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13 SECTION 3 Relative Contribution of Airport-Related Volatile Organic Compound Emissions This section discusses the relative contribution of different Most airport emission inventories classify emission sources airport emission sources to total gas-phase HAP emissions. into identical or similar categories. The emission and activity Emissions of pollutants from airports are quantified in emis- factors from each of these sources will be discussed later in sion inventories. Airports are often required to generate an this report, with the most emphasis placed on aircraft emis- emission inventory for environmental impact statements sions for two main reasons: prepared for airport expansions under the National Environ- mental Policy Act (NEPA), and/or to enable FAA to approve 1. The research reviewed indicates that aircraft emissions are federal actions occurring in nonattainment and maintenance currently the dominant source of gas-phase HAPs when areas where the FAA must show that the federal action con- using a toxicity-emissions weighting method. This obser- forms to the State Implementation Plan. vation will be discussed at length in this section. In a broader sense, quantifications of emissions data (which 2. The aircraft emissions category is the category for which rely on measurements) are used with predictive computer the knowledge base of PM and HAPs has increased models such as the EDMS, so that ambient concentrations can the most in the past few years thanks to recent field be predicted. Any computational model whose purpose is to campaigns. predict the concentration of pollutants relies on various inputs such as emissions data (e.g., grams of NOx emitted by aircraft The first point above is based on the following pieces of during one of the LTO cycle modes) and meteorology information: (e.g., wind speed and direction). The use of emissions data, predictive models, and measurements of ambient concentra- 1. Most airport emission inventories currently report that tions are complementary activities. Emissions data by them- aircraft at idle/taxi are the biggest source (in tons per year) selves (e.g., emission indices) do not predict or determine of gas-phase HAPs. This is quantitatively discussed for ambient concentrations (and risk). Use of emissions data, am- ORD, PHL, and FLL later in this section (see Tables 4 and bient concentration measurements, and models together is 5). Similar conclusions can safely be made for the airports ideal--the measurements can validate the predictions of the of Washington, D.C. (Dulles), Boston, San Diego, and model, the model provides information on which emission Long Beach based on their emission inventories (DOT sources matter most, and if validated the model can, in some and FAA 2005, Vanasse Hangen Brustlin 2006, San Diego cases, be used to predict concentrations. International Airport 2006, City of Long Beach 2005). In this report airport sources are divided into four main 2. Weighting the relative importance of the various emission categories: sources based on both mass emission rates and toxicity of the emitted gas-phase HAPs further increases the impor- 1. Aircraft, tance of aircraft emissions relative to the other airport 2. Auxiliary power units and ground support equipment sources. This is because aircraft VOC emissions contain a (APU/GSE), higher fraction of the most toxic HAPs such as acrolein, 3. Ground access vehicles (GAV), and 1,3-butadiene, and formaldehyde (compared to GSE, 4. Stationary sources. which constitutes the second biggest source of emissions- toxicity-weighted gas-phase HAPs). Stationary sources are The relative contribution of these four emission categories minor sources of HAP emissions (though in some cases for Philadelphia International Airport are shown in Figure 3. they contribute greatly to total VOC emissions). This is
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14 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, Figure 3. Emission inventory for PHL. The pie aircraft are never a minor source of overall HAP emissions chart on the left depicts total VOC emissions (see Figures 3 and 4). from the four emission sources. The pie chart These four topics are discussed in the remainder of Section on the right depicts only the emissions of the 3. It is reemphasized that although gas-phase HAP emissions 10 HAPs considered in the PHL environmental are dominated by aircraft at idle/taxi, the human health risk impact statement. presented by the various emission sources at an airport de- pends on many factors such as particulate matter emissions, visually portrayed for PHL in Figure 4. A report on emis- the exposure group, the physical layout of the airport, and the sions from Santa Monica Municipal Airport, that only local meteorology. considered benzene, formaldehyde, 1,3-butadiene, and The relative importance of the four main emission sources acetaldehyde came to a related conclusion: that aircraft at are first compared for Philadelphia International Airport taxi/idle were the dominant source of carcinogenic risk (PHL) in a series of pie charts (Figures 3 through 5) before (Piazza 1999). looking in greater depth at a few more airports. The contri- 3. It is highly likely that the actual emissions of gas-phase butions of the four main emission sources to VOC emissions HAPs by aircraft are underestimated (due to the current and gas-phase HAP emissions (consisting of formaldehyde, lack of data) in these reports--perhaps by a factor of two toluene, xylene, benzene, acetaldehyde, 1,3-butadiene, ethyl- or more. This is due to two reasons: (a) the assumed use benzene, acrolein, naphthalene, and lead) at PHL are dis- of 7% power level for aircraft idle underestimates actual played in Figure 3. HAPs emissions since commercial aircraft often idle at Figure 3 displays the emission rates of all 10 HAPs com- lower thrust settings resulting in much higher emission bined. If only on-road vehicle miles that occurred within the rates of HAPs, and (b) the HAP emission rates from airport perimeter were counted, the GAV contribution would aircraft engines at idle are much greater at low ambient be smaller. When weighted by the toxicity criteria (see Figure temperatures and this temperature dependence is not 4), it is seen that the importance of the aircraft and GSE reflected in emission inventories. source increases, while the importance of stationary sources 4. Many airports (especially those in nonattainment areas) almost vanishes. This is because alkanes, which are the pre- have had success in reducing emissions from GSE, and the dominant type of VOCs emitted by stationary sources, are action required to reduce GSE emissions is fairly straight- not very toxic. The IRIS toxicity value for formaldehyde was Table 4. Total volatile organic compound emission profiles. 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