Aerosols: Mechanisms of Mischief

By Noah Bussell While many of us have heard about aerosols only in recent years, likely in the context of COVID-19, these micrometer (and even nanometer) sized particles have long been of interest to scientists due in part to their effects on respiratory health and the climate. However, much is still unknown about aerosols:  How do they form, what exact consequences do they have on human health, and are they fortifying or destroying our atmosphere? “Aerosol” generally refers to two phase systems of a gas, namely air, and the solid or liquid particles suspended in the gas. Since this term is a bit nonspecific and includes many classes of particles, their effects are vaguely defined. Accordingly, aerosols can be derived from and associated with natural sources like plants, mineral dust, and sea salt, as well as anthropogenic sources like fossil fuel combustion and hairspray—and the ways by which some aerosols appear in the atmosphere still remains unclear. Whereas primary aerosols emerge directly from events such as volcanic eruptions, secondary aerosols don’t directly come from the earth’s surface, but rather they are produced via continuously occurring atmospheric reactions. To understand the sources and synthesis of secondary aerosol particles, scientists must analyze the individual steps in which a group of molecules progresses from one chemical state to another. Specifically, chemists quantify the amount of energy required for chemical bonds to twist, atoms to switch, rings to break, and functional groups to form. A recent paper published by a group of scientists primarily located in Finland at the University of Helsinki and Tampere University demonstrated how a plant-produced molecule called α-pinene is converted into highly oxygenated organic molecules (HOMs). Because they have low volatility, HOMs prefer to condense into particulate matter and this, as a result, contributes to aerosol formation. On the other side of the spectrum, a highly volatile species would primarily partition into the gas phase instead. α-pinene, specifically, is relevant as it is one of the most prevalent monoterpenes, which is a class of molecules that accounts for a large fraction of global, biogenic volatile organic compound(VOC) emissions—a significant source of HOMs. Previously, much was unknown about the chemical pathway between α-pinene and HOMs,  and thus about aerosols, but the work carried out by these researchers demonstrated how molecular energy gained from a process called ozonolysis—whereby ozone molecules force carbon-carbon bonds to break—can cause further chemical and physical transformation. In the case of α-pinene, the excess energy from ozonolysis breaks apart a 4-atom ring found in the α-pinene molecule. Rings can possess a property called “steric strain,” which forms a molecular roadblock preventing HOMs from forming. Despite this, through the usage of experimental and computational techniques, these Finnish laboratories found that the ozonolysis step provides the energy required for these rings to break and, as a consequence, HOMs to subsequently form. Understanding how aerosols form can help researchers make connections between these particles and various atmospheric processes. For instance, many aerosols directly cool the climate by scattering radiation back into space, meaning that this energy does not reach the earth. They also indirectly cool the climate by acting as cloud condensation nuclei (colloquially known as “cloud seeds”). Aerosol cloud seeds serve as sites for water droplets to coalesce, which leads to the formation of dense clouds; these clouds can then reflect energy away from earth via a phenomenon known as the Twomey effect. While some darker aerosols absorb sunlight and heat the atmosphere, aerosols overall increase the “radiation budget” on earth and thus cause a net cooling effect. These somewhat hopeful findings, however, should be approached with a degree of caution. Aerosols can negatively affect respiratory health and while they help produce remarkable sunsets, their role in atmospheric processes stretches well beyond this rather lovely optical sensation.  Amidst all of these apparent contradictions and correlations, these tiny particles unmistakably make their presence felt in substantial ways.

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