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15 Table 5. Emission inventory for selected hazardous air pollutants. HAP Aircraft GSE/APU GAV Stat Philadelphia International Airport (PHL) (tons/yr) Benzene 3.3 7.24 2.3 0 Formaldehyde 24.5 3.2 2.2 0.2 1,3-butadiene 2.9 1.2 0.4 0 Acrolein 3.7 0.12 0.1 0 Toluene 1.1 11.1 6.6 0.1 Fort Lauderdale-Hollywood International Airport (FLL) (tons/yr) Benzene 8.7 7 0 0 Formaldehyde 66.6 10.7 2.7 0.4 1,3-butadiene 8.1 1.1 1.5 NA Acrolein 8.7 0.4 1.8 NA Toluene 3.8 8.0 1.1 1.8 Chicago O'Hare International Airport (ORD) (tons/yr) Benzene 6.4 20.3 40 0 Formaldehyde 38.8 7.4 16.0 0.1 1,3-butadiene 10.5 4.2 4.8 NA Acrolein 1.1 0.3 0.7 NA Toluene 3.2 38.1 95.6 0 Notes : HAP hazardous air pollutant Air aircraft GSE ground support equipment GAV ground access vehicles Stat stationary sources tons/yr tons per year NA not available (negligible) used for Figure 4. The PHL emission inventory was calculated using 7% thrust for the power level used during aircraft HAP Emissions Toxicity Scaled HAP Emissions (tons/year) 3 (tons/year) ÷ (mg/m ) idle/taxi, as this is the certification value for idle thrust used by the International Civil Aviation Organization (ICAO). Use of a lower, more realistic thrust level (i.e., "ground idle") results in much higher emission rates (see Section 5.1). The "extra" area near the aircraft wedge in Figure 4 represents a 60% increase of gas-phase HAP emissions, assuming that during idle/taxi the aircraft spends equal amounts of time at 7% thrust and 4% thrust. The true distribution of thrust lev- els used during aircraft idle/taxi is highly uncertain (i.e., how 50/50 split of `idle' phase into much time is spent at different thrust levels). ICAO 7% thrust and `ground idle' Figure 5 compares the total mass emission rates of the Figure 4. Comparison of PHL HAP emissions 10 individual HAPs to the toxicity-weighted emissions without scaled by mass and by mass-toxicity. Weighting regard to the emission source (e.g., aircraft versus GSE). The the emissions by the toxicity of the 10 HAPs most important airport-related gas-phase HAPs are acrolein, considered indicates that the aircraft are the 1,3-butadiene, and benzene. Aircraft are the biggest emitters of predominant source of the most important HAPs. acrolein, 1,3-butadiene, and formaldehyde, whereas gasoline The largest wedge represents the HAP emissions GSE can be the biggest source of benzene. This calculation used as calculated using 7% thrust for the idle phase. the IRIS toxicity for formaldehyde, although use of the IRIS The extra "shell" around the aircraft wedge value does not appreciably change the figure. represents a 60% increase in HAP emissions as calculated by assuming that idle thrust is an equal mix (in time) of 7% thrust and 4% thrust. The 3.1 Source Apportionment actual thrust levels used by aircraft are one of the biggest information gaps identified in this In this section the relative importance of the various air- report, and result in uncertainty of the aircraft port emission sources is discussed quantitatively in more contribution to gas-phase HAPs by at least a detail using the emission inventories of three major airports: factor of 2. Philadelphia, Fort Lauderdale, and Chicago O'Hare (KM
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16 Figure 5. Comparison of total HAP emissions at PHL by mass and by mass-toxicity. Relative risk presented by the 10 HAPs considered in the PHL environmental impact statement. The risk is calculated by consideration of both total mass emissions and toxicity criteria. Chng 2005; Landrum and Brown 2007; and FAA 2005). organic matter (POM) (as 16-PAH). Similarly, FLL EIS states These emission inventories were all made as part of environ- that aircraft only account for ~25% of total VOC emissions, mental impact statements pursuant to the National Environ- but still account for more than 50% of the emissions for most mental Policy Act (NEPA) and were created used the FAA's of the compounds listed above. If these inventories are cor- Emissions Dispersion Modeling System, as required. rect, then aircraft appear to be the greatest airport source (in The total VOC emissions reported by these four airports tons per year) of most air toxics, with the most noticeable are compared in Table 4. Table 5 displays the reported emis- exceptions above being benzene and toluene. At ORD, air- sion inventory for five selected gas-phase HAPs from three of craft only account for 10% of benzene emissions. But as the four airports. The five HAPs selected (benzene, formalde- shown below, this estimate includes off-airport emissions hyde, 1,3-butadiene, acrolein, and toluene) have traditionally from GAV. The fractional contribution of aircraft to benzene been considered to be among the most important air toxics emissions that occur at the airport is likely much greater. and other studies have focused on these compounds (e.g., In summary, for PHL, FLL, and ORD, aircraft are the Piazza 1999; FAA 2005). biggest overall emitters of gas-phase HAPs when only emis- Tables 4 and 5 form the basis for the discussion of source sions within the airport perimeter are considered. Of note is apportionment. that reported VOC and HAP emissions at FLL are compara- ble to those at PHL and ORD even though activity levels (e.g., number of flights) at FLL are significantly lower than PHL 3.1.1 Overall Source Apportionment and ORD. The explanation for this is currently unclear. At BOS, the aircraft category is the biggest source of VOCs, whereas PHL and FLL's inventories gave roughly equal 3.1.2 Ground Access Vehicles weight to the four main categories. ORD, meanwhile, only attributes 21% of VOC emissions to aircraft due to the large There is great variation among inventories in the treatment fraction for which GAV account. This is at least in part due to of GAV emissions. While there can be significant differences ORD's inclusion of GAV emissions outside of the airport in the vehicle fleets and number of vehicle visits among the proper as was needed based on the purpose of the emissions airports, the distance to which vehicle miles are counted as inventory. part of the airport inventory is the biggest variable. GAV HAP emissions are not necessarily proportional to total account for only 3% of total VOC emissions in the IAD VOC emissions, however. Indeed as seen in Table 5, aircraft (Washington Dulles Airport, not shown) inventory mainly are reported to be the greatest source of formaldehyde, 1,3- because only on-site vehicle miles are included. Airport- butadiene and acrolein at all four airports, even those in related miles driven on the connecting highways/roadways which aircraft are not reported as the biggest source of VOCs. are explicitly excluded because they are already included else- PHL's full inventory states that aircraft account for well over where in the Transportation Improvement Plan for Fairfax 50% of emissions of the following additional HAPs: acetalde- and Loudoun counties and a redundancy was undesirable hyde; naphthalene, propionaldehyde, styrene, and polycyclic (DOT, FAA et al. 2005). In contrast, Portland International
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17 Airport (PDX, not included above) counts vehicle miles PHL to 24:1 for ATL (not shown above) (Unal, Hu et al. driven up to a distance of 35 miles from the airport, and not 2005). The average time that aircraft spend idling and the surprisingly attributes 50% of its total VOC emissions to composition of the aircraft fleet and GSE fleet are all factors GAV (S. Hartsfield, personal communication). Similarly, the that contribute greatly to this ratio. The low GSE/APU emis- ORD EIS emissions inventory accounted for vehicle miles sions from BOS are likely attributable, at least in part, to driven on a geographic area that extends well beyond the Massachusetts Port Authority's (Massport's) Alternative Fuel bounds of the airport grounds. Such drastic differences are Vehicle Program, which entails the conversion of vehicles to not flaws in any inventory, but do underscore the wide vari- run on natural gas or electricity. ation in methodologies and the need to interpret such data in Even in cases where the reported VOC emissions from GSE context. Not all emission inventories attempt to answer the are equal (in tons per year) to those from the aircraft, such as same questions and these differences are most often associ- at PHL, FLL and ORD, the majority of the most toxic gas- ated with the purpose of the emissions inventory. Consider phase HAPs compounds are emitted by the aircraft. This was the following questions: shown in Figure 4 for PHL and more generally in Table 3 in which aircraft and GSE were assumed to emit equal amounts · What are the total emissions associated with the existence of VOC emissions by mass based on the emission inventories of the airport? of PHL, FLL and ORD. This scenario represents the largest · What change in emissions would be associated with a given plausible contribution by GSE. That is, GSE emissions of construction/upgrade project at an airport? VOCs are at most equal to aircraft emissions of VOCs, and at · What are the total emissions that come from within the air- airports that have modernized the GSE fleet to alternative fuel port perimeter? (CNG, electricity) the GSE emissions of VOCs are substan- tially smaller than aircraft VOC emissions. Nevertheless, the This report focuses on aircraft emissions for the reasons relative toxicity-weighted emissions are largest for aircraft discussed in Section 3. emissions, as shown in Figure 4. For smaller GSE VOC emis- sions (as reported by BOS (Vanasse Hangen Brustlin 2006) and ATL (Unal, Hu et al. 2005), all significant HAP emissions 3.1.3 Stationary Sources are dominated by the aircraft source. Stationary sources such as evaporative emissions from fuel storage, HVAC systems, and generators account for 25% to 3.1.5 Note on Fuel-Based Inventories 30% of reported VOC emissions at BOS and PHL, but are only reported at a minor 2% at ORD. This is one of many Currently it is difficult to assess the accuracy of GSE emis- potential discrepancies among the self-reported airport emis- sions as reported in emission inventories since fuel-based sions that could be elucidated if accurate fuel use inventories inventories are not available. A fuel-based inventory would list were available (Section 3.1.5). At PHL, evaporative emissions the total amount of gasoline, diesel, CNG, and jet fuel dis- account for less than 1% of the total emissions for all 16 toxic pensed at the airport for use in GSE, aircraft, and stationary pollutants (HAPs) listed in its environmental impact state- sources. Some environmental impact statements report some ment, including benzene. One of the few HAPs for which of this information (e.g., at ORD and PHL (URS 2003; KM stationary sources can constitute a nontrivial source is Chng 2005); however, it is not clear if those reported numbers toluene, for which FLL attributes 12% due to painting activ- reflect the true amount of fuel consumed by GSE since outside ities. As discussed later in this report, the toxicity of toluene contractors are usually hired to supply fuel. In principle, the is low compared to other gas-phase HAPs emitted at airports. use of fuel-based inventories is a very appealing approach to The speciation of evaporative emissions is very different from quantifying the emissions from GSE and stationary sources. that for exhaust sources. Evaporative emissions contain a Emission factors for GSE are currently expressed as grams of high fraction of alkanes, which are both relatively benign and pollutant per brake-horsepower hour (see Section 5), and so unreactive photochemically, whereas alkenes and oxygenated calculation of total GSE emissions depends on knowledge of compounds are mostly absent from such emissions. Hence, how much time each type of engine spends at a given work- airport stationary sources are unlikely to be an important load. Fuel-based emission factors (i.e, grams of pollutant per source of the more toxic aviation-related HAPs compounds. kg of fuel) for gasoline and diesel engines have been exten- sively studied. Hence, reasonable calculations could be made regarding HAP emissions from GSE if accurate fuel invento- 3.1.4 Ground Support Equipment ries were available. The ratio of aircraft to GSE VOC emissions varies enor- For example, at some airports aircraft and GSE are reported mously among airport emission inventories: from 1:1 for to emit comparable amounts of total VOCs. Consider a single
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18 LTO cycle for a Boeing 737 with two CFM56-7B22 engines. equal for aircraft and GSE--or much less fuel if the GSE fuel- Calculations using the ICAO certification values for time-in- based emission indices are higher than the aircraft emission mode and fuel flow rate (consisting of 26 min of idling at 7% indices. Without airport fuel inventories, this calculation is thrust, which is not necessarily an accurate portrayal of the very difficult to execute. Of note is the estimate in the ORD en- idle phase) indicate that the aircraft consumes 328 kg of jet vironmental impact statement (FAA 2005, see Section 126.96.36.199) fuel (115 gallons) during the idle/taxi phase, which is when the that the average fuel consumption by GSE vehicles "per air- vast majority of HAP emissions occur (detailed later in Sec- craft operation" is 3.2 gallons. Such a calculation for the entire tion 5, Figure 13). For the GSE emissions of HAPs per LTO to airport would need to reflect the entire fleet of aircraft and be comparable to the aircraft contribution, the GSE would ei- GSE vehicles in use, and the emission indices of VOCs (and ther have to consume a comparable amount of fuel--if the therefore gas-phase HAPs) vary greatly among different air- HAP emission indices (in grams of pollutant per kg of fuel) are craft engines and GSE vehicles.