1Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
2Division of Atmospheric Sciences, Desert Research Institute, Reno Nevada, 89512, USA
3School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138, USA
4Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523, USA
5Air Resources Laboratory, National Oceanic and Atmospheric Administration, Las Vegas, NV 89119, USA
6National Park Service, Colorado State University, Foothills Campus, Fort Collins, Colorado 80523, USA
7Crocker Nuclear Laboratory, University of California Davis, Davis, CA 95616, USA
Abstract. Observations at national parks and other remote sites show that average elemental carbon and fine particle mass concentrations in the United States both decreased by over 25 % between 1990 and 2004. Percentage decreases in elemental carbon were much larger in winter than in summer. These data suggest that emissions controls have been effective in reducing particulate concentrations not only in polluted areas but also across the United States. Despite the reduction in elemental carbon, the simultaneous decrease in non-absorbing particles implies that the overall radiative forcing from these changes was toward warming. The use of a 2005 instead of 1990 as a baseline for climate-relevant emissions from the United States would imply a significantly lower baseline for aerosol emissions. The use of older data will overestimate the possibility for future reductions in warming due to black carbon controls.