Assessing the Influence of NH 4 + Volatilization on δ 15 N Measurements during Active Sampling of Atmospheric NH x

Active samplers such as ChemComb Speciation Cartridge Samplers (CCSC) use Nylon or Teflon filters to collect particulate ammonium (NH 4 + ) for nitrogen isotope analysis (δ 15 N) in source apportionment. However, the NH 4 + volatilization bias or its associated isotope effects on δ 15 N(NH 4 + ) retained on filters are not well characterized, limiting the accuracy of source attribution. Here, NH 3 and NH 4 + were sampled in Beijing using CCSC with backup filters to assess and correct both volatilization bias and isotope fractionation during NH 4 + partitioning (“dual fractionation”), enabling the estimation of corrected initial δ 15 N(NH 3 ) from δ 15 N(NH 4 + ). Significant NH 4 + volatilization from Nylon filters was observed, and dual-fractionation corrections substantially altered estimated contributions of agricultural and nonagricultural NH 4 + sources, revealing that without correction for volatilization bias, and only accounting for isotope fractionation from NH 4 + partitioning, it could underestimate source contributions for agricultural source in July and nonagricultural sources in January by >30%. Sensitivity analysis indicated that volatilization isotope effects are critical for Nylon but negligible for Teflon filters. These results provide practical guidance for designing CCSC sampling strategies, including the use of two acid-coated backup filters, and highlight the necessity of correcting for dual fractionation to improve the accuracy of isotope-based NH 4 + source apportionment.