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Unexpected interactions in the formation of secondary organic aerosols in the atmosphere

Secondary organic aerosols (SOA) consist of very small particles that are generated in the atmosphere from both natural and man-made emissions.

It is a major component of PM2.5 (particulate matter less than 2.5 micrometers in diameter) worldwide known to affect climate and human health.

Sally Lee Ng, the Love Family Professor in Georgia Tech’s College of Chemical and Biomolecular Engineering, College of Earth and Atmospheric Sciences, and College of Civil and Environmental Engineering, led a study to investigate the composition and properties of SOA from the nitrate radical oxidation of two common monoterpenes, compounds found in many Plants, monoterpenes represent an important class of biogenic volatile organic compounds (VOCs) and their oxidation by nitrate radicals are a significant source of SOA globally. Specifically, her research team examined the monoterpenes α-pinene and limonene, both of which are emitted in large amounts by trees.

In results published in Nature Communications, Friqli Ng found that oxidized mixtures of monoterpenes simultaneously produced different results than those observed through oxidation separately in laboratory chamber experiments.

Due to the chemical complexity of VOC reactions and SOA formation, previous experiments have mostly studied only one VOC at a time. In this study, Li Ng’s team used advanced mass spectrometry techniques to investigate the chemistry of multiple VOC reactions from the bulk to the molecular level.

“Our results highlight that, contrary to what is currently assumed in atmospheric models, the reactivity of individual VOC-constituent products must be accurately calculated to describe SOA formation and its climate and health impacts,” Ng said.

One might expect α-pinene and limonene to have the same ability to form SOA when they are oxidized as mixtures simultaneously compared to when they are oxidized separately, Li Ng explained, but what the researchers found was an almost 50% enhancement in SOA formation from α-pinene. and a 20% reduction in limonene composition of SOA.

“In this case, one plus one does not equal two,” she added. “We don’t get a linear reaction in the simultaneous oxidation experiment as we do in the sequential experiment. New products are synthesized through the monoterpene reaction.”

A study examined Ng α-Pinene and limonene in a nocturnal environment when radical nitrate chemistry occurs most prominently. Nitrate radicals are produced by traffic and ozone emissions.

“Aerosol chemistry doesn’t stop at night,” Ng said. After sunset, compounds of nitrogen oxide and ozone can react with emissions from trees to produce organic aerosols. “At night, oxidation still occurs, but nocturnal reactions have not been well studied.”

Further studies
Ng said she hopes the research will lead to more studies examining volatile organic compounds in the atmosphere and how they interact. “Studying the interaction between two VOCs is really just the beginning,” Ng said. “It’s a big step forward, but there’s still a long way to go to understand the complexity of the atmosphere, where hundreds of VOCs are emitted and interacting at the same time. ”



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