The Clean Air Status and Trends Network (CASTNET) has a more than 25-year record of atmospheric nitrogen measurements at rural/remote locations in the United States. The routine nitrogen measurements made at CASTNET sites are nitric acid (HNO₃), nitrate (NO₃), and ammonium (NH₄). CASTNET also provides estimates of dry deposition for these compounds. Many CASTNET sites are located at or near a National Atmospheric Deposition Program (NADP) National Trends Network (NTN) site, which provides measurements of NO₃ and NH₄ in wet deposition. Recent efforts have been made to expand measurements to enable a more complete assessment of contributors to the nitrogen budget. In 2007, CASTNET began participating in the NADP Ammonia Monitoring Network (AMoN), which was initiated to establish a nationwide network of passive ammonia (NH₃) monitors. Recently, CASTNET has also conducted several nitrogen measurement studies to supplement and enhance the filter pack and AMoN samples.

At the coastal site in Beaufort, North Carolina (BFT142), a trace level NO/NO_y system was deployed at the site using a commercially available chemiluminescence analyzer to further characterize reactive oxidized nitrogen. The setup featured a custom modification to use a second molybdenum converter that allowed for the collection of NO_x and, by difference calculation, NO₂. Data analyses from these studies have provided insights into ambient nitrogen levels in coastal North Carolina and helped identify sampling artifacts from this dual converter system.

As a follow on to the Beaufort reactive oxidized nitrogen study, an experimental total reactive nitrogen sampling system has been developed to further analyze the components of the reactive nitrogen sample. The total reactive nitrogen system, deployed at the Beltsville, MD (BEL116) CASTNET site, consists of four converter boxes: a TN_x stainless steel converter, two traditional molybdenum converters (one at 10m for NO_y and a second at the analyzer for NO_x), and an LED-based photolytic NO_x converter. Calculated parameters include NH_x, NO₂ from the molybdenum converter, and NO₂ (true) from the photolytic converter. A solenoid sampling system diverts flow through the various converter boxes to allow for the detection of these species using a single analyzer. This approach reduces the expenses of inter-unit calibration and problematic biases or analytical drifts. Once data have been collected and validated, data comparisons will be made between the total reactive nitrogen sampler, CASTNET filter pack measurements, AMoN, and the MARGA.