Diffusion in large organic aerosols influences growth of ultrafine atmospheric particles
Fast, Jerome D — Pacific Northwest National Laboratory
Area of research:
The dynamics of how aerosol particles grow and their size evolves, along with how they affect Earth’s climate, are poorly understood. New research on secondary organic aerosols formed from oxidation of isoprene—a volatile organic compound released from many plants and trees—provided quantitative insights into how slow diffusion inside viscous organic particles affects their growth kinetics. A team led by researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory discovered that bulk diffusion limits semivolatile organic compounds seeping into large viscous organic particles, effectively promoting the growth of smaller particles that have shorter diffusion timescales.
Physicochemical processes governing secondary organic aerosol formation are more complex than atmospheric models currently represent. The results of this study will enable more accurate predictions of secondary organic aerosol formation by including a better representation of the growth of ultrafine aerosol particles to climatically active sizes in atmospheric models. This will improve simulations of how aerosols affect Earth’s energy balance.
Secondary organic aerosols in the atmosphere are produced when oxidation products from volatile organic compounds condense from the gas phase to the particle phase. These secondary organic aerosols constitute a major fraction of the submicron aerosols in Earth’s atmosphere, and they play a crucial role in the growth of ultrafine particles to sizes larger than about 80 nanometers. At this size, the particles begin to efficiently scatter light and can activate as cloud condensation nuclei. Under dry to moderate relative humidity, secondary organic aerosols can be highly viscous such that slow diffusion of condensing compounds inside these semisolid particles can prolong the time it takes for the gas and particles to equilibrate. Researchers investigated the effects of low bulk diffusivity on the growth kinetics of secondary organic aerosols formed from photooxidation of isoprene in PNNL’s environmental chamber. Mass spectrometric analysis was performed using two instruments in the Environmental Molecular Sciences Laboratory, an Office of Science user facility at PNNL.
The researchers found that isoprene secondary organic aerosols formed from condensation of several semivolatile organic compounds, some of which reversibly react to form high-molecular-weight compounds called oligomers. Model analysis revealed that low diffusivity of semivolatile organic compounds into large viscous secondary organic aerosol particles is responsible for the observed rapid growth of the smaller particles that are less viscous and have shorter diffusion timescales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically active particles via secondary organic aerosol formation under dry to moderately humid conditions.