Both natural and anthropogenic organic molecules can spontaneously self-assemble into supramolecular atmospheric nanoparticles during heat waves, a US study has shown.1 The study explains high levels of new particle formation in hot weather, which could inform climate change models and help explain the number of deaths associated with extreme heat.
Clouds are one of the most important feedbacks in climate change models, and the most uncertain, because they can reflect heat back into space and trap it in Earth’s atmosphere. Cloud formation requires the nucleation of gaseous water molecules by an acid. About half of these are thought to be caused by new particles such as oxidizing pollutants such as sulfur dioxide and volatile organic compounds in the lower atmosphere. Heat waves should simply be expected to increase the evaporation of volatile organic compounds and reduce the formation of new particles, but this has not been well studied.
In 2004, Zhang Renyi Texas A&M University researchers suggested that organic acids, which are produced when sunlight oxidizes naturally occurring volatile organic compounds such as pinene from trees and aromatic hydrocarbons from car engines, may form unusually stable complexes with sulfates in polluted air, promoting aerosol formation.2 The researchers then developed a technique that enables mass spectrometry of atmospheric particles as small as 3 nm (about 60 molecules in total).
Their new study reveals measurements recorded over a one-month period on the Texas A&M campus. The strongest new particle formation occurred when the temperature was well above 30°C. Sulfuric acid was only present in small amounts, suggesting that organic acids can not only form complexes with sulfuric acid but also self-assemble on their own into nanoparticles. “Organic acids can form double hydrogen bonds, which are very stable and have multiple branches for growth,” Zhang explains. “We believe that if it’s happening here, it’s happening elsewhere.”
Since the spontaneous assembly mechanism is not limited by the volatility of organic compounds, Zhang believes this provides a natural explanation for the formation of new particles at high temperatures. Because very small, fully organic particles are unlikely to be highly hygroscopic, questions remain about the extent and direction of their potential feedbacks on climate change. This could potentially prevent the seeding of cloud-forming aerosols. However, there may be other meanings as well. “Heat waves can kill people, but ultrafine particles can penetrate deep into the body,” Zhang said. “The smallest particles are exclusively organic acids…Further research is needed to understand how the heat combined with these ultrafine particles affects human health.”
“It is interesting that we frequently observe the formation of new particles at such high temperatures,” he says. Hamish Gordon from Carnegie Mellon University in Pittsburgh, USA. “In a chemical sense, molecular hydrogen bonding appears to be involved in the formation of nanoparticles in the atmosphere, so while that part is not new, the results are interesting nonetheless.”