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Long-term simulations with the coupled WRF–CMAQ (Weather Researchand Forecasting–Community Multi-scale Air Quality) model have been conducted tosystematically investigate the changes in anthropogenic emissions of SO2and NOx over the past 16 years (1995–2010) across the United States(US), their impacts on anthropogenic aerosol loading over North America, andsubsequent impacts on regional radiation budgets. In particular, this studyattempts to determine the consequences of the changes in tropospheric aerosolburden arising from substantial reductions in emissions of SO2 andNOx associated with control measures under the Clean Air Act (CAA)especially on trends in solar radiation. Extensive analyses conducted by Ganet al. (2014a) utilizing observations (e.g., SURFRAD, CASTNET, IMPROVE, andARM) over the past 16 years (1995–2010) indicate a shortwave (SW) radiation(both all-sky and clear-sky) "brightening" in the US. The relationship ofthe radiation brightening trend with decreases in the aerosol burden is lessapparent in the western US. One of the main reasons for this is that theemission controls under the CAA were aimed primarily at reducing pollutantsin areas violating national air quality standards, most of which were locatedin the eastern US, while the relatively less populated areas in the western USwere less polluted at the beginning of this study period. Comparisons ofmodel results with observations of aerosol optical depth (AOD), aerosolconcentration, and radiation demonstrate that the coupled WRF–CMAQ model iscapable of replicating the trends well even though it tends to underestimatethe AOD. In particular, the sulfate concentration predictions were wellmatched with the observations. The discrepancies found in the clear-skydiffuse SW radiation are likely due to several factors such as the potentialincrease of ice particles associated with increasing air traffic, thedefinition of "clear-sky" in the radiation retrieval methodology, andaerosol semi-direct and/or indirect effects which cannot be readily isolatedfrom the observed data

atmospheric chemistry and physics

Assessment of long-term WRF–CMAQ simulations for understanding direct aerosol effects on radiation &amp;quot;brightening&amp;quot; in the United States