5.1 Introduction
Model simulations conducted for CMIP5 (5th Coupled Model Intercomparison Project) aim to better evaluate the role of atmospheric chemistry in driving climate change as part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), results of which feed in to the Integrated Panel on Climate Change’s (IPCC) 5th Assessment Report (AR5) (Lamarque et al., 2013b). Recommended historical emissions for aviation used by the CMIP5 models, consist only of aviation NOX (nitrogen oxides) and BC (black carbon) mass emissions (Lamarque et al., 2010b). Recent aviation emissions inventories have made efforts to include aviation-borne CO (carbon monoxide), HCs (speciated hydrocarbons), SO2 (sulfur dioxide) and OC (organic carbon) emissions (Wilkerson et al., 2010; Eyers et al., 2004; Quantify Integrated Project, 2005-2012).
Through use of the CMIP5-extended aviation emissions inventory created in Section 4.3 (which considers NOX, CO, HCs, SO2, BC and OC emissions), this Chapter investigates the atmospheric and climatic impacts of aviation as modelled using the nitrate-extended version of the TOMCAT-GLOMAP-mode coupled model (GMV4-nitrate) (Section 5.4.1). As previously discussed in Section 1.1.1 with future projected decreases in global anthropogenic SO2
emissions in tandem with projected increases in ammonia (NH3) nitrate aerosols have the potential to become the more dominant forcing aerosol component, as such the use of a chemistry-aerosol model that considers the formation of nitrate aerosols is of importance, hence the use of the nitrate-extended version of the TOMCAT-GLOMAP-mode coupled model.
Simulated climatic impacts using the CMIP5-extended emissions inventory are compared to those simulated using standard CMIP5 recommended emissions in order to investigate the atmospheric and climatic responses that could be missed through the use of an aviation emissions inventory with fewer emitted species (Section 5.4.2). In Section 5.4.3 sensitivity studies consider the impacts of the inclusion of CO, speciated HCs and SO2 emissions in turn;
thus investigating the relative impacts of the inclusion of these additional aviation emissions species in relation to the CMIP5 recommended aviation emissions inventory. Finally, simulated gas- and aerosol-phase simulated atmospheric concentrations driven by the use of the CMIP5-extended emissions inventory are re-evaluated against observational data (Section 5.5).
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5.2.1 Chemical composition of jet fuel
Through understanding the chemical composition of kerosene, it possible to gain an understanding of why comprehensive aviation emission inventories are required when investigating the atmospheric and climatic impact of civil aviation.
Typically aviation kerosene (Jet A-1/Jet A) is a multicomponent fuel with a carbon chain length of C8–C16 Blakey et al. (2011), with the most common type (Jet A-1) having a chemical formula of C12H23 (Lee et al., 2010).
The main component make up of Jet A-1 is predominately of paraffin origin (with straight chained, isoparaffins and cycloparaffins or naphthenes being present), accounting for 70–85%
of its content. The split between these paraffins is variable based on the different type of raw crude oil used. Aromatics contribute to up to 25% of the fuel blend, containing unsaturated cyclic hydrocarbons (Blakey et al., 2011).The high H:C ratio for n- and iso-paraffin gives a high heat to weight ratio and results in a clean burn. The presence of cycloparaffins reduces the H:C ratio, but their presence helps reduce the fuel freezing point (Blakey et al., 2011).
Additionally, jet fuel also contains trace amounts of sulfur, nitrogen and oxygen containing hydrocarbon species, originating from the raw crude oil feedstock used. Sulfur is present in the form of mercaptans, sulfides, disulfides, thiophenes and other sulfur containing compounds (Blakey et al., 2011). Ultimately the current total sulfur content of Jet A/Jet A-1 fuel is limited to 3000 ppm by ASTM specification ASTM D1655-09a (Blakey et al., 2011; ASTM International, 2012b).
5.2.2 Aviation emitted species
This section summarises aviation emitted species and then briefly discusses the atmospheric and climatic impacts that they have – discussed in greater detail in Section 2.2 and Section 2.4.
Figure 2.6 from Section 2.2 highlights the vast range of aviation emissions species that are release in reality, in relation to the idealised products of combustion. This range in real-world aviation emissions is a product of the different phases of flight aircraft undergo which relates to the engine power settings (Anderson et al., 2006; Knighton et al., 2007; Airbus, 2008;
Baughcum et al., 1996; Commercial Aviation Safety Team, October 2012), along with ambient
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and combustion conditions which effect combustion efficiency (Eastop and McConkey, 1993;
DuBois and Paynter, 2006).
The CMIP5-recommended historical aviation non-CO2 emissions inventory for year 2000 only report datasets for NOX and BC mass emissions. In comparison the CMIP5-extended aviation emissions inventory developed in Section 4.3 report non-CO2 aviation emission datasets for a total of 12 chemical species (inclusive of six speciated hydrocarbons).
The inclusion of CO and speciated HCs have been previously found to show minimal atmospheric impacts in the upper troposphere/lower stratosphere (UTLS), due to the magnitude of HCs released from civil aviation, resulting in negligible changes in aviation-induced O3 (Hayman and Markiewicz, 1996; Lee et al., 2010).
Recent aviation emissions inventories now include CO and HCs (Eyers et al., 2004; Lee et al., 2009; Wilkerson et al., 2010; Olsen et al., 2013b), and with past and projected rates of growth in aviation (Gudmundsson and Anger, 2012; IATA, 2015; Kreutz et al., 2008; Lee et al., 2009;
Penner et al., 1999) the impact of these species (CO and HCs) will increase in magnitude. In this Chapter sensitivity experiments will be conducted to investigate the relative impacts of both aviation-borne CO and speciated HC emission on the aviation-induced radiative effects (O3DRE, aDRE, aCAE and resulting REcomb) (Section 5.4.3).
5.2.3 Ozone radiative impact estimates from aviation NOX only emission driven studies
Figure 5.1 helps put in to context the range in aviation-induced short-lived ozone radiative effect (RE) estimates from studies that only consider aviation NOX emissions. Later in this section, the impact of including CO and speciated HC emissions on the RE from aviation-induced SL-O3 from previous studies that only consider aviation NOX emissions will be revisited, and compared to estimates obtained in this study (Section 5.4.1.2.1).
To aid with the comparison of SL-O3 RE impacts from each study, Figure 5.1 weights REs in terms of Tg of nitrogen emitted (Tg(N)) from aviation. The spread in SL-O3 RE estimates is due to different aviation NOX inventories and chemical transport models (CTMs) used, whether an ensemble of model predictions were assessed, the ozone production efficiencies within the models utilised, and vertical (and horizontal) distribution of aviation NOX emissions (Table 5.1) (Köhler et al., 2008; Myhre et al., 2011; Holmes et al., 2011; Hoor et al., 2009; Frömming et al., 2012; Skowron et al., 2013).
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Figure 5.1: Range in aviation-induced short-lived ozone (SL-O3) radiative effect estimates from studies which only consider aviation NOX emissions (Köhler et al., 2008; Myhre et al., 2011; Holmes et al., 2011; Hoor et al., 2009; Frömming et al., 2012; Skowron et al., 2013).
Table 5.1: Studies which consider aviation NOX emissions only, to quantify the aviation-induced short-lived ozone (SL-O3) radiative effect (RE), and the models and aviation emissions inventories used (Köhler et al., 2008; Myhre et al., 2011; Holmes et al., 2011; Hoor et al., 2009; Frömming et al., 2012; Skowron et al., 2013).
Study
Holmes et al., (2011) Y[22] UCI, LMDz-INCA, p-TOMCAT, TM4, Oslo-CTM2, ULAQ,
Hoor et al., (2009) N[n/a] ECHAM5/MESSy QUANTIFY
Fromming et al., (2012) N[n/a] ECHAM4.L39(DLR)/CHEM TRADEOFF 2000
Skowron et al., (2013) Y[6] MOZART FAA’s AEDT
AEM AERO2k REACT4C TRADEOFF
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From the studies considered in Figure 5.1 and Table 5.1 (Köhler et al., 2008; Myhre et al., 2011;
Holmes et al., 2011; Hoor et al., 2009; Frömming et al., 2012; Skowron et al., 2013) a mean SL-O3 RE of 24.47 mW m-2 Tg(N)-1 is calculated with a standard deviation of 5.81 mW m-2 Tg(N)-1. When a full range of aviation emission species are considered (Section 2.2) the impact of aviation on changes in atmospheric O3 concentrations can be better evaluated. The inclusion of CO emissions will participate in the consumption of OH to form H and CO2 (Reaction 2.21), which results in the cycling of OH to HO2 (Reaction 2.22). In the presence of NOX, the HO2
formed replenishes NO2 and OH concentrations (Reaction 2.1) leading to net O3 formation (Reaction 2.3 – Reaction 2.4) (Fowler et al., 1997; Jenkin and Clemitshaw, 2000).
The inclusion of HCs can lead to reductions in OH (Reaction 2.4), and the formation of NO2
which can aid in the formation of O3 (Reaction 2.24 – Reaction 2.25). Aviation-induced HCHO can lead to increases in HO2 (Reaction 2.26 – Reaction 2.28), which can lead to increases in OH and convert NO to NO2 (Reaction 2.1) (Fowler et al., 1997; Jenkin and Clemitshaw, 2000).
The through the formation of nitric acid (HNO3) from NO2 and OH (Reaction 2.2), aviation-borne emissions can contribute to the formation of nitrates (NO3-) in the troposphere (Reaction 2.20). Additionally gas-phase SO2 reacts with OH in the sulfate formation process (Reaction 2.29 – Reaction 2.31) (Fowler et al., 1997; Jenkin and Clemitshaw, 2000), while SO2 in the aqueous-phase can form sulfates via Reaction 2.32 (Jacobson, 1997).
5.3 Methodology
5.3.1 CMIP5-extended aviation emissions inventory
The investigations carried out in this section use the extended aviation emissions inventory created in Section 4.3. In comparison to CMIP5 recommended aviation emissions (Lamarque et al., 2009) this extended aviation emissions inventory provides 3-D gridded data for CO, speciated HCs (HCHO, C2H6, C3H8, CH3OH, (CH3)2CO, CH3CHO), SO2, OC mass and particle numbers (Table 4.2).
Figure 5.2 and Figure 5.3 show the distribution of species that are not considered in the CMIP5 recommended aviation emissions dataset. Most emissions occur in the Northern Hemisphere (NH) and the cruise region of flight (i.e. between an altitude of ~8–12 km).
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Figure 5.2: Zonal distributions of CMIP5-extended aviation emissions: (a) nitrogen oxides, (b) carbon monoxide, (c) total HCs, (d) sulfur dioxide, (e) black and organic carbon, and (f) particle number.
Also due to the methodology used to create the CMIP5-extended aviation emissions inventory (Section 4.3) the zonal distributions of CO and total HC emissions in comparison follow the distributions shown by NOX and BC. HCs and CO were treated as linearly scalable with fuelburn when in reality these emissions are dependent on ambient and combustor conditions (DuBois and Paynter, 2006; Baughcum et al., 1996; Owen et al., 2010).
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Figure 5.3: Spatial total column distribution of CMIP5-extended aviation emissions: (a) nitrogen oxides, (b) carbon monoxide, (c) total HCs, (d) sulfur dioxide, (e) black and organic carbon, and (f) particle number.
5.3.2 Simulations conducted
In order to investigate the atmospheric and climatic impact of an extended aviation-emissions inventory in comparison to the recommended historical aviation emissions from CMIP5, the first three scenarios outlined from Table 5.2 are simulated. CMIP5 recommended historical aviation emissions are taken from Lamarque et al. (2009), while the CMIP5-extended aviation emissions (relating to the NORM scenario) are developed in Section 4.3.
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These simulations are conducted for year 2000, thus considering year 2000 meteorology from the ECMWF (European Centre for Medium-Range Weather Forecasts), in conjunction with anthropogenic and natural emissions for the associated year.
Table 5.2: Simulations conducted to investigate the impact of aviation based on the use of an extended-emissions inventory in comparison to the CMIP5 recommended emissions in conjunction with seven sensitivity studies.
Scenarios Aviation species considered
NOAVI No aviation emissions
NORM (CMIP5-extended) All aviation emissions (NOX, CO, HCs, SO2, BC and OC)
CMIP5 NOX and BC emissions only
NoCO No CO (Simulation NORM as base)
NoHCs No HCs (Simulation NORM as base)
NoSO2 No SO2 (Simulation NORM as base)
In order to quantify the importance of each of the aviation-emitted species (or species group in the case of HCs) three sensitivity experiments are conducted to investigate each non-CO2
specie in turn (NoCO, NoHCs and NoSO2 simulations).
All simulations were conducted for 16 months from September 1999 to December 2000 inclusive, with the first four months discarded as spin-up time, with results from all simulations being compared against a simulation with aviation emissions excluded (NOAVI).
5.4 Results and Discussion
This section is split in to three parts aiming to investigate: the impact of an extended aviation emission inventory; a comparison between the atmospheric and climatic impact of the use of the CMIP5-extended aviation emissions inventory in relation to the CMIP5 recommended historical aviation emissions inventory for 2000, and; sensitivity studies aiming to assess the contribution of each aviation-borne emission species in turn (with speciated HCs treated as one set).
5.4.1 Impact of an extended aviation emissions inventory
Here aviation-induced atmospheric and climate perturbations driven by the use of the CMIP5-extended aviation emissions inventory developed in Section 4.3 (represented by the simulation NORM in Table 5.2) are discussed. Initially atmospheric perturbations from aviation-borne