Airborne Particles Transport Pollutants Around the World

The symbiotic relationship between airborne particles and pollutants demonstrate how city pollution ends up in faraway places such as the Arctic.

Pollution from fossil fuel burning and forest fires reaches all the way to the Arctic, but the pollution should decay long before it travels that far. A new study can now explain how pollution can travel such great distances. According to the research, the pollutants get inside of the airborne particles and are then protected from the elements as the particles travel. The results from the study will help scientists improve atmospheric air-quality and pollution transport models.

The results also show that the particles holding pollutants inside benefit from this arrangement; the airborne particles, made from natural molecules mostly given off by live or burning plants, last longer with a touch of pollutant packed inside. The pollutants are known as polycyclic aromatic hydrocarbons (PAHs), and are regulated by environmental agencies due to their toxicity.

"What we've learned through fundamental studies on model systems in the lab has very important implications for long-range transport of pollutants in the real world," said physical chemist Alla Zelenyuk of the Department of Energy's Pacific Northwest National Laboratory. "In this study, we propose a new explanation for how PAHs get transported so far, by demonstrating that airborne particles become a protective vessel for PAH transport."

For decades, atmospheric scientists have been trying to explain how atmospheric particles manage to transport harmful pollutants to pristine environments thousands of miles away. The particles collected in areas such as the Arctic also pack higher concentrations of pollutants than scientists' computer models predict.

The predictions are based on the assumption that the particles are like liquid spheres that allows PAHs to escape, but they don’t escape. One recent advance has helped pin down why PAHs are remaining stuck in their particle lairs. Zelenyuk and her colleagues developed an ultra-sensitive instrument that can determine the size, composition, and shape of individual particles.

Called SPLAT II, the instrument can analyze millions of tiny particles one by one, and characterize the individual particles providing unique insight into their property and evolution.

Using SPLAT II to evaluate laboratory-generated SOA particles from alpha-pinene, Zelenyuk has already discovered that SOA particles aren't liquid at all. Her team's recent work revealed they are more like tar -- thick, viscous blobs that are too solid to be liquid and too liquid to be solid.

These results showed the team that PAHs become trapped within the highly viscous SOA particles, where they remain protected from the environment. The symbiotic relationship between the atmospheric particles and pollutants surprised Zelenyuk: SOAs help PAHs travel the world, and the PAHs help SOAs survive longer. Zelenyuk and her colleagues performed comparable experiments with other PAHs and SOAs and found similar results.

In the real world, Zelenyuk said, the evaporation will be even slower than what was shown in their study. These results will help modelers better simulate atmospheric SOA particles and transport of pollutants over long distances. This work was supported by the Department of Energy Office of Science and PNNL's Chemical Imaging Initiative.

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