Looking inside common atmospheric particles provides important clues about their effects on climate and health, according to a new study by chemists at the University of British Columbia. Secondary organic aerosol (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 and help deflect solar radiation or help form clouds. Different types of SOA can mix into a single particle, and their environmental impacts 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 journal Atmospheric Chemistry and Physics, an international team of researchers discovered that two-phase particles can form when different types of SOA are mixed. The finding could help improve current models that predict the climate and health effects of SOA. “Until now, models have often assumed that when SOA types are mixed in the same particle, they have only one phase. But we found that this is not always the case, which means that current models may not be capturing some of these effects correctly,” says lead author Dr. Fabian Mahrt, postdoctoral fellow at the Paul Scherrer Institute and UBC Department of Chemistry. The project was funded by the European Union’s Horizon 2020 research and innovation programme. The team found that six of 15 mixtures of two types of SOA commonly found in the atmosphere resulted in two-phase particles. Importantly, they also found that the number of phases depends on the difference in the average oxygen-to-carbon ratio between the given SOA types. It is a fairly simple but potentially powerful way of representing such effects in models. When this difference is 0.47 or greater, the researchers found that the particles would have two phases. “We can now work with very complex organic molecules, calculate a single parameter that gives us information about the properties of a particular SOA mixture, and then map potential effects at large enough scales,” says aerosol scientist and senior author Dr. Allan Bertram. professor in the Department of Chemistry at UBC. This kind of SOA mixing can happen, Dr. Mahrt says, when clumps of SOA particles, which have been in the atmosphere for a long time, blow from rural environments over cities where SOA particles have recently been emitted. “If we assume that this mixing of plumes forms particles with a single phase, we could overpredict the total organic mass of particles in these regions, and thus, the health effects of these people.” The team of scientists hopes that the finding can help improve models and ultimately ensure that policies and regulations are based on a rigorous scientific understanding. Building on previous work, the researchers used a fluorescence microscope to look inside mixed SOA particles in their current experiments, injecting them with a dye that causes the particle phases to emit a different color of light depending on their polarity. The researchers then used these colors to directly infer the number of phases in the mixtures, providing direct visual evidence of multiple phases. The research team hopes that other scientists will now expand the number of SOA mixtures experimentally, as well as incorporate the findings into atmospheric models in the future. “The study is proof that we need to look at this phenomenon more closely to get the full picture. We have one more piece of the puzzle, but we’re not necessarily done with the puzzle yet,” says Dr. Mahrt. The team found that six of 15 mixtures of two (secondary organic aerosol) types of SOA commonly found in the atmosphere resulted in two-phase particles.
