Assessing the Influence of NH ₄ ⁺ Volatilization on δ ¹⁵ N Measurements during Active Sampling of Atmospheric NH _<i>x</i>

Active samplers such as ChemComb Speciation Cartridge Samplers (CCSC) use Nylon or Teflon filters to collect particulate ammonium (NH4+) for nitrogen isotope analysis (δ15N) in source apportionment. However, the NH4+ volatilization bias or its associated isotope effects on δ15N(NH4+) retained on filters are not well characterized, limiting the accuracy of source attribution. Here, NH3 and NH4+ were sampled in Beijing using CCSC with backup filters to assess and correct both volatilization bias and isotope fractionation during NH4+ partitioning (“dual fractionation”), enabling the estimation of corrected initial δ15N(NH3) from δ15N(NH4+). Significant NH4+ volatilization from Nylon filters was observed, and dual-fractionation corrections substantially altered estimated contributions of agricultural and nonagricultural NH4+ sources, revealing that without correction for volatilization bias, and only accounting for isotope fractionation from NH4+ 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 NH4+ source apportionment.