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Long-term data sets of all-sky and clear-sky downwelling shortwave (SW)radiation, cloud cover fraction, and aerosol optical depth (AOD) wereanalyzed together with surface concentrations from several networks (e.g.,Surface Radiation Budget Network (SURFRAD), Clean Air Status and TrendNetwork (CASTNET), Interagency Monitoring of Protection Visual Environments(IMPROVE) and Atmospheric Radiation Measurement (ARM)) in the United States(US). Seven states with varying climatology were selected to betterunderstand the effects of aerosols and clouds on SW radiation. This analysisaims to assess the effects of reductions in anthropogenic aerosol burdenresulting from substantial reductions in emissions of sulfur dioxide(SO2) and nitrogen oxides (NOx) over the past 16 yr acrossthe US, based on trends in SW radiation. The SO2 and NOxemission data show decreasing trends from 1995 to 2010, which indirectlyvalidates the effects of the Clean Air Act (CAA) in the US. Meanwhile, thetotal column AOD and surface total PM2.5 observations also showdecreasing trends in the eastern US but slightly increasing trends in thewestern US. Moreover, measured surface concentrations of several otherpollutants (i.e., SO2, SO4 and NOx) have similarbehavior to AOD and total PM2.5. Analysis of the observed data showsstrong increasing trends in all-sky downwelling SW radiation with decreasingtrends in cloud cover. However, since observations of both all-sky direct anddiffuse SW radiation show increasing trends, there may be other factorscontributing to the radiation trends in addition to the decreasing trends inoverall cloud cover. To investigate the role of direct radiative effects ofaerosols, clear-sky downwelling radiation is analyzed so that cloud effectsare eliminated. However, similar increasing trends in clear-sky total anddiffuse SW radiation are observed. While significantly decreasing trends inAOD and surface PM2.5 concentrations along with increasing SW radiation(both all-sky and clear-sky) in the eastern US during 1995–2010 imply theoccurrence of direct aerosol mediated "brightening", the increasing trendsof both all-sky and clear-sky diffuse SW radiation contradicts thisconclusion since diffuse radiation would be expected to decrease as aerosolsdirect effects decrease and cloud cover decreases. After investigatingseveral confounding factors, the increasing trend in clear-sky diffuse SW maybe due to more high-level cirrus from increasing air traffic over the US. Theclear-sky radiation observations in the western US also show indications of"brightening" even though the AOD, PM2.5 and surface concentration donot vary drastically. This outcome was not unexpected because the CAAcontrols were mainly aimed at reducing air pollutant emissions in the easternUS and air pollutant levels in the western US were much lower at the onset.This suggests other factors affect the "brightening" especially in thewestern US

atmospheric chemistry and physics

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