Power-dependent speciation of volatile organic compounds in aircraft exhaust

Abstract

As part of the third NASA Aircraft Particle Emissions Experiment (APEX-3, November 2005), whole air samples were collected to determine the emission rates of volatile organic compounds (VOCs) from aircraft equipped with three different gas-turbine engines (an Allison Engine 3007-A1E, a Pratt–Whitney 4158, and a Rolls–Royce RB211-535E4B). Samples were collected 1 m behind the engine exhaust plane of the engines while they were operated at powers ranging from idle up to 30% of maximum rated thrust.

Exhaust emission indices (mass emitted per kilogram of fuel used) for CO and non-methane hydrocarbons (NMHCs) were calculated based on enhancements over background relative to CO₂. Emissions of all NMHCs were greatest at low power with values decreasing by an order of magnitude with increasing power. Previous studies have shown that scaling idle hydrocarbon emissions to formaldehyde or ethene (which are typically emitted at a ratio of 1-to-1 at idle) reduces variability amongst engine types. NMHC emissions were found to scale at low power, with alkenes contributing over 50% of measured NMHCs. However, as the power increases hydrocarbon emissions no longer scale to ethene, as the aromatics become the dominant species emitted. This may be due in part to a shift in combustion processes from thermal cracking (producing predominantly alkenes) to production of new molecules (producing proportionally more aromatics) as power increases. The formation of these aromatics is an intermediate step in the production of soot, which also increases with increasing power. The increase in aromatics relative to alkenes additionally results in a decrease in the hydroxyl radical reactivity and ozone formation potential of aircraft exhaust.

Samples collected 30 m downwind of the engine were also analyzed for NMHCs and carbonyl compounds (acetone, 2-butanone and C₁–C₉ aldehydes). Formaldehyde was the predominant carbonyl emitted; however, the ratio of ethene-to-formaldehyde varied between the aircraft, possibly due to the sampling of transient emissions such as engine start-up and power changes. A large portion of the measured emissions (27–42% by mass) in the plume samples was made up of hazardous air pollutants (HAPs) with oxygenated compounds being most significant.

Introduction

The emission of volatile organic compounds (VOCs) into the atmosphere adversely affects the environment including production of ozone and increased levels of greenhouse gases and hazardous air pollutants (HAPs). The primary anthropogenic source of VOC emissions is the combustion of fossil fuels and related evaporation. Aircraft usage is a minor consumer of fossil fuels, accounting for only 3% of total usage. However, total jet fuel usage in the United States is projected to increase by 130% between 2009 and 2030 (Federal Aviation Agency (FAA), 2010). In addition, aircraft emissions are unique in that they directly affect the atmosphere both at ground level (aircraft taxi, idling, take-off and landing) and at cruise altitudes (upper troposphere and lower stratosphere).

The complete combustion of jet fuel results in the formation of carbon dioxide (CO₂) as the only carbonaceous species. However, because of combustion inefficiencies, other carbon-containing compounds are emitted both as gaseous and particulate species. To quantify the impact of these emissions, the International Civil Aviation Organization (ICAO) requires manufacturers to measure and document carbon monoxide (CO), unburned hydrocarbons, and smoke number emissions from all engines used in civil aircraft at power settings equivalent to idle (7% of rated thrust), airport approach (30%), climb-out (85%) and take-off (100%). While these “certification tests” yield estimates of total carbonaceous species emissions, they provide no information on hydrocarbon speciation and thus no insight into the reactivity or hazardous nature of the exhaust.

A number of investigations have studied the speciation of emitted non-methane hydrocarbons (NMHCs). These include measuring the exhaust directly behind the aircraft engines while on the ground (Spicer et al., 1992, 1994; Anderson et al., 2006; Knighton et al., 2007; Yelvington et al., 2007; Timko et al., 2010a), behind an in-flight aircraft (Slemr et al., 1998, 2001), and in wind-advected aircraft exhaust plumes at airports (Herndon et al., 2006, 2009; Schürmann et al., 2007).

The sum of NMHCs in aircraft exhaust is highly dependent on engine power with the highest emission at power settings typical of ground idle and decreasing as power increases. The organic fraction of the exhaust is composed primarily of short-chain unsaturated hydrocarbons (Anderson et al., 2006) and aldehydes (Spicer et al., 1994). In particular, the predominant species emitted are ethene and formaldehyde (HCHO) at a ratio of 1:1 (Herndon et al., 2009). This is in strong contrast to the composition of unburned jet fuel which is composed primarily of C₁₁–C₁₄ hydrocarbons (Spicer et al., 1994). The short-chained species found in the exhaust are produced from fuel cracking (alkenes) and partial oxidation (forming formaldehyde) during combustion (Warnatz et al., 2006). In a recent study looking at exhaust plumes from multiple aircraft while on an active runway, Herndon et al. (2009) showed that scaling the near-idle power emission indices (EIs) to tracers of fuel cracking (formaldehyde or ethene) eliminated much of the variability between engine types. However, some evidence points to variation in the hydrocarbon speciation profile with changes in engine power setting, with higher engine power settings producing a larger fraction of aromatics and alkanes (Anderson et al., 2006). The formation of aromatics is an intermediate step in the formation of soot (Warnatz et al., 2006) which also increases as aircraft engine power increases. The soot formation process proceeds via hydrogen abstraction/carbon addition (HACA) which first forms aromatic species, then polycyclic aromatic hydrocarbons (PAHs), and finally soot (Bauer and Jeffers, 1988; Wang and Frenklach, 1994; McEnally and Pfefferle, 1997).

The power-dependent speciation of emitted VOCs was a focus of the third Aircraft Particle Emission Experiment (APEX-3) at the NASA Glenn Research Center in Cleveland, Ohio during November 2005 (Wey et al., 2006). This project focused on the quantification and speciation of both particulate and gaseous emissions from different engines. Results from the particulate sampling are given in Timko et al. (2010b) and Kinsey et al. (2011), while gas-phase measurements of nitrogen oxides and formaldehyde are found in Timko et al. (2010a). The current analysis reports emissions of NMHCs directly behind the aircraft engine (non-oxygenated species only) and in exhaust plumes downwind from the engines (including carbonyls).