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Biogenic emissions from trees, vegetation, and grasses are volatile compounds that mix with other compounds in the atmosphere to form particles that can influence the formation of clouds. They also can block, reflect or absorb sunlight energy, affecting the overall energy budget of the planet. Understanding how BVOCs interact with other atmospheric components is one of the biggest challenges to unlocking the mysteries of our climate. Image courtesy of Jerome Fast, PNNL. Enlarge Image.

Results: Our atmosphere—and our climate—are challenged by chemistry. Biogenic organic compounds, or BVOCs, are part of the chemical atmospheric soup that confronts scientists.

Tackling this mystery, researchers at Pacific Northwest National Laboratory and their collaborators used a "mash-up" of several models to simulate BVOCs and compare them to real-life observations. The coupled system reasonably simulated the BVOCs, and using sensitivity experiments, they showed that land-surface formulas in the models do influence how BVOCs are simulated, but the impact is much smaller than the influence of vegetation distribution. (See sidebar, What are BVOCs?)

"Inappropriate vegetation distribution data in climate models may result in large uncertainties when estimating BVOCs," said Dr. Chun Zhao, atmospheric scientist at PNNL. "This can significantly affect our understanding of the atmospheric chemistry and secondary aerosol formation that ultimately influences how these particles affect clouds and Earth's energy budget."

Why It Matters: Current climate models are still struggling to estimate BVOCs. These wily molecular compounds can significantly affect the chemistry in the atmosphere and the consequences are likely significant. Researchers lack complete information about what happens to these compounds when they meet up with human-caused emissions and other natural particles (like dust or ash) and form new particles called secondary organic aerosols (SOAs). The formation of these new, "third-party" particles ultimately influences air quality and the particles' light-reflective talents. The study provides a coupled modeling system with a solid step toward capturing the impact of these cagey compounds.

Methods: In this study, PNNL researchers and their collaborators from the University of California, National Center for Atmospheric Research, Utah State University, NOAA and University of Colorado linked several models to tackle questions around BVOCs. They used the latest version of Model of Emissions of Gases and Aerosols from Nature (MEGAN v2.1) and coupled it with the land-surface model called Community Land Model, or CLM4, in the Weather Research and Forecasting model with chemistry (WRF-Chem). In this implementation, it was helpful that MEGAN v2.1 shares a consistent vegetation map with CLM4 for estimating BVOC emissions. They used this improved modeling framework to investigate the impact of two land-surface models, CLM4 and Noah, on BVOCs and studied how BVOCs reacted to vegetation distributions in California. The measurements collected during the Carbonaceous Aerosol and Radiative Effects Study (CARES) and the California Nexus of Air Quality and Climate Experiment (CalNex) conducted in June of 2010 provided observational sets used to evaluate the simulations of BVOCs.

Uncertainties arose from many factors, including uncertainties in land-surface processes and specification of vegetation types, both of which can affect the simulated near-surface fluxes of BVOCs. However, the results quantified the impacts from land-surface processes and vegetation distributions on the estimated BVOCs in the atmosphere. This study indicates that more effort is needed to obtain appropriate land-cover datasets for climate models rather than improve details on land processes in terms of simulating BVOCs and consequently SOA formation.

"The new modeling framework accounting for the sub-grid variability of vegetation distributions can better capture the BVOC distributions in cities, which is important in studying air pollutants," said Zhao.

The study shows there is more work to do. They need to obtain the most appropriate and accurate land-cover datasets for climate and air quality models that impact BVOCs, pick apart the way chemicals age in the atmosphere, and identify the consequent new particles produced by atmospheric mixing.

What's Next? The newly developed modeling system will be used to investigate the BVOCs impact on SOA formation and radiative forcing. The system will also be applied over other regions such as the Amazon to investigate land processes and the vegetation distribution impact on BVOCs, and consequently, SOA formation.

Acknowledgments

Sponsors: This was work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research's Atmospheric Systems Research (ASR) Program and Atmospheric Radiation Measurement (ARM) Climate Research Facility. A portion of this research was supported by the US NOAA's Atmospheric Composition and Climate Program.

Research Team: Chun Zhao, Maoyi Huang, Jerome D. Fast, Larry K. Berg, Yun Qian, Manish Shrivastava, Ying Liu, and John E. Shilling, PNNL; Alex Guenther, and Dasa Gu, University of California, Irvine; Stacy Walters, and Gabriel Pfister, National Center for Atmospheric Research; Jiming Jin, Utah State University; and Carsten Warneke, National Oceanic and Atmospheric Administration and CIRES at University of Colorado.

Research Area: Climate and Earth Systems Science

User Facility: ARM Climate Research Facility. The simulations required for this work were performed on the National Energy Research Scientific Computing Center, supported by the Office of Science of the U.S. Department of Energy.

Reference: Zhao C, M Huang, JD Fast, LK Berg, Y Qian, A Guenther, D Gu, M Shrivastava, Y Liu, S Walters, G Pfister, J Jin, JE Shilling, and C Warneke. 2016. "Sensitivity of Biogenic Volatile Organic Compounds (BVOCs) to Land Surface Parameterizations and Vegetation Distributions in California." Geoscientific Model Development 9: 1959-1976. DOI: 10.5194/gmd-9-1959-2016