PNNL

The Science

Secondary organic aerosol (SOA) particles are particles in the atmosphere that affect air quality, visibility, human health, clouds, and radiation. Unseen by the naked eye, large quantities of carbon-containing vapors enter the atmosphere as they escape from trees, fossil-fuel burning, and forest fires. The atmosphere acts as a large chemical reactor, "cooking" these emissions to form millions of new carbon-containing molecules. Some of these molecules then condense into SOA particles that can change clouds, precipitation, and the amount of solar energy reaching the Earth. 

SOA particles are extremely complex: they comprise thousands of organic molecules, many of which have not been explicitly identified. Models use simplified SOA yields derived by fitting laboratory  environmental chamber measurements. However, these yields are theory-specific and depend on dynamic SOA processes, which are not routinely included when these yields are derived from  environmental chamber measurements. 

Researchers at the U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL) led the development of a new modeling approach to derive isoprene SOA yields while explicitly including complex nitrogen-oxides-dependent multigenerational chemistry processes of SOA precursors and the losses of SOA vapors and particles to the chamber walls, which were represented by a few tunable parameters. 

Isoprene is the dominant volatile organic carbon emitted from trees on a global basis, and is ideal for studying SOA formation due to its shorter carbon-chain length and simpler SOA chemistry than longer carbon-chain hydrocarbons. The study showed that SOA yields are highly sensitive to processes that are included during fitting of these yields. The findings from this study help fill in important details of SOA formation processes that are still not fully understood.

The Impact

This modeling approach is an important step in parameterizing complex, dynamic SOA chemistry processes based on laboratory measurements for use in three-dimensional chemical transport models in a self-consistent way. Ultimately, this research will improve the representation of complex aerosol processes using empirical approaches within atmospheric chemical transport models, and increase confidence in the models' ability to represent aerosol-cloud-radiation interactions.

Reference: L. Xing, M. Shrivastava, T.-M. Fu, P. Roldin, Y. Qian, L. Xu, N.L. Ng, J. Shilling, A. Zelenyuk, C.D. Cappa, "Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NOx Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases." Environmental Science & Technology 52(16):9225-9234 (2018). [DOI: 10.1021/acs.est.8b00373]