29 August 2013

Atmospheric Bit Players Take Stage

Science

On the atmospheric stage, aerosols are bit players. Identifying each one and their role as change agents was the goal of researchers from Pacific Northwest National Laboratory, Colorado State University and the California Air Resources Board. For the first time, the researchers identified individual types of particles and their relative contribution to influence seasonal warming and cooling effects at the top of the atmosphere over California. Overall, carbon particles contributed up to 95 percent of the total warming throughout the seasons.

Approach

Using the Weather Research and Forecasting model with coupled chemistry (WRF-Chem), the research team showed the model's capability to diagnose the direct radiative forcing of individual aerosol species. They ran the WRF-Chem simulations at a relatively high horizontal resolution of 12 kilometers covering California. The team evaluated model simulations with various datasets of meteorological and aerosol field measurements during 2005 and 2008.

The research team distinguished each particle type by chemical make-up and identified the amount of warming or cooling each particle is responsible for arranged by season and spatial distribution in the atmosphere. Overall, at the top of the atmosphere (TOA), the total aerosol direct radiative effect through all seasons was cooling with sulfate as the largest contributor in winter and summer. Elemental carbon aerosols and dust combined contributed to a TOA warming effect.

Impact

Tiny atmospheric particles such as dust, chemicals and compounds are released from fossil fuel burning and natural sources. These particles can affect visibility, human health and the climate. With a better understanding of how each particle affects the atmosphere, scientists can help assess the success of regional emissions controls of human-caused particles. Of concern in this research was how specific particles directly affect the energy balance over California. The team's use of a popular meteorological-chemistry model to perform this baseline assessment promises to improve understanding of how regional controls limit pollution and other emissions will impact the water cycles and the climate.

Summary

Characterization of speciated aerosol direct radiative forcing over California
Understanding the impact of aerosols and emission control on a California regional climate

A research team led by DOE scientists from Pacific Northwest National Laboratory characterized the spatial and seasonal distribution of speciated aerosol direct radiative forcing over California using a fully coupled meteorology-chemistry model, WRF-Chem. While diagnosing the direct radiative forcing of individual aerosol species, they found that the simulation in 2005 reproduced the observed spatial and seasonal distribution of total PM2.5 mass concentration and the relative contribution from individual aerosol species. Over California on a statewide average, all aerosol species reduced the surface net radiation fluxes in total by about 1.5 W m-2 winter minimum to 3 W m-2 summer maximum. Elemental carbon (EC) was the largest contributor in summer, and sulfate was the largest in winter. In the atmosphere, total aerosol introduced a warming effect from a winter minimum of 0.5 W m-2, to a summer maximum of 2 W m-2. EC and dust contributed about 75-95% and 1-10% of the total warming through the seasons, respectively. At the top of atmosphere (TOA), the overall total aerosol direct radiative effect is cooling through the seasons with sulfate as the biggest contributor of -0.4 W m-2 in winter to -0.7 W m-2 in summer. EC produced a TOA warming of up to about 0.7 W m-2, while all other aerosol species produced a TOA cooling. The researchers found that the diagnostic method implemented in WRF-Chem can be applied to other regions to understand the roles of different aerosols on the direct radiative forcing and regional climate.

Reference: Zhao C, LR Leung, R Easter, J Hand, J Avise. 2013. “Characterization of speciated aerosol direct radiative forcing over California.” Journal of Geophysical Research. DOI:10.1029/2012JD018364. In press.

Funding: California Air Resources Board (CARB); and the U.S. Department of Energy (DOE) Regional and Global Climate Modeling Program. Computing resources from the National Energy Research Scientific Computing Center (NERSCC), supported by the U.S. Department of Energy Office of Science.

Contact: Chun Zhao, (509) 371-6372, chun.zhao@pnnl.gov

Contact
Siyu Chen
Acknowledgments

This work was supported by the California Air Resources Board (CARB) and the U.S. Department of Energy's (DOE's) Office of Science Regional and Global Climate Modeling Program. The study used computing resources from the National Energy Research Scientific Computing Center (NERSC) supported by the U.S. Department of Energy Office of Science, and PNNL's Institutional Computing.

Research Team: Chun Zhao, L. Ruby Leung and Richard C. Easter at PNNL; Jenny Hand at Colorado State University; and Jeremy Avise from the California Air Resources Board.