15N Fractionation of Nitrate Formation Constrained by Dual Oxygen Isotopes Provides Insight Into Its Source Identification in Urban Haze

Abstract

δ¹⁵N-NO₃⁻ is widely used to trace the NO_x/NO₃⁻ emission sources without unique source tracers. However, there is still controversy regarding the ¹⁵N fractionation effects during NO₃⁻ formation, leading to uncertain source apportionment. To address this, this study introduces dual oxygen isotopes (∆¹⁷O and δ¹⁸O) to constrain the ¹⁵N fractionation (∆¹⁵N-∆¹⁷O/∆¹⁵N-δ¹⁸O) of NO₃⁻ formation and compare the impact of δ¹⁵N-NO_x (∆¹⁷O) and δ¹⁵N-NO_x (δ¹⁸O) on NO_x/NO₃⁻ source apportionment. Results found significant differences in ∆¹⁵N-∆¹⁷O (−3.7 ∼ +16.1‰) and ∆¹⁵N-δ¹⁸O (+8.5 ∼ +16.2‰) in haze, reflecting the ∆¹⁵N from three pathways (NO₂ + OH, NO₃ + HC, N₂O₅ hydrolysis) and two pathways (NO₂ + OH and N₂O₅ hydrolysis), respectively. The ¹⁵N fractionation value differences obtained by dual oxygen isotopes increases with the increase of NO₃ + HC contribution (0.02–0.65). Additionally, different results of NO_x/NO₃⁻ sources apportionment were obtained by δ¹⁵N-NO_x($∆$¹⁷O) and δ¹⁵N-NO_x(δ¹⁸O) in NO₃ + HC-induced haze. For example, δ¹⁵N-NO_x($∆$¹⁷O) identified coal combustion (46 ± 8%) and biomass burning (32 ± 3%) as major NO_x/NO₃⁻ sources in Zibo haze. Conversely, δ¹⁵N-NO_x(δ¹⁸O) revealed mobile sources (55 ± 8%) and biomass burning (22 ± 5%) as main contributors. Evidence from diurnal variation of sources and characteristics of source tracers show that δ¹⁵N-NO_x($∆$¹⁷O) analysis is more sensitive and accurate than δ¹⁵N-NO_x(δ¹⁸O). These results highlight the non-negligible role of NO₃ + HC in ¹⁵N fractionation during NO₃⁻ formation and provide insight into improving ¹⁵N tracing techniques for NO_x/NO₃⁻ source identification through the constraint of dual oxygen isotopes.