Nitrogen dioxide and visible light contribute more to the typical urban smog than expected.
Reading a hitherto harmless chemical reaction influences the formation of smog over cities. This is what American scientists around Amitabha Sinha from the University of California in San Diego found out. Thus, the interaction between water vapor and nitrogen dioxide over cities plays a much greater role in smog formation than previously thought. According to the new results, nitrogen dioxide can influence the formation of ozone by absorbing sunlight and then reacting with water vapor. Through several intermediate steps, the reactive ozone forms, which regularly ensures smog alarms, especially in summer. Decisive for the formation of ozone are so-called OH radicals. These molecules, also called hydroxyl radicals, consist of a hydrogen atom and an oxygen atom. In the atmosphere, they play an important role: as they are among the most reactive substances, they combine with a variety of different gases in the air. Because they can make harmless pollutants, the radicals are also referred to as "detergent of the atmosphere". However, OH radicals are also involved in reactions that have negative consequences, especially for city dwellers. If they are close to nitrogen oxides and hydrocarbons, OH radicals initiate a reaction chain that ends in ozone.

So far, scientists assumed that the formation of OH radicals in the atmosphere, especially short-wave, ultraviolet light is necessary. However, in their new study, Sinha and his colleagues experimentally demonstrated that light in the visible spectrum is involved in the production of hydroxyl radicals. If the light hits a nitrogen dioxide molecule, two OH radicals can form if enough water vapor is present in the vicinity. This reaction was discovered as early as 1997, but researchers were unable to detect any OH radicals at that time and considered the influence of this reaction on the atmosphere to be low.

Sinha and his colleagues have now proven the opposite: Although the process is much slower than the OH production in short-wave light. Nevertheless, measurable quantities of hydroxyl radicals are generated. Sinha believes that the impact of this new reaction could be as great as the proportion of other OH radicals generated elsewhere. The development process of urban ozone is thus even more complicated than already thought, adds the researcher.

Amitabha Sinha (University of California, San Diego) et al .: Science, Volume 319, p. 1657 ddp / science.de? Markus Zen's ad

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