Nocturnal Detergents with Side Effects
Nitrate radicals in the air clean the atmosphere at night. Jülich atmospheric researchers together with colleagues from twenty international research institutions have now found out that this probably has unforeseen side effects. Contrary to previous assumptions, the reaction of nitrate radicals with hydrocarbons leads to the increased formation of particles in the low-level troposphere. Since there are almost no systematic investigations into this field, any potential effects on air quality and climate are hard to assess. The scientists' study results were published in the journal Geophysical Research Letters.
The little molecule is called nitrate and consists of one nitrogen atom and three oxygen atoms. In the air, it is present in a special form, namely with an unpaired electron. This makes it very reactive and therefore a radical. The nitrate radical is immensely important for the chemical composition of air - but only at night. During the day, it is decomposed by the sunlight's ultraviolet rays. In darkness, however, it builds up in the atmosphere and reacts with hydrocarbons - it "washes" them out of the atmosphere. Together with ozone and hydroxyl radicals, which take on the same task during daylight, it thus prevents trace gases from accumulating in the air.
An international study initiated by Jülich researchers has now revealed that the nightly atmospheric cleansing probably has side effects of a hitherto unimagined extent. The compounds formed out of nitrate radicals and hydrocarbons - the waste products of the cleansing process, so to speak - lead to the formation of more particles in the air. "The study shows that organic nitrates make up a significant share of the fine dust in the atmospheric layers closest to the Earth's surface," explains Prof. Astrid Kiendler-Scharr from Jülich's Institute of Energy and Climate Research. "Around 40 % of these 'submicron nitrate aerosols' that we measured are organic nitrates."
First study of its kind
For the study, scientists from twenty international research institutions gathered data on aerosols in the troposphere, the lowest layer of the atmosphere. At a total of 18 measuring sites throughout Europe, they collected information with high temporal resolution on the aerosols' compositions using special mass spectrometers during three measurement phases that each lasted several weeks. They were thus able to show that organic nitrates are frequently a component of these aerosols.
The study's scope makes it the first of its kind: until a few years ago, it was viewed as certain that organic nitrates do not contribute to particle formation in the atmosphere. The reason for this is that nitrate radicals and hydrocarbons should not actually encounter each other at all since nitrate radicals in the atmosphere are formed from nitrogen dioxide, whose main source is the combustion of fossil energy carriers - gasoline, gas, coal, and oil. Atmospheric nitrate radicals are therefore anthropogenic, i.e. caused by modern civilization. Hydrocarbons, in contrast, are mainly of biogenic origin since they are the waste products of plant metabolism. "The evidence now increasingly points to these air masses encountering each other after all, particularly in Europe, where the population density is high," says Kiendler-Scharr. "As a result, more complex situations have to be registered in order to understand air quality."
Potential consequences for climate and air quality
The concrete consequences of this discovery are hard to assess. So far, laboratory investigations and simulation experiments have concentrated on other particle sources. The influence of organic nitrates on climate and the health effects of aerosols are therefore not very well known. "And all of the measurements took place at ground level, meaning we can only make a statement on air masses close to the surface," says Kiendler-Scharr. "Neither the lifetime of the substances formed nor their advance into the higher troposphere are known with certainty. This is why climate-relevant consequences are hard to predict."
Using location-dependent modelling, the scientists also analysed how representative their measurements are. They found a general pattern: in regions with lots of industry, i.e. where nitrogen oxide emissions can be expected, the mass of organic nitrates in the aerosol is larger. But Astrid Kiendler-Scharr points out: "There are also deviations, and some processes are not yet understood. Our data now offer the opportunity of reviewing and improving existing models."
Source: Forschungszentrum Jülich