Research / Research Highlights

Nonequilibrium behavior in isoprene secondary organic aerosol

Submitter

Fast, Jerome D — Pacific Northwest National Laboratory

Area of Research

Aerosol Properties

Journal Reference

Chen Y, R Zaveri, G Vandergrift, Z Cheng, S China, A Zelenyuk, and J Shilling. 2023. "Nonequilibrium Behavior in Isoprene Secondary Organic Aerosol." Environmental Science & Technology, 57(38), 10.1021/acs.est.3c03532.

Science

Figure 1. Experiments in the Pacific Northwest National Laboratory (PNNL) Environmental Chamber found that fresh organic vapors established equilibrium with fresh organic particles but were unable to equilibrate with the same particles after they were aged for as little as 20 minutes. PNNL Image.

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.

Impact

This new study shows that the way most large-scale models predict organic aerosol concentration and size is not accurate.

Summary

Our 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 non-equilibrium partitioning behavior and inferred semi-solid phase state were corroborated by offline mass spectrometric analysis on the bulk aerosol particles showing 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 climate-relevant sizes.