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Atmospher Sci & Global Chg
Research Highlights

August 2011

Following the Aerosol Trail

A simplified method to accurately show pollution particles in climate models

Mexico March 2006
A heavy layer of air pollution, a mix of aerosol particles and vapors, obscures the view over Mexico City. The Megacity Initiative: Local and Global Research Observations, or MILAGRO, conducted during March 2006, provided observational data used to study the evolution of primary and secondary organic aerosols. Enlarge Image
VBS Chart
Model results showing very similar 24-day averaged secondary organic aerosol predicted over the Mexico City area using (a) Complex 9-species volatility basis set (VBS) and (b) Simpler 2-species VBS. Enlarge Image

Results: So elusive, tiny aerosol particles in the atmosphere are difficult to follow. Led by scientists at Pacific Northwest National Laboratory, a research team for the first time developed a simplified and computationally efficient way to represent these carbon-based bits in a climate model. They tracked the atmospheric lifecycle of these volatile particles for real-time climate forecasting. Their results show that previous models severely underestimate the presence of these aerosol particles. The results were published in Atmospheric Chemistry and Physics.

Why it matters: Aerosols play a large role in our climate system. The complexity of various physical and chemical processes in the atmosphere makes it very difficult to identify sources of these carbon-containing aerosols. Yet these particles, called secondary organic aerosols, are a dominant atmospheric component in most megacity locations. Scientists employed methods developed in this study to tag each source of aerosol, such as fossil-fuel burning from vehicles and power plants, or biomass burning, and follow its path in the model. Tracking and gathering detailed source-specific information about aerosols is important for regulatory decisions for cleaner air and water. These results can also point to sources of climate change caused by natural and human-caused activities.

Methods: The research team set out to incorporate a detailed physical and chemical mechanism for organic aerosols, called the "volatility basis set framework," in a familiar community climate model. Then they tested the model results using the comprehensive ground and aircraft measurements gathered during the 2006 Megacity Initiative: Local and Global Research Observations in Mexico City.

The research team evaluated model predictions using a detailed 9-species VBS framework and a computationally efficient 2-species VBS framework. The team showed that human activities common in large cities, such as vehicle exhaust and municipal trash burning, are major sources of organic aerosol pollution. On high biomass burning days, both biomass burning and human-based activities equally contributed to levels of secondary organic aerosol downwind of Mexico City. The team also found that the more efficient 2-species VBS framework, which they developed for the first time in this study, predicted very similar results to the detailed 9-species VBS framework both at ground-sampling locations and across several aircraft sampling flight paths in the MILAGRO campaign. In addition, their results show that the 2-species VBS framework is much more computationally efficient, for real-time forecasting.

This study used tools developed at PNNL in combination with data from the MILAGRO field campaign to test and validate the models. The Weather and Research Forecasting (WRF-Chem) model is a community meteorology and chemistry model and was used to simulate the atmospheric conditions within the Mexico City domain. The Aerosol Modeling Testbed toolkit developed at PNNL was used to evaluate WRF-Chem predictions against field measurements.

What's next: Scientists are now working on using new field and laboratory measurements to better understand the physical and chemical processes controlling the formation and atmospheric lifecycle of secondary organic aerosols. The team is also using their 2-species VBS framework to study organic aerosols and cloud-aerosol interactions for other climate science field campaigns.

Acknowledgments: This research was funded by the U.S. Department of Energy Atmospheric Sciences Program. The work was performed by Drs. Manishkumar B. Shrivastava, Jerome D. Fast, Richard C. Easter, William I. Gustafson Jr., and Rahul A. Zaveri of PNNL; Jose L. Jimenez of the University of Colorado; Pablo Saide of the Center for Global and Regional Environmental Research at the University of Iowa; and, Alma Hodzic of the National Center for Atmospheric Research.

Reference: Shrivastava M, JD Fast, RC Easter, WI Gustafson Jr., RA Zaveri, JL Jimenez, P Saide and A Hodzic. 2011. "Modeling Organic Aerosols in a Megacity: Comparison of Simple and Complex Representations of the Volatility Basis Set Approach," Atmospheric Chemistry and Physics 11, 6639-6662. DOI:10.5194/acp-11-6639-2011.

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What are aerosols?

Aerosols are tiny bits of pollution particles dispersed in the air that play an important role in our climate. They can absorb or scatter sunlight, act as cloud droplet seeds, and take part in complex chemical reactions in the atmosphere. The largest portion of these submicron aerosols is organic, or carbon-containing, and is classified as two kinds: primary and secondary. Primary organic aerosols are directly given off from combustion systems like exhaust from vehicles, or biomass burning, such as cooking and forest fires. Like rust forms on iron, secondary organic aerosols are formed in the atmosphere when emitted organic vapors react with sunlight, a process called photochemical oxidation. Aerosols are one of the largest areas of uncertainty in understanding the climate system.