What controls aerosol δ15N-NO3−? NOx emission sources vs. nitrogen isotope fractionation

Atmospheric δ15N-NO3− has been used to reveal NOx (NO + NO2) sources as NO3− is the ultimate sink of NOx. However, it remains questionable whether the nitrogen isotope fractionation among NOy (NO, NO2, NO3, N2O5, HNO3 and NO3−) engender the misjudgment of NOx emission sources by affecting δ15N-NOy. To explore this issue, we integrated the dataset of aerosol δ15N-NO3− values and ratios of fNO2 (fNO2 = NO2/(NO2 + NO)), calculated the nitrogen isotope fractionation factors (Δs) among NOy, 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 Δs displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δ15N-NO3− and Δs, which suggested that Δs have important influence on shaping aerosol δ15N-NO3− and further discriminating NOx emission sources. This study implies that we should refine the Δs when employing atmospheric δ15N-NO3− to quantify NOx source allocation.