PNNL

The Science

Atmospheric aerosol particles modulate climate and the Earth’s energy balance by scattering and absorbing sunlight. They also seed clouds, acting as cloud condensation nuclei (CCN). A key challenge in atmospheric modeling is deciphering how these aerosol particles are initially formed, known as new particle formation. NPF from nucleation of molecular clusters and their subsequent growth is reported to contribute to at least half of the global CCN budget. In forested regions around the world, the atmospheric oxidation of biogenic organic gases emitted by plant foliage are key drivers of NPF. However, in non-forested areas influenced by croplands and urban sources, the processes driving NPF and CCN are not well understood. Based on a test site in the Southern Great Plains (SGP) using measurements from the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) field campaign, this study found that dimethylamines and sulfuric acid are key nucleation drivers in the springtime, but condensation of sulfuric acid alone is not sufficient to explain the observed growth of the molecular clusters. Extremely low volatility organics (ELVOCs) formed by the oxidation of anthropogenic volatile organic compounds (AVOCs) emitted by human sources, such as vehicles, combustion for energy production, and non-combustion volatile chemical products, are critical for explaining the observed particle growth at the SGP site. Researchers also show that clouds might suppress new particle formation at SGP by reducing photochemical activity, thereby reducing production of sulfuric acid and ELVOC gases needed for NPF. Their results imply that cloud and NPF processes are inherently coupled and could affect climate change in greater number of ways than we currently understand.

The Impact

The theory for atmospheric oxidations of AVOCs forming ELVOCs is documented in the literature, but data detailing the chemical and kinetic pathways for their formation are sparse. Atmospheric models have therefore excluded the formation of anthropogenic ELVOCs, resulting in limited ability to predict NPF in urban areas. This study incorporated the formation of ELVOCs by oxidation of AVOCs in the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) for the first time. It showed that ELVOCs are critical for explaining the measured particle growth approaching CCN sizes at the test site, SGP, which is mainly influenced by croplands and urban sources in the springtime when natural biogenic activity is low. With increased emissions controls reducing inorganic particle species, like sulfate and nitrate, the role of anthropogenic ELVOCs on NPF and CCN is expected to increase. This work highlights the critical role of including these missing anthropogenic sources of low volatility organic gases, NPF, and CCN in Earth system models to enable a better predictive understanding of aerosol-cloud-radiation interactions.

Summary

In forested regions around the world, biogenic emissions have been reported to be key drivers of NPF and CCN. However, at locations in the Midwest USA, far from forests and influenced by croplands and urban emissions, the processes driving NPF and CCN are not well understood. Using detailed WRF-Chem regional model simulations, researchers find that dimethylamines and sulfuric acid are key nucleation drivers at the SGP site during two simulated days in the springtime. Their results also show that anthropogenic ELVOCs are critical for explaining the observed particle growth. Treating organic particles as semisolid with strong diffusion limitations for organic vapor uptake in the particle phase brings model predictions into closer agreement with observations. Conversely, simulated non-NPF days at SGP are characterized by low-level clouds, which reduce photochemical activity, sulfuric acid, and ELVOC concentrations, thereby explaining the lack of NPF. At the Bankhead National Forest site in southeast USA, researchers show that nucleation rates are limited by availability of sulfuric acid in this forested area. This study highlights the large potential heterogeneities in nucleation and particle growth mechanisms between forested and urban/farmland-influenced areas.

Contacts

Manish Shrivastava, lead and corresponding author, manishkumar.shrivastava@pnnl.gov

Jerome Fast, ICLASS principal investigator, Jerome.fast@pnnl.gov 

Funding

This research is primarily supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research’s (BER) Atmospheric System Research and Early Career Research programs. Computational resources for the simulations were provided by the Environmental Molecular Sciences Laboratory (a DOE Office of Science user facility sponsored by the Biological and Environmental Research program and located at Pacific Northwest National Laboratory), the PNNL research computing facilities, and the National Energy Research Scientific Computing Center—a DOE Office of Science user facility. Funding for data collection onboard the G-1 aircraft and at the ground sites was provided by the Atmospheric Radiation Measurement Climate Research Facility, a DOE user facility sponsored by BER.