Implications of the Nonequilibrium Behavior of Isoprene Secondary Organic Aerosol on Cloud Formation

The Science 

Atmospheric aerosols play a significant role in Earth’s climate. Understanding the formation of organic particles in the atmosphere is key to understanding cloud properties and formation, and as a result, unraveling future climate change. Isoprene, an organic compound, is produced by many plants and has a large impact on atmospheric chemistry and composition. As described in an earlier research project, the ways isoprene converts to a secondary organic aerosol (SOA) and how anthropogenic, or man-made, pollutants affect this process are researched extensively because it affects Earth’s climate and local air quality. Typically, atmospheric models assume that fresh organic material, generated as a result of chemical reactions in the atmosphere, can dissolve in the existing organic particulate matter. This study revealed that fresh organic vapors are indeed soluble in particulate organics that are actively growing in size. However, if growth halts and the particulate matter ages, fresh organic vapors can no longer mix with the particulate organic matter.  

The Impact 

This new study shows that the way most large-scale models predict organic aerosol concentration and size is not accurate. The unexpectedly short aging timescale necessary for observing non-mixing behavior that was identified during this research will impact model predictions of aerosol particle size distributions. Most atmospheric models will need to be updated to include this mixing limitation. The observed behavior will cause small particles to grow faster and large particles to grow slower than would be otherwise predicted. This will allow more small particles to grow to sizes capable of forming cloud droplets and, thus, impact cloud formation and precipitation and ultimately predictions of climate change.  

Summary 

Previous studies have demonstrated that a rapid segregation of fresh and aged organic particulate matter, derived from tree emissions, has a significant impact on particle growth. However, recent studies have shown that instantaneous gas-particle equilibrium partitioning assumptions fail to predict SOA formation, even at high relative humidity (~85 percent). Photochemical aging seems to be one driving factor. A new study, conducted by researchers from Pacific Northwest National Laboratory (PNNL), examined the minimum aging timescale required to observe nonequilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas- and aerosol-phase at ~50 percent relative humidity. Seed isoprene SOA was generated by photo-oxidation in the presence of effloresced ammonium sulfate seeds at < 1 ppbv NOx, aged photochemically or in the dark for 0.3–6 hours, and subsequently exposed to fresh isoprene SVOCs.  

The team’s results show that the equilibrium partitioning assumption is accurate for fresh isoprene SOA but breaks down after isoprene SOA has been aged for as short as 20 minutes, even in the dark. Modeling results showed that a semi-solid SOA phase state was necessary to reproduce the observed particle size distribution evolution. The observed nonequilibrium partitioning behavior and inferred semi-solid phase state were corroborated by off-line mass spectrometric analysis on the bulk aerosol particles and confirmed the formation of organosulfates and oligomers. The unexpectedly short timescale for the phase transition within isoprene SOA has important implications for the growth of atmospheric ultrafine particles to sizes relevant for cloud condensation nuclei formation, which could impact Earth’s radiative balance. The research team believes the next step is to design experiments that test whether SOA from mixtures of anthropogenic and biogenic volatile organic compounds behaves in a similar manner. 

Contacts 

Jerome Fast, PNNL, Jerome.fast@pnnl.gov 

John Shilling, PNNL  john.shilling@pnnl.gov 

Yuzhi Chen, PNNL, Yuzhi.chen@pnnl.gov 

Funding 

This research was supported by the Atmospheric System Research program as part of the Department of Energy (DOE), Office of Science, Biological and Environmental Research program under a PNNL project. Nanospray Desorption Electrospray Ionization Mass Spectrometry analysis was performed on a Large-Scale Research award from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Biological and Environmental Research program. 

Publication 

Y. Chen, et al. “Nonequilibrium Behavior in Isoprene Secondary Organic Aerosol.” Environmental Science & Technology, 57, 38, 14182–14193 (2023). [DOI: 10.1021/acs.est.3c03532]