Long term observation data analysis of trend in radiation “brightening” in United States during 1995–2010

In this study, we assess long term (approximately 10-15 years depending on the station) observations of total downwelling shortwave (SW) radiation, downwelling clear-sky SW radiation, cloud cover fraction, diffuse ratio, and aerosol optical depth (AOD) together with aerosol concentration from several networks (e.g. SURFRAD, CASTNET, IMPROVE and ARM) in the United States (US). Observations from seven regions (e.g. Illinois, Montana, Mississippi, Colorado, Pennsylvania, Nevada and Oklahoma) with different climatology are selected in this assessment to better understand the spatial and temporal distributions of aerosols and clouds in conjunction with their direct, semi-direct, and indirect effects. This analysis aims to test the hypothesis that the reductions in anthropogenic aerosol burden resulting from substantial reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) associated with Title IV of the Clean Air Act (CAA) has caused an increase in SW radiation near surface. First, we show that the total downwelling SW radiation and clear-sky SW radiation from seven stations across the U.S. have increasing trends except at Penn State, PA (PSU) which shows a decreasing clear-sky SW radiation trend. After investigating several confounding factors such as geography and aerosol spatial distribution (vertically and horizontally), the apparent dimming at PSU can be explained by the site location, heavy air traffic, increasing cloud cover fraction, and aerosol and dust plume events. Moreover, we assess the relationship between total column AOD with surface aerosol concentration to test our hypothesis. In our findings, the trends of SW radiation, AOD, and aerosol concentration from four of the seven regions agree well with our hypothesis that SW is increasing while AOD and surface aerosol concentrations are declining. However, there are three regions (e.g. Table Mountain, CO; Fort Peck Montana and Desert Rock, NV) that show increasing AOD associated with mostly increasing trends in aerosol concentration at surface. Note that these three western sites are often influenced by forest fire and long distance dust transportation events. At these sites, the changes in aerosol burden and / or direct aerosol effects alone cannot explain the observed changes in surface SW radiation, but other factors also need to be considered such as cloud cover, aerosol vertical profiles and elevated plumes.