For more than 25 years, Environmental Protection Agency’s (EPA) Clean Air Status and Trends Network (CASTNET) has collected ambient measurements of nitric acid, nitrate, and ammonium concentrations using a filter pack. As of August 2014, 92 CASTNET sites continue to make these measurements. CASTNET also produces estimates of the dry deposition of these compounds. CASTNET estimates wet deposition of ammonium and nitrate using interpolated measurements from the National Atmospheric Deposition Program’s (NADP) National Trends Network (NTN) and the Parameter-elevation Relationships on Independent Slopes Model (PRISM). However, key contributors to the nitrogen budget have been missing from CASTNET. Two of these components are ammonia (NH₃) and total reactive nitrogen (NO_y). Missing NH₃ measurements have been addressed by CASTNET’s participation in NADP’s Ammonia Monitoring Network (AMoN), which started in 2007. To develop a data set of NO_y measurements, EPA has established an NO_y monitoring network stretching from the east coast to the mountain west, which now features six EPA CASTNET sites. In addition, NPS, a primary sponsor of CASTNET, conducts NO_y measurements at several of their CASTNET sites.

NO_y is defined as NO_x [nitrogen oxide + nitrogen dioxide] plus NO_z [nitric acid, nitrous acid, PAN, other organic nitrates, and nitrite]. Typical concentrations at the six EPA-sponsored CASTNET sites measuring NO_y range from an average of 0.6 parts per billion (ppb) at Huntington Wildlife Forest, NY and 0.7 ppb at Pinedale, WY to 4.5 ppb at Bondville, IL to 10.2 ppb at Beltsville, MD. Comparisons with total nitrate measured by the CASTNET filter pack show similar temporal patterns with total nitrate typically around half of the NO_y concentration. CASTNET NO_y data from select sites are used as part of the NCore program and provide information to atmospheric modelers, policy makers, and scientists studying environmental impacts.