Stability of organic nitrogen in NADP wet deposition samples
Organic compounds represent an important yet largely uncharacterized component of atmospheric nitrogen deposition. Rapid progress in understanding the sources and spatiotemporal patterns of organic nitrogen (ON) deposition will require the use of existing large-scale monitoring infrastructure, such as the National Atmospheric Deposition Program's National Trends Network (NADP/NTN). The purpose of this study is to investigate the analytical and sampling requirements for adding ON measurements to the NTN, with specific interest in examining ON stability during sampling and storage. The analytical technique for total nitrogen (TN) used by the NADP's Central Analytical Laboratory (CAL) and associated quality assurance data are described. We then compare TN, inorganic nitrogen (IN = NH4+ + NO3-), and ON (ON = TN - IN) concentrations in a field study between standard weekly NADP/NTN samples (unrefrigerated during sampling and storage), daily event samples collected using the Atmospheric Integrated Research Monitoring Network protocol (AIRMoN, unrefrigerated during sampling but refrigerated during storage), and daily event samples that were preserved via refrigeration in the field upon collection (AIRMoN_Ref, refrigerated during sampling and storage). Using AIRMoN_Ref as the reference for comparison, total loss of ON in weekly NTN samples in the field and during laboratory storage is approximately 40%. This bias is likely dominated by losses of ON in the collection bucket. However, additional loss may occur during laboratory storage at room temperature prior to analysis. Loss of ON was also observed in AIRMoN samples, though differences relative to AIRMoN_Ref (10.8%) were less than weekly NTN samples. Biases in ON are more consistently negative at higher ambient temperatures. Storage experiments indicated that refrigeration at 4 degrees C at the CAL was sufficient to stabilize ON concentrations. We conclude that weekly sampling for ON is feasible if precipitation is refrigerated or frozen immediately upon collection. Samples should be kept refrigerated or frozen prior to analysis. Preliminary results indicate that NO2-, an additional inorganic species not currently measured by the CAL, makes a small contribution to TN (<1%), but if neglected may cause significant negative bias in ON determined as TN - IN. We recommend that CAL include NOT quantification as a component of IN for bulk ON determination. Published by Elsevier Ltd.