PNNL

The Science

Thousands of tiny particles in the atmosphere unseen by the naked eye scatter and absorb solar radiation and seed clouds. A lot of these particles are not directly emitted but are formed by reactions of gases with atmospheric oxidants in multiple phases, including gas-phase and liquid water contained in clouds and other particles. Organic gases, such as phenols emitted in wildfires, could also be taken up by clouds and subsequently react with cloud water in the presence of sunlight-forming secondary organic aerosols (SOA). However, a quantitative understanding of cloud chemistry-forming SOA is limited due to the complexity of their liquid-phase reactions and challenges in measurements. Researchers developed a stand-alone box model to predict aqueous and cloud chemistry of biomass-burning phenols based on laboratory measurements. They found that in cloudy environments, the cloud chemistry of soluble and reactive organic gases like phenols is likely a dominant source of SOA, which is overlooked in climate models.

The Impact

As biomass-burning emissions with organic gases and aerosols are transported in the atmosphere, they encounter clouds and undergo aqueous chemistry. Researchers showed that this cloud chemistry could be a major source of SOA in cloudy environments. Neglecting this process has limited our understanding of aerosol impacts on the Earth’s energy balance, clouds, and climate change. In addition, computationally efficient treatments of cloud chemistry are needed for representing it in three-dimensional models. Researchers parameterized and optimized this complex chemistry so that it is amenable for representation in regional and global Earth system models. In the future, they will include cloud chemistry of biomass-burning organic gases in Earth system models, which is expected to fill a major gap in the process understanding of SOA and its impacts on clouds and the Earth’s radiation budget.

Summary

Wildfires continue to increase in frequency and severity, transporting smoke for thousands of miles. The smoke contains soluble organic gases. The reactions of these gases that form aqueous-phase SOA in aerosols and clouds might be important. So far, a quantitative and predictive understanding of the cloud chemistry of biomass-burning organic gases has been missing. Using mechanisms derived from laboratory measurements, researchers simulated the multiphase chemistry of water-soluble organic gases emitted by biomass burning that includes their dissolution within aerosol and cloud liquid water followed by their aqueous-phase reactions to form SOA. Within cloud layers, they found that highly soluble and reactive multifunctional phenols are almost entirely dissolved and react in cloud water, causing SOA formation that greatly exceeds their previously known gas-phase chemistry. Even near the surface in the absence of clouds, these highly soluble phenols form significant amounts of SOA in aqueous aerosols because they can be easily dissolved in aerosol liquid water and are available for aqueous aerosol chemistry. Their research is expected to open new frontiers in the understanding of how cloud chemistry increases SOA loadings in the atmosphere and its ability to seed clouds and to scatter and absorb the sun’s radiation. In addition, this work will aid in the design of laboratory-based cloud chamber measurements that aim to understand how SOA cloud chemistry causes detectable changes in cloud droplet residuals and aerosols that serve as cloud condensation nuclei.

Computational resources provided by the Environmental Molecular Sciences Laboratory and Pacific Northwest National Laboratory (PNNL) Research Computing.

PNNL Contact

Manish Shrivastava, Pacific Northwest National Laboratory, manishkumar.shrivastava@pnnl.gov, corresponding author

Funding

This research is primarily supported by the U.S. Department of Energy (DOE), Office of Science, Atmospheric System Research project and the DOE Office of Science, Biological and Environmental Research program, Early Career Research Program at PNNL. 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 located at PNNL) and the PNNL Research Computing facilities.