Researchers at the University of Colorado at Boulder found invisible, reactive gases hovering above the ground form the bulk of organic haze in both urban and rural environments.

It was believed sources that emit soot and other tiny particles directly into the air were the primary cause of organic haze being formed.

But, following tests at 37 sites in 11 countries, a team at the university’s Co-operative Institute for Research in Environmental Sciences (CIRES) found aerosols formed chemically in the air account for about two-thirds of the total organic haze in urban areas and more than 90% of organic haze in rural areas.

The scientists compared concentrations of directly emitted, or primary, aerosols with chemically formed, or secondary aerosols. They surveyed urban areas, areas downwind of urban areas and rural areas.

The study was led by Qi Zhang, a former CIRES scientist now at the Atmospheric Sciences Research Centre at the State University of New York, and CIRES researcher Jose-Luis Jimenez.

“What we’re seeing is that concentrations of secondary organic aerosols decrease little downwind from urban areas,” Jimenez said. “That tells us there has to be an extended source or continuous formation for the pollution.”

The scientists believe the extended source of particle pollution is reactive, colourless gases called Volatile Organic Compounds (VOCs), the same gases that form smog. It is now believed these VOCs undergo a chemical transformation that causes them to stick to particles and increase such pollution.

“We think the gases react over a few days as the air travels downwind into more rural regions, producing more organic haze,” said Jimenez.

Among other groups involved in the study was the University of Manchester.

Paul Humphries

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