The Los Angeles region has some of the most polluted air in the United States, failing to meet standards set by the Environmental Protection Agency for the last decade. Now, Caltech researchers have quantified the levels of a component of smog called ammonium nitrate, a molecule that has been notoriously difficult to measure, and have found that there is much more of it than previously calculated, especially on the most polluted days. The findings emphasize the need for the continued reduction