Active vs. passive isotopic analysis: Insights from urban Beijing field measurements and ammonia source signatures

Active collection methods such as ChemComb Speciation Cartridge samplers (CCSCs) are considered more accurate for determining the isotopic composition of NH3 (i.e., δ15N(NH3)), especially in ambient air. However, it is uncertain whether the difference in δ15N values remains consistent when comparing ALPHA to a lab-verified CCSCs with different filter types (Teflon, cellulose, and glass fiber). Here, we deployed two CCSCs and ALPHA side-by-side to collect ambient NH3 samples and compare their δ15N(NH3) values in urban Beijing during July 2019. Furthermore, NH3 source samples from urban traffic and livestock waste were collected using the ALPHA sampler under different scenarios, including various sampling locations, and sampling intervals to assess their representativeness. The results showed that NH3 concentrations measured by ALPHA were 27.6% and 8.5% lower than the concentrations measured by CCSCs A and B, respectively. CCSCs A showed lower δ15N(NH3) values (−16.4 ± 8.0‰) compared to CCSCs B, whereas the δ15N(NH3) values obtained from ALPHA (−23.9 ± 7.7‰) were not significantly different from CCSCs A. This variation in δ15N(NH3) values was attributed to reduced isotopic fractionation (31.5 ± 22.9‰) caused by decreased NH3 volatilization using Teflon and cellulose filter pack configurations. Moreover, this study revealed a consistent 15.1‰ lower δ15N(NH3) value with ALPHA compared to CCSCs B, when Teflon and glass fiber filter pack configurations were used for daily sampling. Additionally, this study revealed that ALPHA sampler might reach saturation when used for longer sampling durations, in contrast to shorter intervals of 8–12 h specifically for livestock waste. This saturation led to positive δ15N(NH3) values (2.9 ± 0.4‰), which were significantly higher compared to samples collected within the 8–12 h range. Further studies should conduct co-located measurements using both passive and active methods, enabling more accurate comparisons of sampling techniques at each location and ensuring representative samples for source apportionment studies.