What controls aerosol δ¹⁵N-NO₃^-? NO_x emission sources vs. nitrogen isotope fractionation

Atmospheric δ¹⁵N-NO₃^- has been used to reveal NO_x (NO + NO₂) sources as NO₃^- is the ultimate sink of NO_x. However, it remains questionable whether the nitrogen isotope fractionation among NO_y (NO, NO₂, NO₃, N₂O₅, HNO₃ and NO₃^-) engender the misjudgment of NO_x emission sources by affecting δ¹⁵N-NO_y. To explore this issue, we integrated the dataset of aerosol δ¹⁵N-NO₃^- values and ratios of f_NO2 (f_NO2 = NO₂/(NO₂ + NO)), calculated the nitrogen isotope fractionation factors (Δ_s) among NO_y, compared the total energy consumption in Beijing-Tianjin-Hebei region (BTH) from 2013 to 2018. Results showed that, although the total energy consumption structure changed from 2013 to 2018 in BTH, there were fewer interannual variances of aerosol δ¹⁵N-NO₃^- values. Nitrogen isotope fractionation factors between NO and NO₂ (Δ₀), NO₂ and NO₃ (Δ₂), NO₂ and N₂O₅ (Δ₃), NO₂ and ClONO₂ (Δ₄) also displayed less interannual variations from 2013 to 2018 in BTH. But both aerosol δ¹⁵N-NO₃^- and Δ_s displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δ¹⁵N-NO₃^- and Δ_s, which suggested that Δ_s have important influence on shaping aerosol δ¹⁵N-NO₃^- and further discriminating NO_x emission sources. This study implies that we should refine the Δ_s when employing atmospheric δ¹⁵N-NO₃^- to quantify NO_x source allocation.