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

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31.1 Report Motivation Understanding the emission sources, ambient concentra- tions, human exposure parameters, and health risk factors of hazardous air pollutants associated with an airport is neces- sary to fully protect the health of personnel working at the air- port, minimize exposure for the traveling public, and avoid adverse impacts on the air quality of nearby neighborhoods. The goal of this project is to produce a prioritized agenda for future research that will address critical information gaps as- sociated with airport-related hazardous air pollutants (HAPs). 1.2 Report Overview A literature review was conducted on the following topics: • Aircraft/airport HAP emission factors, • Airport activity and fuel use factors, • Related ambient HAP concentrations, • Dispersion modeling of airport emissions, and • Toxicology of aviation-related HAPs. This information was used to • Assess the current state of knowledge regarding airport- related HAPs, • Identify the information gaps that limit our ability to assess fully the impact of airport HAP emissions on human health, • Determine which compounds are likely to present the most risk to human health, and • Determine which aviation-related emission sources are most significant. The literature survey drew on several sources including: (1) Boolean searches using Google Scholar and Web of Science, and (2) communication (telephone/email) with aviation researchers and airport operators. These searches yielded information in several forms including articles pub- lished in the peer-reviewed literature, available emission inventories, reports from recent aircraft emission measure- ment campaigns, airport environmental impact statements, and unpublished data (recent experimental results that are not yet published and emissions data collected by airport op- erators). Our findings are used to create a prioritized research agenda that will address the identified information gaps. 1.3 Background Information on Hazardous Air Pollutants The U.S. Environmental Protection Agency (EPA) has six designated “criteria” air pollutants, which are known to be damaging to public health: ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb), and particulate matter (PM). There is a national network of moni- toring stations that make continuous measurements of these pollutants. The concentrations of these pollutants must not ex- ceed pre-set National Ambient Air Quality Standards (NAAQS) established by EPA to protect public health and welfare. In addition to the criteria pollutants, there are more than one hundred other air pollutants that are either known or suspected to be hazardous. These are known as “hazardous air pollutants” or HAPs, or alternately “air toxics.” Two com- monly known examples are benzene and formaldehyde. As dictated by the Clean Air Act, the EPA maintains a list of these HAPs. Additionally, for mobile source emissions the EPA maintains a “Master List of Compounds Emitted by Mobile Sources,” which is available at www.epa.gov/otaq/toxics.htm. Aside from federal standards, many individual states also have programs governing emissions of HAPs. In many in- stances, the list of state HAPs differs with the USEPA desig- nation. The nature and structure of these programs differ considerably from state to state. Measurements of ambient HAP concentrations are not as widespread as those of the S E C T I O N 1 Introduction

criteria pollutants. Descriptions of individual HAPs and their affects on human health have been published in recent docu- ments (URS 2003; FAA 2005). There are many sources that emit HAPs—ground transportation, construction, power generation, dry cleaning, and aviation—to name a few. At airports there are several sources of HAP emissions. A partial list of “airside” sources (vehicles that do not leave the airport) includes baggage tugs and other ground support equipment (GSE), solvent use, and the aircraft themselves. There are additional “roadside” sources, also called ground access vehi- cles (GAVs), that consist mainly of on-road vehicles (cars, buses, shuttles, etc.). This report focuses on gas-phase HAPs, which in the air- port context is mainly a subcategory of volatile organic com- pounds (VOCs). A related report, ACRP Report 6: Research Needs Associated with Particulate Emissions at Airports, addresses airport particulate matter emissions. 1.4 Approach Used for Identifying Information Gaps Associated with Airport-Related Hazardous Air Pollutants What is the risk presented by airport-related hazardous air pollutants? Information gaps preclude a definitive answer to this question; however, knowledge of which airport sources matter the most and which hazardous air pollutants are most important will surely assist in addressing that question. This section discusses the approach used to determine which pieces of information are most crucial to determining the rel- ative risk presented by airport-related HAPs. Note that this report is not a human health risk assessment and therefore does not evaluate knowledge gaps based on exposure risk, which is explained in more detail further in this section. In this report airport emission sources are divided into four main categories: 1. Aircraft, 2. GSE (baggage tugs, belt loaders, etc.) and auxiliary power units (APU), 3. GAV (cars, buses, shuttles, etc.), and 4. Stationary sources (power generation, HVAC systems, solvent use, etc.) Quantification of the “risk” posed to a particular exposure group by the emissions of HAPs by these numerous emission sources at an airport is complicated. The risk presented to human health by any particular compound is a function of exposure and toxicity. Exposure and toxicity are in turn both a function of the exposure pathway (e.g., ingestion, dermal contact, inhalation). For the inhalation pathway, exposure is a function of exposure time, and the concentration of a pol- lutant, which is determined by the quantity emitted from the various sources and how these emissions disperse and react in the air, as depicted in Figure 1. The emission sources and pollutants that present the great- est risk to the health of a baggage tug operator may differ greatly from those that affect the health of a nearby resident. Consider a few exposure groups related to an airport (Figure 2). A baggage tug operator’s exposure to airport pollutants is presumably greatly influenced by those emission sources that she or he is closest to, for example, baggage tugs, belt loaders, and possibly idling aircraft engines and auxiliary power units. Residential Neighborhood 3 is sandwiched between the air- port runways and the ocean. Since there are few busy roads nearby, the emission source to which these residents experi- ence the most exposure might be the aircraft themselves during the various phases of the landing take-off (LTO) cycle. Residential Neighborhood 1 is near the airport ground traf- fic, the downtown business district, and a number of busy roads. Residents of Neighborhood 1 are presumably exposed to a wider range of emission sources—airport and non- airport related. Furthermore, the local meteorology (e.g., wind direction and speed) is another parameter that greatly affects the relative importance of these various airport emis- sion sources. As evident by the example of just this one airport and a few exposure groups, there are many parameters that determine risk. A full health risk assessment that is relevant for the clos- est exposure groups must be based on exposure and toxicity, which must be done on a case-by-case basis and may be different from airport to airport. This is beyond the scope of this report. An example of such an undertaking is the 2003 supplemental environmental impact report for Oakland International Airport (OAK) (CDM 2003), which explicitly calculated the risk presented to a number of exposure groups due to emissions from OAK. This report provides a preliminary assessment of which airport sources and which individual hazardous air pollutants should be closely considered in future research. This assess- ment is based on both the toxicity of the individual pollutants and the amount emitted. It is not possible to identify human health risk generically, as such effects require an extensive amount of information regarding exposure that is uniquely site-specific. Thus, the approach deployed in this evaluation resulted in the identification of airport-related sources, their associated HAPs, and the relative toxicity of the individual HAPs. With this approach, key HAPs and associated sources were identified. The rationale for using this approach is that HAPs inventories can be quantified much more easily than exposure, and thus the results of the analysis presented herein can be considered relevant to most commercial airports. The 4

disadvantage of the approach is that exposure is essential for determining true risk of any population, and thus, the approach identified in this report would not enable identifi- cation of absolute human health risk at any airport. Rather, this preliminary assessment is intended to guide the identifi- cation of information gaps and the formation of a prioritized research agenda, as noted below. The greatest source of airport-related source gas-phase HAPs is aircraft operations, as described in Section 3 of this report. As a result, this report focuses on the current state of knowledge regarding HAPs emitted by aircraft and the associ- ated information gaps. In Section 2 the emissions and toxicity- weighted relative rankings of HAPs emitted by aircraft and GSE are described. In subsequent sections the state of knowl- edge regarding emissions from the various airport-related sources (e.g., aircraft, auxiliary power units, GAVs, stationary sources, and GSE) and the toxicity of these emissions are discussed. 1.5 Main Findings The research team reported the following findings: 1. Aircraft emissions at idle/taxi power are the predominant source of significant gas-phase HAP emissions at an airport. The actual emissions can vary by more than a factor of two depending on the actual power thrust levels used (e.g., real- world ground idle versus International Civil Aviation Orga- nization [ICAO] 7% idle), time spent idling, and ambient conditions, especially temperature. (Yelvington, Herndon et al. 2007). It is likely that many current airport emission inventories underestimate aircraft HAP emissions as the 5 Figure 1. Schematic depiction of the factors that determine risk. Emissions of pollutants from various sources undergo chemistry and dispersion in the atmosphere to determine a concentration. Risk is determined by the toxicity of these pollutants and the total exposure. This report focuses on emissions and toxicity, rather than exposure and toxicity, since to a first approximation total exposure due to a given source is proportional to emission rates, which are much easier to quantify and less situation-specific. There is also a “background” concentration of most pollutants that is determined by regional to global processes.

required emissions and dispersion modeling system (EDMS) assumes use of 7% thrust during the idle and taxi phase (HAP emission rates are higher at lower thrust levels). 2. Consideration of both the quantity of emissions and the toxicity of the individual pollutants yields a list that ranks the relative importance of airport-related HAPs. This list is similar to those produced by two previous studies for many compounds (acrolein, benzene, 1,3-butadiene, etc.), but differs significantly for others (e.g., toluene, xylene). The compounds in Table 1 are listed in order of relative im- portance. This ranking reflects that some HAPs emitted in great quantity have relatively low toxicity, while other HAPs emitted in lower quantities have high toxicity. The “rank” of formaldehyde on the list prepared for this report depends on which of two values is used for its inhalation unit risk (IUR). Use of the value currently listed on EPA’s Integrated Risk Information System (IRIS) database results in the placement as shown in Table 1 (i.e., second only to acrolein in importance). Use of the value by the Chemical Industry Institute of Toxicology (CIIT), which is used by 6 This ACRP Review FAA 2003 ORD 2005 Acrolein Formaldehyde* 1,3-Butadiene Naphthalene Benzene Acetaldehyde Ethylbenzene Formaldehyde Acetaldehyde Benzene Toluene Acrolein 1,3-Butadiene Xylene Lead Naphthalene Propanal (Propionaldehyde) Acrolein 1,3-Butadiene Formaldehyde Benzene Acetaldehyde Naphthalene Toluene Notes: HAP hazardous air pollutant FAA Select Resource Materials and Annotated Bibliography on the Topic of Hazardous Air Pollutants (URS 2003) ORD O’Hare Modernization Environmental Impact Statement (FAA 2005) * Using EPA IRIS value Table 1. Comparison of aviation-related HAPs lists. Figure 2. Map of a hypothetical airport. The factors that determine risk for any given exposure group depend on numerous factors such as the proximity of emission sources to the location of exposure.

the EPA’s National Air Toxics Assessment, results in a much less significant role for formaldehyde. Table 2 lists additional HAPs identified by this analysis that may also be important to consider for further evalua- tion. These HAPs have not undergone a formal toxicity evaluation. A comparison with structurally similar HAPs, however, indicates they may be important in terms of relative toxicity and emissions. Note that glyoxal and methylglyoxal are currently not classified by the EPA as HAPs, but are included on the EPA’s “Master List of Com- pounds Emitted by Mobile Sources.” This analysis indicates they may be among the most important airport HAPs. 3. The two gas-phase HAPs for which non-aircraft sources (GSE, GAV, and stationary sources) are most important, when viewed in the same type of emissions-toxicity weighting, are benzene and 1,3-butadiene, both of which are primarily emitted by gasoline engines (as opposed to those that run on diesel or compressed natural gas). 4. Speciation profiles for aircraft-emitted HAPs that were based on the work of Spicer, Holdren et al. (1994) are ac- curate, as the relative speciation of HAPs in aircraft exhaust has been found to be constant among numerous types of engines subsequently characterized (see Section 5.1). Recent field measurements (EXCAVATE, APEX 1, 2, 3) have greatly expanded the knowledge base regarding the quantification and speciation of aircraft HAPs emissions. 1.6 Prioritized Research Agenda Based on the results of this research, the following areas of research have been identified that would be most beneficial in addressing these information gaps. The projects listed below should be able to address the identified information gaps and substantially decrease the uncertainty associated with airport- related HAPs. More detail is presented in Section 9. Research Item 1: Identify the effects of ambient condi- tions (temperature, pressure, humidity) and engine technol- ogy on HAP emissions at various idle/taxi power settings. This project would measure the emission rates of various HAPs compounds from commercial aircraft as a function of thrust level near idle and as a function of environmental vari- ables such as temperature, humidity, and pressure. The goal of this project would be to improve our quantitative un- derstanding of the largest airport-related HAPs emission source—jet engines operating at low power. Emissions data for commercial aircraft are only available in the narrow tem- perature range (8° to 35°C, 46° to 95°F), and show that HAP emissions increase greatly with decreasing temperatures. The lack of knowledge regarding the temperature dependence of HAP emissions, and how it depends on engine technology, currently results in uncertainties of more than a factor of 2 in modeled HAP concentrations and risk. This is especially im- portant for airports located in cold environments. Research Item 2: Quantification of the “real-world” thrust values used at airports during the idle phase. This project would investigate and quantify the actual thrust values used by commercial aircraft at several airports. Recent research indicates that the standard power setting (7% rated thrust) that is used to calculate emission inventories does not represent actual thrust levels used during aircraft taxi and idle. This introduces large uncertainties in emission inventories and risk assessment, since HAP emissions are very sensitive to small changes in engine power. 7 HAP Basis for Concern Crotonaldehyde Crotonaldehyde is structurally similar to the highly reactive compound acrolein, and its airport emissions may be comparable to those of benzene and 1,3-butadiene. Glyoxal Glyoxal is a mutagenic aldehyde with two carbonyl groups, and has been shown to act as a tumor promoter in rats. Airport emissions of glyoxal are comparable to those of benzene and 1,3-butadiene. a Methylglyoxal Methylglyoxal is a mutagenic aldehyde with two carbonyl groups that has a DNA adduct formation potency 20-fold greater than acetaldehyde. Airport emissions of methylglyoxal are comparable to those of benzene and 1,3-butadiene. b Propanal (propionaldehyde) Propanal (pr opionaldehyde) is structurally similar to acetaldehyde and its emissions are comparable to naphthalene. a IARC 1991; NEG 1995. b IARC 1991; Vaca, Nilsson et al. 1998. Table 2. Aviation-related HAPs of potential concern.

Research Item 3: Characterization of HAP emissions from general aviation (e.g., piston engine aircraft). This project would support emissions measurements of general aviation aircraft. HAP emissions (with the exception of lead) from piston engine aircraft, turbojet engines, and low-bypass turbofan engines (such as business jets) are largely unknown and should be quantified. This is most important at general aviation airports. Research Item 4: Identify the emission sources most im- portant to on-airport and off-airport exposure. The purpose of this project is to identify which emission sources (aircraft, GSE, terminal traffic, etc.) most greatly affect potential receptors (e.g., nearby residents, airport- based workers, passengers). This should be done using dispersion/chemistry models tested against complementary measurements. Conclusive deduction of source apportion- ment will require coordinated measurements of the main combustion gases (CO2, CO, NO, NO2), speciated HAPs (e.g., benzene, formaldehyde, etc.) and particulate matter (PM) (characterized by number/size, mass, and chemical composi- tion). Source apportionment is required to evaluate proposed emissions mitigation strategies. This project will help airport operators to identify the “low-hanging fruit” with regards to minimizing the health risk presented by the various emission sources present at an airport. 8