Atmospheric chemist and RADICAL team member, Professor John Wenger, describes the role that atmospheric radicals play in air quality, and why we want to improve our radical detection methods.
All over the world, vast quantities of gases are continuously being released into the air from a range of man-made and natural sources. These emissions range from combustion products like nitrogen dioxide and carbon monoxide to volatile organic compounds (VOCs) released from vegetation and automobile fuels. Thankfully, the atmosphere has a natural ability to remove almost all of these emissions, helping to clean the air in the process.
This self-cleaning ability of the atmosphere is mainly controlled by two naturally-occurring radical species in the atmosphere – hydroxyl (OH) and nitrate (NO3). Radicals are atoms or molecules with an unpaired electron, which makes them highly reactive to other species.
A remarkable feature of these radicals is that they dominate the chemistry of the first 10 km of the atmosphere (troposphere) even though they are only present at concentrations around 1 part per trillion, i.e., there is only one radical in a trillion (1,000,000,000,000) molecules of air!
Day-time radical chemistry
Hydroxyl is often called the “detergent of the atmosphere” because it is extremely effective at removing gases in the atmosphere. It does this through a series of chain reaction cycles which both use up and regenerate the reactive species (see figure below). Hydroxyl is mainly produced by the action of sunlight on ozone and other species in air and peak concentrations therefore occur around mid-day.
A hydroxyl radical typically lives for around one second before colliding and reacting with a gas molecule, initiating its oxidation in the atmosphere. For example, over a few days, hydroxyl converts nitrogen and sulfur dioxides to nitric and sulfuric acid respectively, which can dissolve in cloud water droplets and be removed from the air as acid rain. Alternatively, nitric and sulfuric acid can also be involved in the formation of atmospheric particulate matter (PM), which impacts on both health and climate.
Interestingly, hydroxyl does not react with carbon dioxide (CO2), resulting in a very long atmospheric lifetime for CO2 (20-200 years) and its status as the most important greenhouse gas. While methane is also a greenhouse gas, it does react with hydroxyl very slowly, yielding a lifetime of around 12 years. The overall removal of methane from the atmosphere occurs through a sequence of reactions which first produce formaldehyde, then carbon monoxide and finally carbon dioxide and water. The hydroxyl radical is the key reactant in each of these chemical reactions.
In addition to methane, thousands of other volatile organic compounds are emitted into the air and also react with the hydroxyl radical. The reactions of these VOCs with hydroxyl is generally much faster, typically occurring over hours to days. Similar to the methane reaction, the atmospheric transformation of VOCs also occurs in a stepwise manner, often producing a large number of oxidized products. Some of these products remain in the gas phase and react further with hydroxyl to eventually produce carbon dioxide and water. However, oxidation products with lower volatility tend to produce secondary particles which can scatter light and are responsible for the blue haze often seen hanging in the air over forests and the hazy pollution or photochemical smog first observed in Los Angeles.
The chemistry of VOCs is very complex and currently one of the most challenging areas of atmospheric chemistry research.
Night-time radical chemistry
While the hydroxyl radical is the main chemical agent controlling atmospheric composition, it is important to remember that it only operates during the day. At night, another species – the nitrate radical- takes centre stage in driving chemical transformations in the troposphere. Nitrate is formed by reaction of nitrogen dioxide with ozone and although this reaction occurs at all times, nitrate is easily broken down by sunlight, meaning that appreciable concentrations can only build up at night.
The nitrate radical behaves like hydroxyl when reacting with VOCs, producing a range of oxidation products including nitrates and secondary particles. It also reacts readily with nitrogen dioxide to produce the reservoir compound N2O5 which interacts with water to produce nitric acid. However, when it comes to the other main pollutants (methane, carbon monoxide, nitrogen dioxide and sulfur dioxide), their reaction with the nitrate radical is too slow to be of atmospheric importance.
It is clear that hydroxyl and nitrate radicals play a pivotal role in determining the chemical composition of Earth’s lower atmosphere and drive the key processes that affect both air quality and climate.
However, the chemistry is very complex and the methods currently available for measuring these atmospheric radicals are highly specialised, very expensive and difficult to deploy (see review article by Wang et al 2021).
The development of low-cost sensors for hydroxyl and nitrate radicals, as envisaged in the RADICAL project, would be real game changer by facilitating a much wider range of laboratory studies and atmospheric observations for advancing our knowledge in this critically important area of atmospheric science.
About the author: John Wenger is Professor of Chemistry at University College Cork and director of the Centre for Research into Atmospheric Chemistry. His main research interests are in the atmospheric degradation of volatile organic compounds and the chemical composition and sources of atmospheric aerosols. He has been involved in a large number of National and European projects involving atmospheric simulation chambers and field measurement campaigns to improve our understanding of atmospheric processes and their links with air pollution and climate. Prof. Wenger is an Associate Editor for the journal Atmospheric Environment and the National Contact Person for the pan-European research infrastructure ACTRIS (Aerosol, Clouds and Trace Gases Research Infrastructure).
Prof. Wenger leads the RADICAL sensor evaluation and validation work package.