Gazing inside common atmospheric particles provides important clues to their climate and health effects, according to a new study by chemists at the University of British Columbia.

Secondary organic aerosols (SOA) particles are ubiquitous in the atmosphere and play an important role in air quality and climate. They can increase air pollution and damage the lungs, as well as help scatter solar radiation or help form clouds.

Different types of SOA can mix together into a single particle and their environmental effects are governed by the physical and chemical properties of the new particles, particularly the number of phases—or states—in which they can exist.

In a new research letter published in the Open Access European Journal of Geosciences Atmospheric Chemistry and Physics, an international team of researchers found that two-phase particles can form when different types of SOA are mixed. This finding could help improve current models that predict climate and health impacts on SOA.

“So far, models often assume that when SOA types mix in the same particle, they have only one phase,” says the lead author. “But we have found that this is not always the case, which means that current models may not correctly capture some of these effects.” . Fabian Mert, Postdoctoral Fellow in the Paul Scherer Institute and University of British Columbia’s Department of Chemistry. The work was funded by the European Union’s Horizon 2020 Research and Innovation Programme.

The team found that six out of 15 mixtures of two types of SOA commonly found in the atmosphere resulted in biphasic molecules. Importantly, they also discovered that the number of phases depends on the difference in the average oxygen-to-carbon ratio between the selected SOA species. It is a fairly simple but potentially powerful way of representing such effects in models. When this difference is 0.47 or higher, the researchers found that the particles have two phases.

“We can now work with very complex organic molecules, compute a single parameter that gives us information about the properties of a given mixture of SOA, and then map potentially large-scale effects,” says aerosol scientist and senior author Alan Bertram, professor in the UBC Department of Chemistry.

This type of SOA mixing may occur when plumes of SOA particles, which have been in the atmosphere for some time, blow from rural environments over cities where newly produced SOA particles are released, Mahrt explains.

“If we assume that this blending of plumes forms single-phase particles, we might over-predict the total mass of organic particles in these regions, and thus the effects on the health of these people.” The team of scientists hope that the results will help improve the models and ultimately ensure that policies and regulations are based on rigorous scientific understanding.

Building on previous work, the researchers used fluorescence microscopy to look inside the mixed SOA particles in their current experiments, injecting them with a dye that causes the phases of the particles to emit different colored light depending on their polarity. The researchers then used these colors to directly infer the number of phases of mixtures, providing direct visual proof of multiple phases.

“The study is evidence that we need to look at this phenomenon more carefully to get the full picture,” Mahert says. “We have another piece of the puzzle but we haven’t necessarily finished the jigsaw yet.” The research team hopes that other scientists will now expand the number of SOA mixtures experimentally, as well as include the results in atmospheric models going forward.

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