Organic compounds coat airborne particles like a lacquer of spray paint, and make up as much as 90% of all fine particle mass in the atmosphere. These particles influence cloud formation and therefore rainfall. They also affect human health and can lead to illnesses like asthma, heart disease and lung cancer.

But so far only about 10 to 30% of the thousands of individual compounds have been identified. Past research has focused on following specific molecules with the idea that these compounds remain relatively static once they enter the atmosphere. However, recent discoveries show that the life cycle of these compounds is much more complex, with organic molecules reacting many times in several different ways. Attempts by atmospheric scientists to track this life cycle often leave researchers with a sea of divergent paths to follow.

Through a series of worldwide field observations and lab experiments, the researchers found that organic matter ultimately tends to evolve toward a similar end, regardless of the source or where it occurs in the atmosphere. The scientists present a solution that will improve the speed and accuracy of prediction models used to understand how aerosols affect climate and human health.

The researchers focused on two key properties—volatility, or the tendency to evaporate, and the oxygen-to-carbon ratio—that evolve as aerosols make their way through the atmosphere. They used that the volatility and oxidation state of organics to build a two-dimensional (2D) modeling framework that maps the evolution of atmospheric OA.

The measurements and model reveal OA to be a highly dynamic system, tightly coupled to gas-phase oxidation chemistry. Gas-phase reactions transform OA constituents, and the OA itself is an intermediate, often forming from gas-phase precursors and ultimately returning, in part, to gas-phase products. The framework, though computationally inexpensive, allows an accurate representation of OA in regional and global climate and air-quality models used for policy assessments.

...OA is dynamic and continually evolves in the atmosphere; this evolution strongly influences the effects of particulate matter on climate and air quality. The complex evolution of OA contrasts with the simpler behavior of sulfate, which is irreversibly oxidized and condensed. Current modeling frameworks for OA are constructed in an analogous way to those for sulfate, with either no aging or one-step oxidation. Here we have presented a unifying framework describing the atmospheric evolution of OA, which is directly connected to worldwide observations and experimentally verifiable and can be used to evaluate and form the basis of practical phenomenological modeling approaches.

The combination of measurements and the modeling framework implies that most OA is an intermediate state of organic material, between primary emissions of reduced species and highly oxidized volatile products (CO and CO₂). Future models, inventories, and measurements will almost certainly need to account for the dynamic sources and sinks of OA to accurately predict regional and global OA distributions and properties and thus the associated health and climate effects.

—Jimenez et al.

Resources

• J. L. Jimenez et al. (2009) Evolution of Organic Aerosols in the Atmosphere. Science Vol. 326. no. 5959, pp. 1525 - 1529 doi: 10.1126/science.1180353

• Meinrat O. Andreae (2009) A New Look at Aging Aerosols. Science Vol. 326. no. 5959, pp. 1493 - 1494 doi: 10.1126/science.1183158