Characterizing airplane plumes on the runway
Three new papers provide a detailed look at emissions spewed by commercial airplanes when they idle, taxi, take off, and descend.
Airport expansions are on the rise worldwide, and so are questions about the contribution of commercial-aircraft emissions to poor air quality. Now, a suite of papers published in ES&T (DOI: 10.1021/es072029+; 10.1021/es072050a; 10.1021/es071926a) examines commercial-airplane exhaust plumes and finds that levels of pollution emitted while planes sit idling are higher than is usually assumed. These emissions can easily be reduced, the coauthors note.
The papers examine several pollutants: particulate matter, total hydrocarbons, and carbon monoxide (CO)—all of which are emitted when a hydrocarbon fuel doesn't burn completely—and nitrogen oxides (NOx), which are formed as a result of the hot temperatures of combustion. Aircraft emit various amounts of these pollutants when they take off, descend, idle, and taxi.
The papers also examine how the emissions stack up against international standards used to certify aircraft-engine emissions performance, instituted approximately 25 years ago by the International Civil Aviation Organization (ICAO), a 54-year-old group with 190 government participants. The ICAO data-bank values reflect engine-emissions performance at four powers and are commonly used to estimate an airport's emissions.
All three studies provide real emissions data that are likely more credible than using the ICAO certification numbers to estimate actual performance, and the new data can be used by air-quality regulators tasked with keeping local air healthy and in line with federal standards, notes Philip Whitefield, chemistry professor and director of the Center of Excellence for Aerospace Particulate Emissions Reduction Research at the Missouri University of Science and Technology.
This is especially important today because aircraft travel is terrifically popular, adds Whitefield, who is a coauthor of one of the papers (DOI: 10.1021/es072029+). In that study, Scott Herndon, a researcher with the instrumentation and research group Aerodyne Research, Inc.; Whitefield; and colleagues examined 376 aircraft exhaust plumes and found that the NOx emissions were 18% lower than estimates based on using ICAO's certification data for full-power takeoff.
In the fall of 2004, the group set up two mobile laboratories 1–2 minutes downwind of runways at the Hartsfield-Jackson Atlanta International Airport. The CO concentration in plumes from idling planes was much greater (by almost 50%) than estimates based on ICAO idle data, and significant differences were observed in emissions of black carbon and particulate matter. The disparity between the study's CO results and the ICAO idle data wasn't a surprise, Herndon says, because planes on a taxiway idle at a lower power setting than the settings used for certification reporting to ICAO.
The second paper (DOI: 10.1021/es072050a) draws from studies carried out for a few weeks each at Oakland International Airport in August 2005 and at Cleveland Hopkins International Airport in October and November of the same year. Coauthor Ezra Wood, also an Aerodyne researcher, and colleagues examined the speciation and chemical evolution of NOx in the aircrafts' exhaust plumes. The researchers found that, during idling, the emitted NOx is mostly in the form of NO2, which can have a greater impact on downwind NO2 and ozone concentrations than the usual form of nitric oxide (NO). Wood's research indicates that the current assessments of the impact of aircraft NOx emissions on downwind concentrations of NO2 and ozone may have been underestimated, "but by how much remains to be seen," he says. The analysis of NO and NO2 provides new data about aircraft exhaust plumes that are lacking from the ICAO standards and that few studies report, Wood says.
The papers by Herndon and Wood also detail the high concentration of pollutants emitted during idling. A Boeing 737 jet uses approximately 80 gallons of fuel during 20 minutes of idling, Wood says, which adds up to substantial costs for the airlines. Some airport owners and airlines are beginning to put programs in place that safely reduce these emissions with no change in engine or fuel technology, Herndon notes.
In the third study (DOI 10.1021/es071926a), researchers uniquely applied an existing technique to examine plume samples at Heathrow International Airport (U.K.), notes coauthor Karl Ropkins at the Institute for Transport Studies (ITS). The team sampled 5618 plumes using a monitor located 180 meters from the runway and looked at the planes' takeoff plumes. They found that aircraft operational factors, such as takeoff weight and engine-thrust setting, likely "have a measurable and important effect" on NOx emissions that is not accounted for in the ICAO data bank or airport emission inventories, adds coauthor David Carslaw of ITS.
Don Wuebbles, professor and executive coordinator of the School of Earth, Society, and Environment at the University of Illinois Urbana–Champaign, says he welcomes the research. Yet, minimal data illustrate the link between airport emissions and human-health effects on nearby communities, he says. "Some studies have suggested the airport emissions are important to air quality in the local communities, and studies at other airports have suggested that the contribution is very little," he says. "Our research indicates that ultrafine particles and organic vapors [from airplane exhaust] are really what are harmful to health," adds John Froines, director of the Southern California Particle Center, a nationally funded research institute.
