The study says the basic assumptions of air quality need to be redefined

Thomas Carl at the Innsbruck Atmospheric Observatory over the city of Innsbruck, Austria. Credit: University of Innsbruck

The 40-meter-tall observation tower at the Innsbruck Atmospheric Observatory near the city center of Innsbruck, Austria, continuously provides data on the composition of the atmosphere near the surface. Every hour, 36,000 data points are recorded. Using a special measurement method – the so-called vortex covariance method – the concentration of air components can be continuously monitored.

An international team led by Thomas Karl of the University of Innsbruck’s Department of Atmospheric and Cryospheric Sciences used this data to study in detail the chemistry of ozone, nitrogen monoxide and nitrogen dioxide in urban areas. The high percentage of diesel vehicles in European cities leads to strong concentrations of nitrogen monoxide. This reacts with ozone to produce nitrogen dioxide. In the atmosphere, nitrogen dioxide decomposes back into nitrogen monoxide and atomic oxygen, which immediately combines with atmospheric oxygen to form ozone.

The common assumption needs to be redefined

This chemical cycle was described mathematically more than 60 years ago in the first air pollution textbook by Philip Leighton. The relationship between the two processes has since been referred to as the Leighton ratio. Computer models of atmospheric chemistry use the Leighton ratio to reduce complexity by deriving the concentrations of ozone, nitric oxide, and nitrogen dioxide from the concentrations of each of the other two.

In practice, this has been used, for example, to derive ozone concentrations in areas polluted by nitrogen oxides. The data of the Innsbruck researchers now show that in the presence of high emissions of nitrogen monoxide, Leighton’s computational simplification leads to incorrect results. “In cities with high nitrogen monoxide emissions, this proportion can be overestimated by as much as 50%, which could lead to model calculations that overestimate ground-level ozone concentrations in urban areas,” says Thomas Karl. The effect of chemistry – turbulence interactions – plays an important role in the lower atmosphere, up to 200 meters above the Earth’s surface.

Responsible for the effect studied in Innsbruck is a combination of severe disturbance in urban areas in the presence of high emissions of nitrogen monoxide. The mixing of the gases with relatively rapid chemical processes converts more ozone into nitrogen dioxide. The researchers’ data also show that direct emissions of nitrogen dioxide from urban traffic are largely negligible compared to secondary formation.

“It remains important to note that environmental regulations are not based on model calculations but come into effect depending on actual measured pollutant concentrations,” said Thomas Karl.

The results are now published in Science advances.

more information:
Thomas Carle, Rising NOx in Urban Areas Leads to a Great Downward Chemical Flux of Ozone, Science advances (2023). DOI: 10.1126/sciadv.add2365.

Provided by the University of Innsbruck

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