Technical note: Refining δ¹⁵N isotopic fingerprints of local NOx for accurate source identification of nitrate in PM_2.5

Abstract. Stable nitrogen isotopic composition (δ¹⁵N) has proven to be a valuable tool for identifying sources of nitrates (NO₃^–) in PM_2.5. However, the absence of a systematic study on the δ¹⁵N values of domestic NOx sources hinders accurate identification of NO₃^– sources in China. Here, we systematically determined and refined δ¹⁵N values for six categories of NOx sources in the local Tianjin area using an active sampling method. Moreover, the δ¹⁵N values of NO₃^– in PM_2.5 were measured during pre-heating, mid-heating and late-heating periods, which are the most heavily polluted in Tianjin. Results shown that the representative nature and region-specific characteristics of isotopic fingerprints for six categories of NOx sources in Tianjin. The Bayesian isotope mixing (MixSIAR) model demonstrated that coal combustion, biomass burning, and vehicle exhaust collectively contributed more than 60 %, dominating the sources of NO₃^– during sampling periods in Tianjin. However, failure to consider the isotopic signatures of local NOx sources could result in an underestimation of the contribution from coal combustion. Additionally, the absence of industrial sources, an uncharacterized source in previous studies, may directly result in the contribution fraction of other sources being overestimated by the model more than 15 %. Notably, as the number of sources input to the model increased, the contribution of various NOx sources was becoming more stable, and the inter-influence between various sources significantly reduced. This study demonstrated that the refined isotopic fingerprint in a region-specific context could more effectively distinguish source of NO₃^–, thereby providing valuable insights for controlling NO₃^– pollution.