Digest hydrocarbons

Volatile organic compounds can be found in the air, everywhere. A wide range of sources, including plants, cooking fuels and household cleaners, emit these compounds directly. They can also form in the atmosphere through a complex network of photochemical reactions.

Researchers from Sandia National Laboratories and colleagues from other institutions have studied the reactions of hydroxyl and methylperoxy radicals in order to understand their impact on the ability of the atmosphere to process pollutants.

This work, which was published in Nature Communications, showed that reactions can affect the levels of a key chemical marker used to badess understanding of pollutant treatment and abundance. It ultimately helps us understand how nature and human activity affect the chemical composition of the atmosphere.

Recent studies in this field have shown that the reaction of methylperoxy with the hydroxyl radical occurs more rapidly than expected, and this reaction could therefore modify current knowledge about the chemistry of both low temperature combustion and atmospheric earthly.

The hydroxyl radical, an important molecule of combustion and atmospheric chemistry, initiates the oxidation or treatment of fuel molecules and pollutants. When this radical reacts with the fuel molecules in the presence of oxygen, a new clbad of radicals, called peroxy radicals, is formed. In the Earth's atmosphere, when the hydroxyl radical reacts with methane (which is both a greenhouse gas and the most abundant hydrocarbon), a methylperoxy is created.

Impacts on combustion

Rebecca Caravan, Sandia's postdoctoral fellow and principal investigator of the new collaborative effort, said that studying the subsequent reactions of peroxy radicals is essential to understanding low-temperature combustion, as the fate of the latter determines to what extent the fuel will undergo auto-inflammation. The researchers wanted to understand how the reaction of hydroxyl and methylperoxy radicals could impact it, for example, the possible inhibition of auto-inflammation through the elimination of reactive radicals. and the production of relatively unresponsive chemicals.

"Determining the impact of any specific reaction in a given environment requires knowing both the speed of the reaction and the products of the reaction," she said. "Careful quantification of products is often the most difficult task.A relatively small change in these reactions can dramatically alter the magnitude and even the direction of the impact of a reaction in a given environment. "

Recent theoretical work has indicated that a possible product of the reaction of the hydroxyl radical and methylperoxy could be methanol and oxygen. These products would have a significant impact on our understanding of the chemistry of the Earth's troposphere – the part of the atmosphere between zero and 10 km (6 miles), which contains about 75% of the mbad of the atmosphere.

Caravan has stated that methanol has long been largely underestimated in the troposphere by atmospheric modelers. As methanol can be formed from multiple sequences of tropospheric oxidation reactions, understanding how chemical reactions contribute to methanol levels in the atmosphere helps to better understand how the atmosphere processes the methanol levels in the atmosphere. hydrocarbons emitted by nature and human activities, which helps us understand the influence of both on the chemical composition of the atmosphere.

Craig Taatjes, a combustion chemist at Sandia, lead researcher for this research effort, said, "We recognized that our fundamental measurements of methanol yield from the hydroxyl radical and the methylperoxy reaction could impact Modeled abundance of atmospheric methanol could focus on the consequences of our investigations. "

International collaboration

The difference between modeled methanol and methanol measured is particularly important in the remote troposphere, areas where the influence of human activity is relatively limited.

Dwayne Heard, professor of atmospheric chemistry at the University of Leeds in the UK, said that an understanding of these regions was needed before one could understand human changes.

"We know that changes in human emissions cause a warming of the atmosphere and a deterioration in the quality of the air we breathe," Heard said. "However, there are also natural and dominant processes that occur everywhere, for example in oceans where human influence is relatively low."

Radical chemistry studies are complicated; multiple side reactions should be understood with the reaction of interest. To address this, researchers at Sandia and NASA's Jet Propulsion Laboratory have used the world-renowned capabilities of the Sandia Combustion Research Center and the advanced light source at the Lawrence Berkeley National Laboratory.

The researchers used Sandia multiplex photoionization mbad spectrometry instruments developed by Sandia researchers David Osborn and Lenny Sheps. The team also used adjustable vacuum ultraviolet ionizing radiation from the Chemical Dynamics light line to the advanced light source to observe and characterize chemistry and reaction products.

The researchers then worked to interpret their experimental observations via models and calculations. They examined the role of longer-term chemistry on reaction products by collaborating with partners from the University of Lille in France, who used their atmospheric simulation chamber. Additional members of the University of Bristol team in the UK used a global chemical model to evaluate experimental results on the troposphere.

"It was a highly collaborative international project, each party bringing its own world-clbad capabilities," Caravan said.

The Sandia team was funded by the Energy Sciences Base Office of the Department of Energy. The co-authors of the document were supported by NASA and British and French agencies.

Impact on the atmosphere

Thanks to this collaborative effort, it is now understood that in the troposphere, about 25% of the methylperoxy radicals are removed by the rapid reaction with the hydroxyl radical, which means that fewer peroxy radicals are subjected to other reactions. known to lead to methanol. To counterbalance this, the methanol yield resulting from the reaction of the hydroxyl radicals with methylperoxy should be about 15%, but the yields measured by the authors are between 6 and 9%.

The consequences of this result on the understanding of tropospheric methanol are important. The reaction of the hydroxyl radical and methylperoxy does not resolve the imbalance between the modeled higher and lower methanol abundances; in fact, this gap is now exacerbated. Methanol in remote areas is now underestimated by about a factor of 1.5 in global atmospheric models.

"This work highlights our incomplete understanding of the key chemical reactivity of the troposphere, and we lack significant responses, opening the door to further investigation," Caravan said.

Alexander Archibald, a Cambridge University professor and domain expert, explains that Caravan's experiments demonstrate that methanol has other secrets to reveal.

"Although the reaction between methylperoxy radicals and hydroxyl radicals is not a major source of methanol, the models still underestimate the amount of methanol," said Archibald. "The exciting work done by Caravan and his staff closes a chapter in history, but the book remains unfinished and further work is needed to complete our understanding of this important compound in the atmosphere."

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More information:
Rebecca L. Caravan et al. The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol, Nature Communications (2018). DOI: 10.1038 / s41467-018-06716-x