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Yale study finds that organic aerosols more complicated than previously thought

Yale researchers have found that organic aerosols—a type of air pollution—are much more complicated than previous studies indicated. Using high-powered equipment to analyze air samples, the researchers were able to get a detailed look at the molecular makeup of organic aerosols, which have a significant presence in the atmosphere.

The atmospheric evolution of organic compounds encompasses many thousands of compounds with varying volatility, polarity, and water solubility. The molecular-level chemical composition of this mixture plays a major, yet uncertain, role in its transformations and impacts. Here we perform a non-targeted molecular-level intercomparison of functionalized organic aerosol from three diverse field sites and a chamber. Despite similar bulk composition, we report large molecular-level variability between multi-hour organic aerosol samples at each site, with 66 ± 13% of functionalized compounds differing between consecutive samples.

Single precursor environmental laboratory chamber experiments and fully chemically-explicit modeling confirm this variability is due to changes in emitted precursors, chemical age, and/or oxidation conditions. These molecular-level results demonstrate greater compositional variability than is typically observed in less-speciated measurements, such as bulk elemental composition, which tend to show less daily variability. These observations should inform future field and laboratory studies, including assessments of the effects of variability on aerosol properties and ultimately the development of strategic organic aerosol parameterizations for air quality and climate models.

—Ditto et al.

Posing risks to health and climate, these airborne particles generally fall into two categories: Primary organic aerosols that can form during combustion, such as in car and truck exhaust, and secondary organic aerosols that result from oxidation of organic gases and particles in the air.

For the open-access study, published in Nature’s Communications Chemistry, the researchers used a combination of liquid chromatography, which separates thousands of compounds in a sample, and a mass spectrometer, which identifies and analyzes those compounds.

Here, we can actually differentiate molecules that would otherwise appear to be very similar. In past studies, they had less information on molecular identities across the complex mixtures present. With these instruments, we can determine molecular formulas with more accuracy.

—senior author, Drew Gentner, assistant professor of chemical & environmental engineering

That’s a significant advancement, the researchers note, since knowing what harmful elements are in the air is critical to finding ways to reduce them, added Gentner.

If you develop an air pollution control policy based on less specific information for organic aerosol, there may be much more variability in the molecular-level composition than you might expect, which could influence aerosol properties and impacts.

—Jenna Ditto, a Ph.D. candidate in Gentner’s lab and lead author of the study

Collected over three weeks at each site in the summer, samples were taken from a forest in Michigan, and from urban environments in Atlanta and New York City. Most surprising, said the researchers, were the variations they saw in samples taken from the same sites. In most cases, up to 70% of the compounds at a site were distinct from each other in consecutive samples.

Even if certain causes remain constant, the researchers said, a number of factors could be driving the variability.

The different types of compounds emitted from cars and plants could vary from car to car or tree to tree.

—Jenna Ditto

Other factors, such as weather patterns and chemical oxidation conditions can also change. Individually, these variations are usually slight, but they can add up to significant differences, said the researchers.

Building off of this study, the researchers said they expect to analyze the results further to get a better sense of what types of health and climate effects these variabilities may pose.

Resources

  • Jenna C. Ditto, Emily B. Barnes, Peeyush Khare, Masayuki Takeuchi, Taekyu Joo, Alexander A. T. Bui, Julia Lee-Taylor, Gamze Eris, Yunle Chen, Bernard Aumont, Jose L. Jimenez, Nga Lee Ng, Robert J. Griffin & Drew R. Gentner (2018) “An omnipresent diversity and variability in the chemical composition of atmospheric functionalized organic aerosol” Communications Chemistry volume 1, Article number: 75 doi: 10.1038/s42004-018-0074-3

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