Organic nitrogen in precipitation over Eastern North America

A measurement technique was developed to reliably quantify organic nitrogen (ON) in ambient, wet-only precipitation. Samples were frozen during collection and subsequently divided into two aliquots. One set was stored at −170°C and analyzed for total N (TN) via high-temperature combustion to NO and detection by chemiluminescence; the other set was sterilized with CHCl3, stored refrigerated, and analyzed for NH4+ by automated colorimetry and for NO3−, and NO2− by ion chromatography. ON was inferred by difference. Analysis of paired, untreated aliquots stored for 30 and 41 days at different temperatures revealed substantial conversion of NH4+ to ON at room temperature and significant losses (16% and 23%) of NH4+ (presumably to biota growing on bottle walls) in refrigerated samples. Analytes in frozen and sterilized samples were stable. Volume-weighted ON concentrations for precipitation sampled at Charlottesville, Virginia (VA), Newark, Delaware (DE), and New Castle, New Hampshire (NH; 3.1, 4.2, and 0.6 μM N, respectively) and corresponding contributions to volume-weighted TN (6.5%, 7.8%, and 2.6%, respectively) are at the lower limit of published values for eastern North America and elsewhere. Methodological differences contribute to the apparent variability among these reported sample statistics. Volume-weighted ON concentrations were generally highest during spring and were lowest during summer. Due to the combined influence of unmeasured ON and loss of NH4+ from inadequately preserved samples, current estimates for the wet deposition of atmospheric N to eastern North America based on data from national networks may be underestimated by 10–20%.