) and geometric standard deviation (σ) particle size distributions can be evaluated for BC and OC. This methodology allows for a range of particle sizes to be considered. This is of great importance when aiming to understand aviation-induced impacts on climate as well as human health (Eyers et al., 2004).
Aviation-borne OC emissions are found to be related to the emission of aviation-borne BC emissions, and dependant on the combustion process (Bond et al., 2004). Aviation-borne carbonaceous particulate emissions are found to be largely consisting of black carbon, with a ratio between BC and OC emissions of 4:1 (Bond et al., 2004). As such the OC emissions index to be used in this study will be a quarter of the BC emissions index used by Eyers et al. (2004), i.e. 0.00625 g kg(fuel)-1.
4.3 Methodology for extending the CMIP5 aviation emissions inventory for year