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Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis (2008)

Chapter: Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions

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Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
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Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
×
Page 14
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Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
×
Page 15
Page 16
Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
×
Page 16
Page 17
Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
×
Page 17
Page 18
Suggested Citation:"Section 3 - Relative Contribution of Airport-Related Volatile Organic Compound Emissions." National Academies of Sciences, Engineering, and Medicine. 2008. Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis. Washington, DC: The National Academies Press. doi: 10.17226/14168.
×
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13 This section discusses the relative contribution of different airport emission sources to total gas-phase HAP emissions. Emissions of pollutants from airports are quantified in emis- sion inventories. Airports are often required to generate an emission inventory for environmental impact statements prepared for airport expansions under the National Environ- mental Policy Act (NEPA), and/or to enable FAA to approve federal actions occurring in nonattainment and maintenance areas where the FAA must show that the federal action con- forms to the State Implementation Plan. In a broader sense, quantifications of emissions data (which rely on measurements) are used with predictive computer models such as the EDMS, so that ambient concentrations can be predicted. Any computational model whose purpose is to predict the concentration of pollutants relies on various inputs such as emissions data (e.g., grams of NOx emitted by aircraft during one of the LTO cycle modes) and meteorology (e.g., wind speed and direction). The use of emissions data, predictive models, and measurements of ambient concentra- tions are complementary activities. Emissions data by them- selves (e.g., emission indices) do not predict or determine ambient concentrations (and risk). Use of emissions data, am- bient concentration measurements, and models together is ideal—the measurements can validate the predictions of the model, the model provides information on which emission sources matter most, and if validated the model can, in some cases, be used to predict concentrations. In this report airport sources are divided into four main categories: 1. Aircraft, 2. Auxiliary power units and ground support equipment (APU/GSE), 3. Ground access vehicles (GAV), and 4. Stationary sources. The relative contribution of these four emission categories for Philadelphia International Airport are shown in Figure 3. Most airport emission inventories classify emission sources into identical or similar categories. The emission and activity factors from each of these sources will be discussed later in this report, with the most emphasis placed on aircraft emis- sions for two main reasons: 1. The research reviewed indicates that aircraft emissions are currently the dominant source of gas-phase HAPs when using a toxicity-emissions weighting method. This obser- vation will be discussed at length in this section. 2. The aircraft emissions category is the category for which the knowledge base of PM and HAPs has increased the most in the past few years thanks to recent field campaigns. The first point above is based on the following pieces of information: 1. Most airport emission inventories currently report that aircraft at idle/taxi are the biggest source (in tons per year) of gas-phase HAPs. This is quantitatively discussed for ORD, PHL, and FLL later in this section (see Tables 4 and 5). Similar conclusions can safely be made for the airports of Washington, D.C. (Dulles), Boston, San Diego, and Long Beach based on their emission inventories (DOT and FAA 2005, Vanasse Hangen Brustlin 2006, San Diego International Airport 2006, City of Long Beach 2005). 2. Weighting the relative importance of the various emission sources based on both mass emission rates and toxicity of the emitted gas-phase HAPs further increases the impor- tance of aircraft emissions relative to the other airport sources. This is because aircraft VOC emissions contain a higher fraction of the most toxic HAPs such as acrolein, 1,3-butadiene, and formaldehyde (compared to GSE, which constitutes the second biggest source of emissions- toxicity-weighted gas-phase HAPs). Stationary sources are minor sources of HAP emissions (though in some cases they contribute greatly to total VOC emissions). This is S E C T I O N 3 Relative Contribution of Airport-Related Volatile Organic Compound Emissions

visually portrayed for PHL in Figure 4. A report on emis- sions from Santa Monica Municipal Airport, that only considered benzene, formaldehyde, 1,3-butadiene, and acetaldehyde came to a related conclusion: that aircraft at taxi/idle were the dominant source of carcinogenic risk (Piazza 1999). 3. It is highly likely that the actual emissions of gas-phase HAPs by aircraft are underestimated (due to the current lack of data) in these reports—perhaps by a factor of two or more. This is due to two reasons: (a) the assumed use of 7% power level for aircraft idle underestimates actual HAPs emissions since commercial aircraft often idle at lower thrust settings resulting in much higher emission rates of HAPs, and (b) the HAP emission rates from aircraft engines at idle are much greater at low ambient temperatures and this temperature dependence is not reflected in emission inventories. 4. Many airports (especially those in nonattainment areas) have had success in reducing emissions from GSE, and the action required to reduce GSE emissions is fairly straight- forward—upgrade the GSE fleet (e.g., to run on com- pressed natural gas [CNG] or electricity, see Section 5.2.2). In contrast, the most appropriate methods to re- duce aircraft emissions are not as straightforward. Ground access vehicles (GAV) only make an appreciable contribution to total airport gas-phase HAP emissions in emission inventories when off-airport miles driven are con- sidered (as is required for some environmental impact state- ments). Even when off-airport miles driven are considered, aircraft are never a minor source of overall HAP emissions (see Figures 3 and 4). These four topics are discussed in the remainder of Section 3. It is reemphasized that although gas-phase HAP emissions are dominated by aircraft at idle/taxi, the human health risk presented by the various emission sources at an airport de- pends on many factors such as particulate matter emissions, the exposure group, the physical layout of the airport, and the local meteorology. The relative importance of the four main emission sources are first compared for Philadelphia International Airport (PHL) in a series of pie charts (Figures 3 through 5) before looking in greater depth at a few more airports. The contri- butions of the four main emission sources to VOC emissions and gas-phase HAP emissions (consisting of formaldehyde, toluene, xylene, benzene, acetaldehyde, 1,3-butadiene, ethyl- benzene, acrolein, naphthalene, and lead) at PHL are dis- played in Figure 3. Figure 3 displays the emission rates of all 10 HAPs com- bined. If only on-road vehicle miles that occurred within the airport perimeter were counted, the GAV contribution would be smaller. When weighted by the toxicity criteria (see Figure 4), it is seen that the importance of the aircraft and GSE source increases, while the importance of stationary sources almost vanishes. This is because alkanes, which are the pre- dominant type of VOCs emitted by stationary sources, are not very toxic. The IRIS toxicity value for formaldehyde was 14 BOS 2005 PHL 2003 FLL 2005 ORD 2002 Source tons/yr % tons/yr % tons/yr % tons/yr % Aircraft 240 47 141 26 155 32 424 21 GSE/APU 71 14 142 26 137 28 414 20 GAV 47 9 118 22 154 31 1,149 57 Stationary 156 30 134 25 46 9 34 2 Total 514 100 536 100 492 100 2,021 100 Notes: BOS Boston Logan International Airport (Vanasse Hangen Brustlin 2006) PHL Philadelphia International Airport (KM Chng Environmental 2005) FLL Fort Lauderdale-Hollywood International Airport (Landrum & Brown 2007) ORD Chicago O’Hare International Airport (FAA 2005) tons/yr tons per year GSE/APU ground support equipment/auxiliary power unit GAV ground access vehicles Table 4. Total volatile organic compound emission profiles. Figure 3. Emission inventory for PHL. The pie chart on the left depicts total VOC emissions from the four emission sources. The pie chart on the right depicts only the emissions of the 10 HAPs considered in the PHL environmental impact statement.

used for Figure 4. The PHL emission inventory was calculated using 7% thrust for the power level used during aircraft 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 much time is spent at different thrust levels). Figure 5 compares the total mass emission rates of the 10 individual HAPs to the toxicity-weighted emissions without regard to the emission source (e.g., aircraft versus GSE). The most important airport-related gas-phase HAPs are acrolein, 1,3-butadiene, and benzene. Aircraft are the biggest emitters of acrolein, 1,3-butadiene, and formaldehyde, whereas gasoline GSE can be the biggest source of benzene. This calculation used the IRIS toxicity for formaldehyde, although use of the IRIS value does not appreciably change the figure. 3.1 Source Apportionment In this section the relative importance of the various air- port emission sources is discussed quantitatively in more detail using the emission inventories of three major airports: Philadelphia, Fort Lauderdale, and Chicago O’Hare (KM 15 Toxicity Scaled HAP Emissions (tons/year) ÷ (mg/m3) 50/50 split of ‘idle’ phase into ICAO 7% thrust and ‘ground idle’ HAP Emissions (tons/year) Figure 4. Comparison of PHL HAP emissions scaled by mass and by mass-toxicity. Weighting the emissions by the toxicity of the 10 HAPs considered indicates that the aircraft are the predominant source of the most important HAPs. The largest wedge represents the HAP emissions as calculated using 7% thrust for the idle phase. The extra “shell” around the aircraft wedge 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 actual thrust levels used by aircraft are one of the biggest information gaps identified in this report, and result in uncertainty of the aircraft contribution to gas-phase HAPs by at least a factor of 2. 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/y r tons per y ear NA not available (negligible) Table 5. Emission inventory for selected hazardous air pollutants.

Chng 2005; Landrum and Brown 2007; and FAA 2005). These emission inventories were all made as part of environ- mental impact statements pursuant to the National Environ- mental Policy Act (NEPA) and were created used the FAA’s Emissions Dispersion Modeling System, as required. The total VOC emissions reported by these four airports are compared in Table 4. Table 5 displays the reported emis- sion inventory for five selected gas-phase HAPs from three of the four airports. The five HAPs selected (benzene, formalde- hyde, 1,3-butadiene, acrolein, and toluene) have traditionally been considered to be among the most important air toxics and other studies have focused on these compounds (e.g., Piazza 1999; FAA 2005). Tables 4 and 5 form the basis for the discussion of source apportionment. 3.1.1 Overall Source Apportionment At BOS, the aircraft category is the biggest source of VOCs, whereas PHL and FLL’s inventories gave roughly equal weight to the four main categories. ORD, meanwhile, only attributes 21% of VOC emissions to aircraft due to the large fraction for which GAV account. This is at least in part due to ORD’s inclusion of GAV emissions outside of the airport proper as was needed based on the purpose of the emissions inventory. HAP emissions are not necessarily proportional to total VOC emissions, however. Indeed as seen in Table 5, aircraft are reported to be the greatest source of formaldehyde, 1,3- butadiene and acrolein at all four airports, even those in which aircraft are not reported as the biggest source of VOCs. PHL’s full inventory states that aircraft account for well over 50% of emissions of the following additional HAPs: acetalde- hyde; naphthalene, propionaldehyde, styrene, and polycyclic organic matter (POM) (as 16-PAH). Similarly, FLL EIS states that aircraft only account for ~25% of total VOC emissions, but still account for more than 50% of the emissions for most of the compounds listed above. If these inventories are cor- rect, then aircraft appear to be the greatest airport source (in tons per year) of most air toxics, with the most noticeable exceptions above being benzene and toluene. At ORD, air- craft only account for 10% of benzene emissions. But as shown below, this estimate includes off-airport emissions from GAV. The fractional contribution of aircraft to benzene emissions that occur at the airport is likely much greater. In summary, for PHL, FLL, and ORD, aircraft are the biggest overall emitters of gas-phase HAPs when only emis- sions within the airport perimeter are considered. Of note is 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 and ORD. The explanation for this is currently unclear. 3.1.2 Ground Access Vehicles There is great variation among inventories in the treatment of GAV emissions. While there can be significant differences in the vehicle fleets and number of vehicle visits among the airports, the distance to which vehicle miles are counted as part of the airport inventory is the biggest variable. GAV account for only 3% of total VOC emissions in the IAD (Washington Dulles Airport, not shown) inventory mainly because only on-site vehicle miles are included. Airport- related miles driven on the connecting highways/roadways are explicitly excluded because they are already included else- where in the Transportation Improvement Plan for Fairfax and Loudoun counties and a redundancy was undesirable (DOT, FAA et al. 2005). In contrast, Portland International 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.

Airport (PDX, not included above) counts vehicle miles driven up to a distance of 35 miles from the airport, and not surprisingly attributes 50% of its total VOC emissions to GAV (S. Hartsfield, personal communication). Similarly, the ORD EIS emissions inventory accounted for vehicle miles driven on a geographic area that extends well beyond the bounds of the airport grounds. Such drastic differences are not flaws in any inventory, but do underscore the wide vari- ation in methodologies and the need to interpret such data in context. Not all emission inventories attempt to answer the same questions and these differences are most often associ- ated with the purpose of the emissions inventory. Consider the following questions: • What are the total emissions associated with the existence of the airport? • What change in emissions would be associated with a given construction/upgrade project at an airport? • What are the total emissions that come from within the air- port perimeter? This report focuses on aircraft emissions for the reasons discussed in Section 3. 3.1.3 Stationary Sources Stationary sources such as evaporative emissions from fuel storage, HVAC systems, and generators account for 25% to 30% of reported VOC emissions at BOS and PHL, but are only reported at a minor 2% at ORD. This is one of many potential discrepancies among the self-reported airport emis- sions that could be elucidated if accurate fuel use inventories were available (Section 3.1.5). At PHL, evaporative emissions account for less than 1% of the total emissions for all 16 toxic pollutants (HAPs) listed in its environmental impact state- ment, including benzene. One of the few HAPs for which stationary sources can constitute a nontrivial source is toluene, for which FLL attributes 12% due to painting activ- ities. As discussed later in this report, the toxicity of toluene is low compared to other gas-phase HAPs emitted at airports. The speciation of evaporative emissions is very different from that for exhaust sources. Evaporative emissions contain a high fraction of alkanes, which are both relatively benign and unreactive photochemically, whereas alkenes and oxygenated compounds are mostly absent from such emissions. Hence, airport stationary sources are unlikely to be an important source of the more toxic aviation-related HAPs compounds. 3.1.4 Ground Support Equipment The ratio of aircraft to GSE VOC emissions varies enor- mously among airport emission inventories: from 1:1 for PHL to 24:1 for ATL (not shown above) (Unal, Hu et al. 2005). The average time that aircraft spend idling and the composition of the aircraft fleet and GSE fleet are all factors that contribute greatly to this ratio. The low GSE/APU emis- sions from BOS are likely attributable, at least in part, to Massachusetts Port Authority’s (Massport’s) Alternative Fuel Vehicle Program, which entails the conversion of vehicles to run on natural gas or electricity. Even in cases where the reported VOC emissions from GSE are equal (in tons per year) to those from the aircraft, such as at PHL, FLL and ORD, the majority of the most toxic gas- phase HAPs compounds are emitted by the aircraft. This was shown in Figure 4 for PHL and more generally in Table 3 in which aircraft and GSE were assumed to emit equal amounts of VOC emissions by mass based on the emission inventories of PHL, FLL and ORD. This scenario represents the largest plausible contribution by GSE. That is, GSE emissions of VOCs are at most equal to aircraft emissions of VOCs, and at airports that have modernized the GSE fleet to alternative fuel (CNG, electricity) the GSE emissions of VOCs are substan- tially smaller than aircraft VOC emissions. Nevertheless, the relative toxicity-weighted emissions are largest for aircraft 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 are dominated by the aircraft source. 3.1.5 Note on Fuel-Based Inventories Currently it is difficult to assess the accuracy of GSE emis- sions as reported in emission inventories since fuel-based inventories are not available. A fuel-based inventory would list the total amount of gasoline, diesel, CNG, and jet fuel dis- pensed at the airport for use in GSE, aircraft, and stationary sources. Some environmental impact statements report some of this information (e.g., at ORD and PHL (URS 2003; KM Chng 2005); however, it is not clear if those reported numbers reflect the true amount of fuel consumed by GSE since outside contractors are usually hired to supply fuel. In principle, the use of fuel-based inventories is a very appealing approach to quantifying the emissions from GSE and stationary sources. Emission factors for GSE are currently expressed as grams of pollutant per brake-horsepower hour (see Section 5), and so calculation of total GSE emissions depends on knowledge of how much time each type of engine spends at a given work- load. Fuel-based emission factors (i.e, grams of pollutant per 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- ries were available. For example, at some airports aircraft and GSE are reported to emit comparable amounts of total VOCs. Consider a single 17

LTO cycle for a Boeing 737 with two CFM56-7B22 engines. Calculations using the ICAO certification values for time-in- mode and fuel flow rate (consisting of 26 min of idling at 7% thrust, which is not necessarily an accurate portrayal of the idle phase) indicate that the aircraft consumes 328 kg of jet fuel (115 gallons) during the idle/taxi phase, which is when the vast majority of HAP emissions occur (detailed later in Sec- tion 5, Figure 13). For the GSE emissions of HAPs per LTO to be comparable to the aircraft contribution, the GSE would ei- ther have to consume a comparable amount of fuel—if the HAP emission indices (in grams of pollutant per kg of fuel) are equal for aircraft and GSE—or much less fuel if the GSE fuel- based emission indices are higher than the aircraft emission indices. Without airport fuel inventories, this calculation is very difficult to execute. Of note is the estimate in the ORD en- vironmental impact statement (FAA 2005, see Section 5.1.1.7) that the average fuel consumption by GSE vehicles “per air- craft operation” is 3.2 gallons. Such a calculation for the entire airport would need to reflect the entire fleet of aircraft and GSE vehicles in use, and the emission indices of VOCs (and therefore gas-phase HAPs) vary greatly among different air- craft engines and GSE vehicles. 18

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TRB’s Airport Cooperative Research Program (ACRP) Report 7: Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis examines the state of the latest research on aviation-related hazardous air pollutants emissions and explores knowledge gaps that existing research has not yet bridged.

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