Volatile compounds from personal products drive urban ozone chemistry

Personal care products like deodorant and lotion contain volatile organic compounds (VOCs) that add fragrance or texture—and these can be emitted into the air. Now, research suggests that in densely populated areas, these emissions have significant implications for air quality. During summer 2018, volatile chemical products played just as important a role in driving high ozone pollution levels in New York City as did volatile organic compounds (VOCs) produced by fossil fuel burning (Proc. Natl. Acad. Sci. USA 2021, DOI: 10.1073/pnas.2026653118).

Ground-level ozone is a respiratory irritant and results when nitrogen oxides, mostly from vehicle tailpipes, react with VOCs in the presence of sunlight. Historically, most of those VOCs, such as benzene, have also come from fossil fuel combustion. VOCs such as pinene and limonene emitted by forests in the summer can also drive ozone chemistry. Decades of regulation have reduced vehicle emissions and have led to better air quality, says Matthew Coggon, an atmospheric chemist at the National Oceanic and Atmospheric Administration and a leader of the research. “But we still have ozone exceedances in megacities. What’s the cause?”

Chemists have been examining the role of volatile chemical products in ozone formation since the 1990s. These compounds, he says, are petrochemicals, and are found in a huge variety of products, including industrial solvents and personal care products. But Coggon says figuring out the importance of these compounds relative to VOCs from other sources has been challenging without sensitive measurements.

Coggon and his team drove a mobile lab, equipped with a proton transfer reaction time-of-flight mass spectrometer, around American cities including New York and Chicago, and European cities including Bern and Vienna. The team used relative ratios of tracer molecules to assign VOC signatures to different sources. High levels of benzene meant measured volatiles were likely a product of fossil fuel combustion. And high levels of decamethylcyclopentasiloxane (D5-siloxane), whose primary sources are antiperspirants and hair care products, meant other associated volatiles were also probably from personal care products. Volatile personal care product emissions, Coggon says, were correlated with population density.

Armed with these experimental data, they modeled the contributions of various VOC sources to ozone production in New York City in the summer of 2018. They looked in detail at July days when ozone levels exceeded regulatory standards. “These are the days when the chemistry is hot, and you can see all those reactions on a much grander scale,” says Coggon. They found that VOCs from chemical products played as large a role as VOCs from fossil fuel combustion in driving ozone production. Half of these chemical emissions were from personal care products, and were primarily monoterpenes, ethanol, and oxygenates. The other half were aromatics and alkanes from coatings and adhesives. Coggon says this is likely to be the case in other megacities with dense populations that commonly use these products.

“This clearly makes the case that emissions from volatile consumer products are really important in our urban areas now that we’ve cleaned up other sources like cars and trucks,” says Allen Goldstein, an atmospheric chemist at the University of California, Berkeley, who was not involved with the research. “If we want to make further progress we have to look elsewhere. We need to re-evaluate the most important contributors to VOCs that lead to ozone and aerosol formation.”