**The Unanticipated Assembly of Organic Molecules into Atmospheric Nanoparticles During Heat Waves**
Recent findings from the United States have indicated that both naturally occurring and human-made organic molecules can spontaneously self-assemble into supramolecular atmospheric nanoparticles during heat waves. This revelation sheds light on the elevated levels of new particle creation noted during warm weather, potentially affecting climate change models and informing our understanding of the health ramifications of extreme heat events.
Clouds serve a dual function in climate dynamics by reflecting heat back into space and retaining it within Earth’s atmosphere, thus establishing a crucial feedback mechanism within climate models. However, the formation of clouds depends on the nucleation of gaseous water molecules combined with acids, which remains one of the least predictable elements of these models. It is estimated that around 50% of cloud-seeding particles stem from new particles such as oxidized pollutants, including sulfur dioxide or volatile organic compounds in the troposphere. Contrary to expectations that heat waves would enhance the evaporation of volatile organic compounds, thereby diminishing new particle formation, this relationship has not been thoroughly explored.
In 2004, research led by Renyi Zhang at Texas A&M University suggested that organic acids—resulting from the photochemical oxidation of natural volatile organic compounds like pinene or human-made hydrocarbons from automotive emissions—might create exceptionally stable complexes with sulfates in polluted air, thereby increasing aerosol production. Subsequent advancements in analytical methodologies have permitted the mass spectrometric examination of atmospheric particles as small as 3 nm, roughly consisting of 60 molecules.
In their most recent investigation, Zhang’s team carried out month-long observations on the Texas A&M campus, noting significant new particle formation when temperatures surpassed 30°C. Sulfuric acid was found in trace amounts, indicating that organic acids are capable not only of forming complexes with sulfuric acid but also of independently self-assembling into nanoparticles. Zhang claims that organic acids can create stable double hydrogen bonds and possess multiple growth branches. This occurrence is probably not restricted to Texas but is also present in other areas under comparable circumstances.
The spontaneous assembly process seems to be unaffected by the volatility of organic compounds, providing a plausible rationale for new particle formation at elevated temperatures. However, there remain unresolved questions about the feedback effects of these small, purely organic particles on climate change, as their low hygroscopicity may inhibit their capacity to seed cloud-forming aerosols. Still, these particles might have significant health implications. As Zhang points out, the ultrafine particles emitted from organic acids can infiltrate deep into the human body, potentially heightening the health risks associated with heat waves.
Hamish Gordon from Carnegie Mellon University notes that while hydrogen bonding is a well-known mechanism in nanoparticle formation, the prevalence of new particle formation at high temperatures observed in this study is particularly noteworthy. The ongoing research clarifies the vital interactions between heat waves and atmospheric particle dynamics, emphasizing the need for further examination into their wider environmental and public health effects.