Vehicle tailpipes may be a more important source of smog-causing ammonia that hovers over big cities than agriculture emissions, a new study suggests.
Researchers from Princeton University in the US outfitted vehicles with sophisticated sensors to detect ammonia levels and focused on six cities - Philadelphia, Denver and Houston in the US, and Beijing, Shijiazhuang and Baoding in China.
Holes were drilled into the bodies of the vehicles to attach sensors.
By measuring ammonia levels during various times of the day at different points of entry into the cities, the team was able to paint a picture of a "breathing" city, where levels of pollutants rise and fall, depending on traffic and conditions.
Researchers found that ammonia emissions from cities were much larger than recognised and occurred at the very times when unhealthy particulate matter is at its worst, and when agricultural emissions are at daily or seasonal lows.
"It is actually coming from the vehicles in the cities themselves," said Mark Zondlo, associate professor at Princeton University.
Researchers noted that vehicle emissions of ammonia were co-emitted with nitrogen oxides.
These chemicals combine to form ammonium nitrate, which can be seen from the brown colour in urban haze, researchers said.
Ammonia emissions from vehicles are especially important during cold weather (eg during winter or the morning rush hour) when agricultural emissions are at their lowest and when haze pollution is at its worst, researchers said.
"Vehicle tailpipes actually are a more important source of ammonia's contribution to the haze that hovers over big cities," researchers said.
"Ammonia does not have to come all the way from the Midwest to Philadelphia or New York, much of it is being generated here," Zondlo said.
The study was published in the journal Environmental Science and Technology.
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