Complex sentence: Previously, he was the editor of the "Bulletin of the Atomic Scientists", winner of the 2007 National Magazine Award for General Excellence (in the under 100,000 circulation category) awarded by the American Society of Magazine Editors.
Split into simpler sentences: Previously, he was the editor of the "Bulletin of the Atomic Scientists". As editor of the Bulletin, he was awarded the 2007 National Magazine Award for General Excellence (in the under 100,000 circulation category) by the American Society of Magazine Editors.