Three major types of aerosols form up to three different liquid phases simultaneously when they mix in the atmosphere, a once-poorly understood interaction that can better inform future modeling of air pollution and the climate, according to recent research.

Publishing the findings April 20 in PNAS, a team of chemists and atmospheric scientists was the first to study the liquid phases that form when the three kinds of airborne particles interact. Most research has found only up to two coexisting phases of liquid in atmospheric aerosol particles, and none explained what causes three phases to form.

Aerosols, suspensions of small liquid or solid particles in the air, are prevalent in the atmosphere and naturally occur in forms such as breath, fog, pollen and volcanic fumes. But they are often the result of human-created emissions and contribute to poor air quality and alterations to the climate, whether by warming the planet by absorbing sunlight or cooling it by acting as seeds for cloud formation.

Atmospheric aerosols, for instance, cause more than 3 million premature deaths each year, according to a 2015 study. But accurately modeling how aerosols affect humans and their environment requires understanding how they interact with each other in the atmosphere.

"Our understanding of the phase behavior of atmospheric aerosols is far from complete, resulting in large uncertainties when predicting aerosol effects on air quality and climate," the new paper's authors wrote.

Led by Canadian scientists, the research team focused on three major categories of atmospheric aerosols: primary organic aerosols, secondary organic aerosols and secondary inorganic aerosols. Organic aerosols contain carbon-hydrogen bonds, and inorganic aerosols are usually salts or mineral dust. Primary aerosols are emitted directly into the atmosphere, such as from fossil fuel emissions, while secondary aerosols are created by reactions in the atmosphere. 

Behaviors of the groups have been studied individually or in pairs, but interactions between all three have never been investigated, said Fabian Mahrt, a postdoctoral chemistry researcher at the University of British Columbia and an author of the paper.

"The goal of this study was to see, how many phases can we actually form when we combine all these three major aerosol components and individual particles?" Mahrt said. "That hasn't been studied before."

Running tests on all three aerosol categories, Mahrt and his co-authors changed the relative humidity and used several different secondary organic aerosols. Borrowing a method from chemistry and biology, they identified liquid phases using the dye Nile red, which changes color depending on the liquid's polarity. The molecules of polar liquids such as water are electrically neutral but have positively and negatively charged regions.

The scientists discovered that the particles can contain up to three phases of liquid: a low-polarity organic-rich phase, a higher-polarity organic-rich phase and an aqueous inorganic-rich phase. When all three were present, the inorganic-rich phase formed the core of the particle while the organic-rich phases surrounded it in stacked layers.

The presence of three liquid phases depended on the ratio between oxygen and carbon atoms in the secondary organic aerosols being below 0.8. A high ratio means oxygen atoms are more prevalent and the polarity of the liquid they are in is higher, which appeared to prevent some phase transitions that led to the triple-phase state.

The researchers also replicated the experiment in an atmospheric chamber, where a more complex mix of aerosols was tested and also produced three liquid phases.

"That is sort of a proof of concept that we can really take all the things that we learned from our clean single-component laboratory proxies and model more complex atmospheric particles," Mahrt said.

The authors noted that other scientists had identified three liquid phases in secondary organic and inorganic aerosol mixtures in a 2019 study, but said that the trio did not appear in most of the experiments and that the cause of the three phases was unclear.

The liquid phases affect how aerosol particles form and behave in the atmosphere. Follow-up experiments revealed that the mixed aerosols took longer to reach a stable size and were more likely to become the basis of cloud droplets because of changes in surface tension.

"These particles can have enhanced cloud-formation ability, which then feeds into cloud-cover properties, radiative properties of clouds," Mahrt said. "I think there's really future laboratory, field and also modeling studies needed to fully explore the extent of such effects on a global scale."

The researchers said follow-up work should explore the large-scale effects of these liquid phases in air-quality and climate models. They also recommend experiments that use primary organic aerosols with high oxygen-to-carbon ratios, which are created by cooking and burning plant matter and contribute to widespread indoor air pollution. Mahrt is continuing tests on primary and secondary organic aerosols and what combinations yield a single liquid phase.

"The study that we did is one small puzzle piece to really understand this complex Earth that we're living on," Mahrt said. "It's fundamental, not so much the big picture — that is what hopefully comes and builds up on our research."

The study, "Coexistence of three liquid phases in individual atmospheric aerosol particles," published April 20 in PNAS, was authored by Yuanzhou Huang, Shaun Xu and Allan Bertram, University of British Columbia; Fabian Mahrt, University of British Columbia and Paul Scherrer Institute; Manabu Shiraiwa, University of California, Irvine; and Andreas Zuend, McGill University.