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1 SUMMARY Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis Information has been collected regarding airport-related hazardous air pollutants (HAPs) via a literature review and communication with experts in the field. The state of knowledge has been assessed, information gaps have been identified, and research topics to address these gaps have been proposed. A prioritized list of gas-phase HAPs emitted by airport emission sources has been constructed based on the product of the compounds' toxicities and emission rates. This list consists of acrolein (propenal), formaldehyde, 1,3-butadiene, naphthalene, benzene, acetaldehyde, ethylbenzene, and propanal (propionaldehyde). Glyoxal, methylglyoxal, and crotonaldehyde (butenal), although not officially hazardous air pollutants, may be compa- rable in importance to the compounds listed above. Within the airport perimeter, aircraft engines during idle/taxi are the largest emission source for most of these compounds, although gasoline engines (used in ground access vehicles and some ground service equipment) can in some cases emit comparable amounts of benzene and 1,3-butadiene. The sources that contribute the most to human exposure depend heavily on the particular exposure group and airport and cannot be easily general- ized. For example, nearby residents' exposure may be most affected by the aircraft or ground access vehicles depending on meteorology and the relative location of busy roadways, air- port runways, and residential areas. Future studies such as health risk assessments, however, should not limit themselves to the above compounds and should consider all types of particulate matter (refer to ACRP Report 6: Research Needs Associated with Particulate Emissions at Airports) and site-specific parameters such as the relative location of emission sources and the relevant exposure groups. It is recommended that ACRP fund the following research topics. Research on these topics would enable airport operators to develop more accurate HAP emission inventories and/or to reduce emissions most cost effectively. 1. Quantify the dependence of HAP emissions from aircraft as a function of ambient conditions (temperature, pressure, humidity) and engine technology. Gas-phase HAP emissions increase greatly with decreasing temperatures; however, they have never been measured at sub-freezing temperatures. This can result in over a factor of two uncertainty in emission inventories. 2. Quantify the actual thrust levels used by aircraft during the idle/taxi phase of a land- ing and/or take-off cycle. The current uncertainty in the actual thrust levels used during taxi/idle results in up to a factor of two uncertainty in HAP emission inventories. 3. Quantify HAP emissions from general aviation aircraft. With the exception of lead, air- craft emissions from piston engine aircraft, which are unregulated, remain unquantified.

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2 4. Identify the emission sources most important to on-airport and off-airport exposure. Exposure (and therefore human health risk) depends on several factors such as meteor- ology and the relative location of emission sources and exposure groups. Such a project will help airport operators to identify the "low-hanging fruit" with regards to minimiz- ing the health risk presented by the various emission sources present at an airport. In addition to the information gaps identified in the research statements above, there are information gaps related to the current state of knowledge regarding the toxicity of the following two classes of compounds: alkenes and certain aldehydes (including glyoxal, methylglyoxal, and crotonaldehyde). Of these compounds, addressing information gaps for glyoxal, methylglyoxal, and crotonaldehyde is most critical, as they may be emitted in size- able quantities, and limited toxicological information for these compounds indicates their toxicity could be comparable to that of formaldehyde, acetaldehyde, and acrolein. The toxicity of glyoxal, methylglyoxal, and crotonaldehyde is highly uncertain.