Poorly quantified trends in ammonium nitrate remain critical to understand future urban aerosol control strategies.

Despite decades of progress in reducing nitrogen oxide (NO) emissions, ammonium nitrate (AN) remains the primary inorganic component of particulate matter (PM) in Los Angeles (LA). Using aerosol mass spectrometry over multiple years in LA illustrates the controlling dynamics of AN and their evolution over the past decades. These data suggest that much of the nitric acid (HNO) production required to produce AN in LA occurs during the nighttime via heterogeneous hydrolysis of NO. Further, we show that US Environmental Protection Agency-codified techniques for measuring total PM fail to quantify the AN component, while low-cost optical sensors demonstrate good agreement. While previous studies suggest that declining NO has reduced AN, we show that HNO formation is still substantial and leads to the formation of many tens of micrograms per cubic meter of AN aerosol. Continued focus on reductions in NO will help meet the PM standards in the LA basin and many other regions.