Airports can pose a far bigger threat to local air than previously recognized, thanks to the transformative power of sunlight.

In the first on-tarmac measurements of their kind, researchers have shown that oil droplets spewed by idling jet engines can turn into particles tiny enough to readily penetrate the lungs and brain.

Allen Robinson of Carnegie Mellon University in Pittsburgh and his team collected the pollution spewed from a plane powered by one of the most common types of commercial jet engines as it operated at different loads. Though jet engines operating at full power produce mostly solid particles, at low engine loads — such as when a plane idles at the gate or on the runway — emissions are predominantly in the form of microscopic droplets.

The researchers piped the engine’s exhaust into a 7-cubic-meter covered Teflon bag. When the bag was full the researchers uncovered it, allowing sunlight to fire up chemical reactions that would normally occur in the open air.

Within minutes solid particles were generated by interactions between the oily microdroplets and gases. “Driving this chemistry,” Robinson notes, “was hydroxyl radical,” or OH — the oxidant that’s most effective at catalyzing the breakdown of oily hydrocarbons. “To create this hydroxyl radical, you need sunlight,” he explains.

Sunlight’s oxidation of the exhaust emitted at idling can generate 35 times more particles than the engine originally emitted and 10 times what computer models have typically predicted, the researchers report online May 5 in Atmospheric Chemistry and Physics. Robinson says he found these new data “unbelievable. It sort of blew our minds.”

He points out that at busy airports, “If you’re number 46 in line awaiting takeoff, you could spend an hour idling.”

Sunlight’s boost for submicrometer particles from exhaust at higher engine loads was smaller — but always many times the particle count present in the jet’s starting exhaust.

Although researchers have measured emissions directly coming out the back of an aircraft, this is the first to look at what happens when those emissions age in sunlight, notes Richard Miake-Lye of Aerodyne Research in Billerica, Mass. The new data are “very important to be clear on how aircraft emissions impact local and regional air quality,” he says.

When scientists considered jet pollution in the past, observes George Thurston of the New York University Langone Medical Center in Tuxedo, “I don’t think they looked much at the particle implications.” This generation of particles by aging gases and hydrocarbons can be hard to figure out, he says, “but it’s where a lot of the action is in atmospheric chemistry.”

As pollution from cars, trucks and most smokestacks has fallen, airport emissions have tended to climb, notes Ronald Henry of the University of Southern California in Los Angeles. Although pollution from airports tends to be localized within a few kilometers, he says, “It now appears it could be a significant source of ultrafine particles — and health effects.”

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