Gases from Burning Biomass React within Clouds, Forming Secondary Organic Aerosols

The Science                                 

Thousands of tiny particles in the atmosphere unseen by the naked eye scatter and absorb solar radiation and become one of the bases for the formation of clouds. A lot of these particles are not directly emitted into the atmosphere from the land or ocean surface but are formed by reactions of gases with oxidants that are in multiple phases in the atmosphere, including the gas-phase, and in water that is in the liquid form in clouds and other particles. Organic gases, such as phenols emitted in wildfires, could also be taken up into existing clouds, and they can then subsequently react with cloud water in the presence of sunlight to form secondary organic aerosols (SOA). However, a quantitative understanding of the chemistry of the formation of SOA in clouds is limited due to the complexity of their liquid-phase reactions and challenges in measurements. A multi-institutional team of 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 biomass burning 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 chemical reactions. A multi-institutional team of researchers showed that this cloud chemistry could be a major source of SOA in cloudy environments. Neglecting this process results in limiting the understanding of aerosol impacts on the Earth’s energy balance, clouds, and climate change. In addition, computationally efficient treatments of cloud chemistry are needed to appropriately represent such processes in three-dimensional models. The team 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. Their research 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, which form aqueous-phase SOA in aerosols and clouds, might be important for understanding global and regional climate modeling. So far, a quantitative and predictive understanding of the cloud chemistry of biomass-burning organic gases has been missing from such models. Using mechanisms derived from laboratory measurements, a multi-institutional team of researchers simulated the multiphase chemistry of water-soluble organic gases emitted by biomass burning, and 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 formation due to gas-phase chemistry within these layers. 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. The team’s 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.

Contacts

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

Shaima Nasiri, Atmospheric System Research Program, Shaima.Nasiri@science.doe.gov

Jeff Stehr, Atmospheric System Research Program, Jeff.stehr@science.doe.gov

Sally McFarlane, Atmospheric Radiation Measurement Program, sally.mcfarlane@science.doe.gov

Funding

This research is supported by the 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, as part of a Large-Scale Research award and the PNNL Research Computing facilities.

Publication

J. Zhang, et al. “Modeling novel aqueous particle and cloud chemistry processes of biomass burning phenols and their potential to form secondary organic aerosols.” Environmental Science and Technology 024, 58, 8, 3776–3786 (2024). [DOI: 10.1021/acs.est.3c07762]