Power-dependent speciation of volatile organic compounds in aircraft exhaust
Highlights
► At low power, scaling hydrocarbons to ethene reduces variability amongst engines. ► At high power, there is a difference in composition with aromatics dominating. ► At low power, thermal cracking dominates. ► At high power, aromatic species are formed via the HACA process in the exhaust. ► In downwind samples, HAPs make up a large portion of measured emissions (27–42%).
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).
Section snippets
Sample collection
During APEX-3, aircraft were parked and chocked on a side portion of the tarmac at Cleveland Hopkins International Airport in Cleveland, Ohio. The thrust of the engines was then varied from conditions typical of idle (4% of full rated thrust) to take-off (93%). Samples were collected using an inlet probe positioned one meter behind the engine exit plane. The sample was immediately diluted with dry nitrogen, resulting in a variable dilution ratio between 4:1 and 90:1. Dry nitrogen dilution was
Sample analysis
After the experiment, the direct exhaust samples were analyzed at the University of California, Irvine (UCI) using gas chromatography for the measurement of CO2, NMHCs and halogenated hydrocarbons. However, only CO2 and NMHC data are presented here because emission rates of halocarbon species were found to be small. Carbon dioxide was separated by GC and quantified by a thermal conductivity detector. Non-methane hydrocarbons and halocarbons were measured using a three GC system featuring a
Data analysis procedures
The dilution factors for the direct exhaust samples were determined by measuring CO2 in the raw exhaust (by non-dispersive infrared gas analyzers) and in the diluted samples (by the GC system described above). The background mixing ratio of each compound was subtracted from the dilution-corrected mixing ratio to determine the enhancement in compound X measured in the exhaust (ΔX). Background measurements for the VOCs were made by the EPA during periods with no aircraft activity.
The enhancement
Results – direct exhaust samples
Power dependent emission indices for the NMHCs measured in the direct exhaust and ICAO certification values for the AE3007, PW4158 and RB211 engines are given in Table 2. Halogenated compounds had small background enhancements and are therefore not reported. The total measured NMHC is lower than ICAO estimates of unburned hydrocarbons at idle except for PW4158. This is likely due to the fact that the ICAO measurements include oxygenated species not measured in the direct exhaust during APEX-3.
Results – EPA plume samples
The samples collected downwind from the aircraft represent a naturally diluted sample of the exhaust. Emission indices were determined using the method described above (Table 3), but the plume samples were collected over a prolonged period encompassing multiple engine powers and thus represent an average emission sample. Time-weighted powers were between 32 and 41%. However, the non-oxygenated NMHC emissions (Fig. 7) were closer to levels measured in the direct exhaust at 7 and 15%. These high
Discussion – combustion processes
As mentioned previously, a shift is seen in the NMHC speciation (towards production of aromatics) as engine power increases. This is likely the result of increased engine temperatures and pressures as engine power increases (Wey et al., 2006) causing a shift in combustion processes (Schulz et al., 1999). Fuel cracking is believed to be the primary source of short-chain alkenes and dominates at idle power, while formation of larger compounds (e.g., toluene) can occur at the higher power settings
Discussion – ozone formation potential
The shift in hydrocarbon composition as the power changes also affects the reactivity of the exhaust. To analyze this shift, the Maximum Incremental Reactivity (MIR) scale for ozone production is used (Carter, 1994; using updated values from Carter, 2010). The MIR scale relates the amount of ozone (O3) produced from an incremental increase in the amount of a specific hydrocarbon to the urban atmosphere. This allows for a scale of ozone production from individual hydrocarbons. As ozone
Discussion – HAPs emission
Additionally, aircraft exhaust includes a number of compounds the United States classifies as HAPs (EPA, 2008). These compounds are identified in Tables 2 and 3. In the plume samples, HAPs made up 27–42% of the measured emissions with formaldehyde dominating. The contribution of HAPS in the direct exhaust is lower (8–28% with an increase at higher power) because oxygenated species were not measured. Due to differences in measurement techniques and engine power settings, the direct and plume
Conclusions
The current research shows that the speciation of hydrocarbon emissions from aircraft is power dependent. Low engine powers (representative of idle conditions) produce a larger amount of total NMHCs composed primarily of alkenes and alkynes. As power increases, total emissions decrease and the speciation shifts towards the production of aromatics. The increasing importance of aromatics observed as power is increased is likely the result of a shift from cracking (forming predominantly alkenes)
Acknowledgements
This research was supported by the NASA Aeronautics Research Mission Directorate. Assistance during the field campaign was provided by Charles Hudgins (NASA Langley) and was facilitated by Southwest Airlines, Continental Airlines, FedEx and NASA Glenn Research Center. Analytical assistance was provided by Changlie Wey at NASA Glenn (CO2 data) and Gloria Liu, Brent Love and Simone Meinardi at UC Irvine. John Kinsey at the EPA provided the APEX report and helpful suggestions during manuscript
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