Systematic low bias of passive samplers in characterizing nitrogen isotopic composition of atmospheric ammonia

Identifying the sources of increased ammonia (NH₃) in the atmosphere has important implications for air quality, ecosystems, and climate. While analysis of the nitrogen isotopic composition (δ¹⁵N) can aid in the apportionment of NH₃ emission sources, the widely-used passive collection methods have not been verified for their ability to accurately characterize δ¹⁵N-NH₃. In this study, three types of passive samplers (ALPHA, Analyst, and Radiello) were employed and collocated with a denuder based active sampler (DELTA, serves as a reference method) to compare and contrast their ability in collecting atmospheric NH₃ for subsequent concentration and δ¹⁵N-NH₃ analysis in urban Beijing during the summer of 2018. The results showed that NH₃ concentrations determined using passive collection samplers that included ALPHA (n = 11), Analyst (n = 12), and Radiello (n = 7) samplers were 13.4%, 14.4%, and 27.5% lower than that of the active sampler (n = 12), respectively. The low NH₃ concentration bias in the passive collection samplers was suggested to be the result of inaccurate effective sampling rates due to incorrect mass transfer correction factors for the environmental conditions of this study. The δ¹⁵N-NH₃ values did not show significant differences between passive samplers, with weekly means of −31.0 ± 1.0‰ (n = 4), −28.4 ± 5.5‰ (n = 4), and −29.1 ± 0.9‰ (n = 4) for the ALPHA, Analyst, and Radiello, respectively. Notably, the overall mean δ¹⁵N-NH₃ values collected by these passive samplers (−29.5 ± 3.2‰, n = 12) was significantly lower than that of the active sampler (−14.1 ± 1.6‰, n = 4), with a difference of 15.4 ± 3.5‰. The large underestimation of δ¹⁵N-NH₃ values by passive samplers can be explained by the diffusive isotope fractionation of NH₃ isotopologues, which is driven by relative mass differences of ¹⁴NH₃ and ¹⁵NH₃. Overall, these findings highlight a potential low bias in NH₃ concentrations and a substantial uncertainty in previous source interpretation using δ¹⁵N-NH₃ as previous studies have almost exclusively characterized δ¹⁵N-NH₃ using passive samplers. We suggest that the denuder-based active sampler should be used in the future for characterizing δ¹⁵N-NH₃ whether at the source or in an atmospheric environment with mixed sources. This work also has important implications for characterizing δ¹⁵N of other gaseous nitrogen compounds.