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1 Summary Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions This project characterized the dependence of gaseous hazardous air pollutants (HAP) and other partially combusted fuel emissions from aircraft on ambient and near-idle engine conditions, including temperature and engine thrust setting. The measurement work focused on characterization of on-wing CFM56 engines installed on commercial aircraft and operated according to airline practice. This project also developed a simple empirical frame- work (based on measurement data collected on variants of the CFM56 engine) that can be used in emissions scenario analysis. Three measurement campaigns were conducted at Chicago Midway Regional Airport (MDW), Chicago O'Hare International Airport (ORD), and Dallas Love Airport (DAL) in 2009 and 2010. Measurement tests focused on the CFM56-7B24 and -3B1 engine types. A single V2527 engine and a PW4090 engine were also characterized. The temperatures during these tests ranged from -8C to 25C (18F to 77F). Engine conditions examined ranged from the minimum fuel flow rates used at airports (ground idle with zero bleed air demand) to the International Civil Aviation Organization (ICAO) certification thrust setting for taxi/idle (7% of maximum rated thrust). Exhaust from the measured aircraft engines was collected using state-of-the-art probe/sampling schemes and analyzed using instrumentation housed in a mobile laboratory. Speciated measurements of HAP compounds such as formaldehyde, benzene, and 1,3-butadiene were measured using high-time-response, research grade instrumentation. The main HAP emissions from aircraft engines are volatile organic compounds (VOCs), with the important HAPs being formaldehyde, benzene, and acetaldehyde. Both the fuel-based emission indices (grams of benzene per kilogram of fuel combusted) and emission rates (grams of benzene per second) of VOCs increase with decreases in ambient temperature and fuel flow rate. A major accomplishment of this project's research is the formulation of a simple model (currently only applicable to selected variants of the CFM56 engine type) that outputs the relative change in VOC emission index compared with ICAO's reference temperature condition (15C). Findings The findings of this project are summarized as follows: 1. Temperature dependence: VOC emission indices are approximately twice as high under cold conditions (-8C to 0C) as they are at 15C for the CFM56-7B24.