Polycyclic aromatic hydrocarbons (PAHs) are among the most common hydrophobic organic pollutants with well documented toxic effects on human health and the ecosystem. Appearing on the list of persistent organic pollutants (POPs), they are regulated under the POPs protocol of the convention on long-range trans-boundary air pollution and by the convention for the protection of the marine environment of the northeast Atlantic (OSPAR convention). Most PAHs are anthropogenic byproducts of energy production and use and of biomass burning.

Although the majority of PAH sources are in developing countries, they reach remote regions, including pristine environments, through long-range transport (LRT). While it is clear that gas-particle partitioning of semivolatile PAHs strongly influences their atmospheric distribution, transport pathways, degradation and environmental fate, the fundamental aspects that determine this partitioning remain unclear. A number of studies have reported an unexpectedly high LRT potential of particle-bound PAHs—for example gas-phase PAH concentrations in the Arctic are orders of magnitude lower than in Europe, while the concentration of particle-bound PAHs is only slightly lower.

Moreover, based on current understanding of gas-particle partitioning and atmospheric degradation of PAHs some species, like benzo[α]pyrene and fluoranthene, should not undergo LRT at all yet are found in the Arctic at concentrations similar to those in Europe. In general, existing gas-particle partitioning models severely underpredict observed LRT of particle-bound PAHs, highlighting large knowledge gaps in kinetic partitioning models.

—Zelenyuk et al.

The results also show that the particles that envelop pollutants also benefit from this arrangement. The new study shows that the airborne particles last longer with PAHs packed inside.

Perhaps the most surprising finding is the observed synergetic relationship between PAHs and SOA. The presence of even a small amount of hydrophobic organics inside SOA significantly decreases the SOA evaporation rate and amplifies the effect of aging, thus creating conditions that ensure efficient LRT of both SOA particles and PAHs, consistent with observations. This synergy between PAHs and SOA particles has important implications not only for human health but also for climate change.

—Zelenyuk et al.

Created from carbon-based molecules given off by trees, vegetation, and fossil fuel burning, airborne particles known as secondary organic aerosols travel in the air and contribute to cloud formation. The PAHs have long been thought to coat the particles on their surface.

For decades, atmospheric scientists have been trying to explain how atmospheric particles manage to transport harmful pollutants to pristine environments thousands of miles away from their starting point. 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, whose fluidity allows PAHs to escape. But they don’t escape, and one recent advance has helped to pin down why PAHs are remaining stuck in the particles. Alla Zelenyuk and her colleagues at EMSL, DOE’s Environmental Molecular Sciences Laboratory at PNNL, 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. The ability of this novel instrument to characterize individual particles provides unique insight into their property and evolution.

Using SPLAT II to evaluate laboratory-generated SOA particles from α-pinene, the molecule that gives pine trees their piney smell, 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.

Armed with this data, Zelenyuk and researchers from Imre Consulting in Richland and the University of Washington in Seattle set out to determine the relation between the SOA particle and the PAHs. Again they used α-pinene for the SOA. For the PAH, they used pyrene, a toxic pollutant produced by burning fossil fuels or vegetation such as forests.

They created two kinds of particles. The first kind exemplified the classical SOA: first they produced the particles with alpha-pinene and then coated them with pyrene. The second kind resembled what likely happens in nature: they mixed α-pinene and pyrene and let the particles form with both molecules present. Then they sent the particles through SPLAT and watched what happened to them over time.

With the pyrene-coated particles, the team found the PAH pyrene evaporating off the surface of the particle quickly, all of it gone after four hours. By the next day, the particle itself had shrunk by about 70%, showing that the alpha-pinene SOA also evaporates, although more slowly than pyrene.

When they created the particles in the presence of both SOA and PAH, the PAH evaporated much more slowly. 50% of the original PAH still remained in the particle after 24 hours. In addition, the SOA particle itself stayed bulky, losing less than 20% of its volume.

These results showed the team that PAHs become trapped within the highly viscous SOA particles, where they remain protected from the environment. 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. These results will help modelers better simulate atmospheric SOA particles and transport of pollutants over long distances.

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. 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.

—Alla Zelenyuk, PNNL, first author

This work was supported by the Department of Energy Office of Science and PNNL’s Chemical Imaging Initiative.

Resources

• Alla Zelenyuk, Dan Imre, Josef Beránek, Evan Abramson, Jacqueline Wilson and Manish Shrivastava (2012) Synergy between Secondary Organic Aerosols and Long-Range Transport of Polycyclic Aromatic Hydrocarbons, Environmental Science & Technology doi: 10.1021/es302743z