What controls aerosol δN-NO? NO emission sources vs. nitrogen isotope fractionation.

Atmospheric δN-NO has been used to reveal NO (NO + NO) sources as NO is the ultimate sink of NO. However, it remains questionable whether the nitrogen isotope fractionation among NO (NO, NO, NO, NO, HNO and NO) engender the misjudgment of NO emission sources by affecting δN-NO. To explore this issue, we integrated the dataset of aerosol δN-NO values and ratios of f (f = NO/(NO + NO)), calculated the nitrogen isotope fractionation factors (Δ) among NO, 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 δN-NO values. Nitrogen isotope fractionation factors between NO and NO (Δ), NO and NO (Δ), NO and NO (Δ), NO and ClONO (Δ) also displayed less interannual variations from 2013 to 2018 in BTH. But both aerosol δN-NO and Δ displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δN-NO and Δ, which suggested that Δ have important influence on shaping aerosol δN-NO and further discriminating NO emission sources. This study implies that we should refine the Δ when employing atmospheric δN-NO to quantify NO source allocation.