Tracking urban volatile organic compound sources

Half of the volatile organic compounds (VOCs) in the northern hemisphere are man-made, but the levels of these sources have been unclear.

Even though about half of all the volatile organic compounds (VOCs) found in the northern hemisphere are man-made, the individual levels of these sources have been unclear. But now, researchers at the University of Innsbruck have produced the first chemical fingerprint of urban VOC emission sources.

The research team measured a large number of VOCs on Innsbruck’s campus from July to October in 2015. Using statistical methods, they were able to draw conclusions about individual emission sources from the measurement data.

The EU has been regulating VOCs from organic solvents in paints for the past 15 years. Many of these toxic solvents have been replaced by water-soluble substances, and the transformation can be seen in the results of the study.

“We find smaller amounts of compounds such as benzene or toluene,” said study co-author Thomas Karl. “On the other hand, water-soluble substances are much more ubiquitous. These are less reactive, which can have a positive effect on the formation of ground-level ozone.”

The investigation also revealed that the overall global amount of urban emissions is significantly underestimated, which is due to an extremely high proportion of oxygen-containing compounds.

“If the figure calculated for Innsbruck is also representative of Asian cities – which is rather optimistic – then this would at least double the number globally,” said Professor Karl.

The researchers say that this new estimate of urban emissions will result in higher levels of particulate matter in the atmosphere as well, and climate models may need to be adjusted accordingly.

The team measured a broad range of compounds at very low quantities and were able to establish the fingerprint of VOC emission sources. Many of the trace gases were odorous and reflected the characteristic scent of the city.

“In this respect Innsbruck is a quite ordinary city,” said Professor Karl. “We find mainly traces of food preparation – from coffee roasting to frying – and solvents that humans associate with the particular smell of a city. The sources of emissions range from bakeries to the regional hospital.”

The scientists were surprised to detect compounds derived from cosmetics and detergents.

“In our data, we found clear evidence of silicone oils contained in many cosmetic and cleaning products,” said Professor Karl. “We were surprised that these compounds leave such a characteristic fingerprint in urban air.”

The research is published in the Proceedings of the National Academy of Sciences.

By Chrissy Sexton, Earth.com Staff Writer