The exhaust gases from road traffic trigger chemical reaction chains that produce air pollutants such as nitrogen oxides and ground-level ozone. But new measurements suggest that the models describing this conversion need to be corrected for urban air. Because there, turbulence and the exhaust gas mixture from traffic lead to the fact that the ozone close to the ground is overestimated by the atmospheric chemistry models. However, the urban air quality measurements are still valid and correct.
In many cities, the air is thick: pollutants such as particulate matter, nitrogen dioxide or ground-level ozone pollute the air and increase the risk of many diseases. While some air pollutants are released directly from traffic and smoke from buildings, power plants and industrial plants, others are created through chemical reactions in the air. The nitrogen monoxide emitted by many diesel vehicles reacts with ozone to form nitrogen dioxide. Over time, this in turn breaks down into nitrogen monoxide and oxygen radicals, which lead to the formation of ground-level ozone.
Air Pollutants and the Leighton Relationship
This chemical cycle of air pollutants has been known for a long time: it was described mathematically by the chemist Philip Leighton more than 60 years ago in the first textbook on air pollution. Since then, the relationship between the two processes – the formation of nitrogen dioxide and the formation of ozone – has been referred to as the Leighton relationship. Computer models of atmospheric chemistry use this equation to derive the concentrations of ozone, nitric oxide, and nitrogen dioxide from the concentrations of the other two—reducing complexity. In practice, this is used, for example, to derive the ozone concentration in areas polluted by nitrogen oxides.
Thomas Karl from the University of Innsbruck and his colleagues have now checked how accurate the Leighton relationship is, especially in densely populated urban areas. To do this, they evaluated the data from a 40-metre-high air measurement tower located in downtown Innsbruck. The tower’s instruments record readings on a wide range of air pollutants and weather parameters, collecting 36,000 data points per hour. This allows the concentration of air components to be continuously monitored.
Ozone formation overestimated
The evaluation of these long-term measurements revealed that Leighton’s model becomes inaccurate when nitrogen monoxide emissions are high and leads to incorrect results. Because with higher monoxide concentrations and the stronger turbulence typical of urban street canyons, more ozone is converted into nitrogen dioxide, as the data showed. If one uses atmospheric models based on the Leighton relationship to estimate urban ozone values, the proportion of ground-level ozone can tend to be overestimated. “In cities with high nitrogen monoxide emissions, this ratio is overestimated by up to 50 percent,” warns Karl.
In order to avoid such misjudgments in future, for example in urban air quality forecasts, the models must be adjusted accordingly. The researchers emphasize that this is particularly true when modeling the lowest layer of the atmosphere, up to 200 meters above the ground. However, this correction does not mean that previous measurements of air pollutants are incorrect. Only forecasts or estimates derived from them using models are affected. The same applies to environmental protection measures: “It is important to note that environmental policy regulations do not refer to model calculations, but come into force depending on the concentrations of pollutants actually measured,” stresses Karl.
Source: University of Innsbruck; Specialist article: Science Advances, doi: 10.1126/sciadv.add2365