Assessment of long-term simulations with various observations for better understanding of aerosol effects on radiation “brightening” in the United States

Assessment of long-term simulations with various observations for better understanding of aerosol effects on radiation “brightening” in the United States.

 

Chuen-Meei Gan¹, Jonathan Pleim¹, Rohit Mathur¹, Christian Hogrefe¹, Charles N. Long², Jia Xing¹, David Wong¹, Robert Gilliam¹, Shawn Roselle¹ and Chao Wei¹

 

(1) Atmospheric Modeling and Analysis Division, National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, North Carolina, USA

(2) Climate Physics Group, Pacific Northwest National Laboratory, Richland, Washington, USA

Corresponding author: Chuen-Meei Gan, AMAD, NERL, US EPA (chuenmeei@gmail.com, Gan.Meei@epa.gov)

 

Multi-decadal simulations with the coupled WRF-CMAQ model have been conducted to systematically investigate the changes in anthropogenic emissions of SO₂ and NO_x (Xing et al, 2013) over the past 21 years (1990-2010) across the United States (US), their impacts on anthropogenic aerosol loading over North America, and subsequent impacts on regional radiation budgets. In particular, this study attempts to determine the consequence of the changes in tropospheric aerosol burden arising from substantial reductions in emissions of SO₂ and NO_x associated with control measures under the Clean Air Act especially on trends in solar radiation. Extensive analyses of available observations (e.g. SURFRAD, CASTNET, IMPROVE and ARM) over the past two decades (Gan et al. 2014) indicate a shortwave (SW) radiation (both all-sky and clear-sky) “brightening” over the past 16 years (1995-2010) in the US (see Figure 1 and 2). The relationship of the radiation brightening trend to decreases the aerosol burden (see Figure 3) is less apparent in the western US. Model versus observation comparisons for annual and seasonal trends of aerosol optical depth (AOD), aerosol concentration, and radiation will be discussed. Discrepancies between trends inferred from the observations and models will be presented and possible causes for these differences will be discussed.

 

 

Figure 1: All-sky SW radiation (W/m²/year) annual trend based on a 21 year coupled WRF-CMAQ simulation with observed trends from SURFRAD and ARM at seven sites overlaid in circles.

 

 

Figure 2: Clear-sky SW radiation (W/m²/year) annual trend based on a 21 year coupled WRF-CMAQ simulation with observed trends from SURFRAD and ARM at seven sites overlaid in circles.

 

Figure 3: AOD (unitless) annual trend based on a 21 year coupled WRF-CMAQ simulation with observed trends from SURFRAD and ARM at seven sites overlaid in circles.

 

 

Reference

Gan, C.-M., Pleim, J., Mathur, R., Hogrefe, C., Long, C. N., Xing, J., Roselle, S., and Wei, C.: Assessment of the effect of air pollution controls on trends in shortwave radiation over the United States from 1995 through 2010 from multiple observation networks, Atmos. Chem. Phys., 14, 1701-1715, doi:10.5194/acp-14-1701-2014, 2014.

 

Xing, J., et al. "Historical gaseous and primary aerosol emissions in the United States from 1990 to 2010." Atmospheric Chemistry and Physics 13.15 (2013): 7531-7549.