Technical note: Quantifying the Nitrogen Isotope Difference between Ammonium in the Atmosphere and Ammonia Emitted from Sources

Abstract. The difference (δ¹⁵N_4a-3s) in nitrogen isotopes (δ¹⁵N) between NH₄⁺ and source - emitted NH₃ is a crucial factor influencing the source apportionment of atmospheric NH₄⁺. This δ¹⁵N_4a-3s is mainly due to isotopic fractionation during NH₃ - NH₄⁺ gas - particle conversion and atmospheric deposition. The impact of isotope fractionation on δ¹⁵N_4a-3s had been well quantified by simplified method, but that of atmospheric deposition had often been overlooked. This study developed a model to assess δ¹⁵N_4a-3s variations by considering both the atmospheric deposition and isotope fractionation. The results of six model scenarios showed the difference between δ¹⁵N_4a-3s values under both influences and under isotope fractionation alone increased with the rise of ξ_A (the molar fraction of NH₄⁺ to NH_x in the atmosphere). At 20 °C, when ξ_A = 0.9, the maximum gap could reach 10.7%. δ¹⁵N_4a-3s was insensitive to NH₃ and NH₄⁺ deposition rates, NH₄⁺ generation rate, and temperature, but it was sensitive to ξ_A. A prediction function for δ¹⁵N_4a-3s was constructed and applied for atmospheric NH₄⁺ source apportionment in the Yellow River Delta. Compared with the simplified method, the fitted equation could more reasonably reflect the contribution of agricultural sources (e.g., fertilizer application). The constructed equation could be used for tracing atmospheric NH₄⁺origin, thus improving the accuracy of atmospheric NH₄⁺ source apportionment.