air pollution

Vehicles pump out more ammonia in winter

RICE U. (US) — The seasons play a role in the amount of ammonia produced by cars and trucks, with output greatest during the winter months, according to a new study.

The findings are not cause for immediate concern, says Robert Griffin, associate professor of civil and environmental engineering at Rice University. “There may not be a health risk from ammonia itself, but the fact that ammonia is a precursor to particles is a big deal. They can get into your lungs and do some damage.”

While ammonia’s role in air quality draws minimal oversight from the Environmental Protection Agency (EPA), Griffin and scientists from the University of Houston measured plumes of airborne ammonia from isolated incidents in the Houston area.

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Ammonia quickly combines with other airborne elements: sulfuric acid to make ammonium sulfate salts, or, in cooler conditions, nitric acid to make ammonium nitrate. The particles can impact air quality as well as atmospheric visibility, cloud formation, climate patterns, and nutrient cycling, Griffin says.

Found throughout the atmosphere in levels ranging from parts per trillion to parts per billion (ppb), ammonia is detected at five to 50 parts per million (ppm). Concentrations above 100 ppm are uncomfortable to most people. Sources include industry, motor vehicles, agriculture (as a major component of fertilizer) and livestock. Even humans produce ammonia. (Household ammonia is highly diluted with water—but one should still avoid the pungent fumes.)

For their study, published in the journal Atmospheric Chemistry and Physics, the researchers gathered data 24 hours a day over two weeks in February and six weeks in late summer, 2010.

Readings were taken atop the University of Houston’s tallest building, North Moody Tower. The residence hall is ideally situated to pick up changes in the wind not only from the nearby Houston Ship Channel and its associated industries to the east, but also power generation facilities to the southwest and Houston traffic in every direction.

Griffin’s colleagues Frank Tittel, professor of electrical and computer engineering, and Rafal Lewicki, a co-author and graduate student in Tittel’s laser science group, designed and built an apparatus to collect the data. Their external-cavity quantum cascade laser-based sensor is finely tuned to pick up signs of ammonia from air samples continuously cycled through the closed system. Real-time readings were taken with a resolution of less than five parts per billion and autonomously monitored at Rice via the Internet.

Sampling at a single site produced results that at first seemed contradictory, Griffin says.

For example, while overall levels were highest in the summer, ammonia emissions from vehicles were found to be highest in winter when harder-working car and truck engines reduced the performance of catalytic converters. (Carbon monoxide levels recorded by UH instruments on the tower correlated nicely, the study showed.)

Part of the answer was found in the wind. Prevailing winds during winter morning rush hours came from the southeast—past several major highways and Houston’s William P. Hobby Airport—and carried a high level of vehicle emissions.

During summer morning rush hours, the wind came in from the northeast, passing the ship channel and increasing readings from industrial activity and including occasional spikes, including a nearby traffic accident, that raised the average.

Winter levels of airborne ammonia ranged from 0.1 to 8.7 ppb with a mean of 2.4 ppb. A larger range—0.2 to 27.1 ppb with a mean of 3.1 ppb—was observed during the summer.

The accident, that occurred Aug. 14. involved the collision of two 18-wheeled tankers on Interstate 45 two miles north of the tower. One was carrying fertilizer and pesticide, and the fumes from the resultant chemical fire reached the sensor, which recorded a spike in airborne ammonia to about 21 ppb.

“If the wind was blowing the other way, we wouldn’t have captured it,” says Owen Gong, a graduate student in Griffin’s lab and first author of the paper. “There is a bit of luck associated with this kind of field work.”

A similar spike occurred a few weeks later when winds from Hurricane Hermine in the Gulf of Mexico blew emissions from industries in and around Texas City—40 miles south of downtown Houston—to the tower. The next week, ammonia levels reached 27 ppb, but no source of the emissions was identified.

Griffin says he appreciated having access to the site and the assistance of Barry Lefer, associate professor of  atmospheric science and researcher James Flynn at the University of Houston. “Without their data to give us wind direction and other chemical information, analysis of the ammonia time series would have been difficult,” he says.

The team’s next study will track the source and fate of other components in airborne particulate matter. Griffin did not foresee the EPA monitoring ammonia for the sake of establishing a standard. “But because it can be such a significant precursor to particulate matter, the EPA needs to keep an eye on it.”

The Mid-InfraRed Technologies for Health and the Environment Center and the National Science Foundation supported the research.

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