Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO 80521, USA

now at: Complex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA

Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, MD 20771, USA

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80407, USA

The USDA UV-B Monitoring and Research Program, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80521, USA

Biospheric Sciences Branch, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA

The USDA UV-B Monitoring and Research Program, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80521, USA

Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Rd, Houston, TX 77204-5007, USA

Biospheric Sciences Branch, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA

Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO 80521, USA