Routine monitoring of atmospheric organic nitrogen (ON) is needed to better characterize spatiotemporal patterns and their drivers as well as provide data for more accurately assessing total atmospheric N deposition. This study evaluates an extension of the Clean Air Status and Trends Network (CASTNET) methodology to quantify water soluble total N (WSTN) and ON (WSON) in particulate matter (PM). Routine samples were collected for 12 months at a subset of 25 CASTNET sites. Results demonstrate that WSON in PM can be quantified using routine monitoring techniques. CASTNET storage and handling procedures represent a small (<5%) source of uncertainty with respect to potential changes in concentrations between sample collection and analysis. Field blanks were sufficiently low that a blank correction for WSON was not required and 75% of observations were above the method detection limit. Precision of WSON derived as the median absolute difference of field duplicates was 8.9% at a site representative of network-wide observations. Mean and median % contributions of WSON to WSTN were 15.3 and 13.3, respectively. On average, NO₃^- contributed about twice as much to WSTN than WSON and NH₄⁺ contributed about four times as much as WSON. WSON concentrations were highest in summer and lowest in winter. Concentrations of WSON in smoke-impacted samples were ~ 2X larger than non-impacted samples, as was the % contribution to WSTN (23.7% vs 12.8%). Establishing a method for routine monitoring of WSTN and WSON in precipitation for collocation with the method demonstrated here would rapidly advance understanding of ON deposition.