Sources and formation pathways of particulate nitrate over the western India: Insights through δ15N and δ18O isotopes

Oxides of Nitrogen (NO_x) are key precursors of tropospheric ozone and particulate nitrate (NO₃^-), both of which contribute to air quality degradation and climate forcing. NO_x is oxidized to nitric acid (HNO₃), which partitions into particulate NO₃^-, an important component of PM_2.5. The formation of HNO₃ and ultimately NO₃^- occurs through multiple chemical pathways and is influenced by atmospheric chemistry as well as meteorological parameters like temperature, relative humidity, and boundary layer dynamics. India has been recognised a global hotspot for NO_x emission, owing to rapid urbanization and population growth. Major sources of NO_x include emissions from traffic, agriculture, biomass burning, and combustion. Despite being a major contributor of NO_x, large uncertainties exist in regional emission inventories due to limited observational constraints. Stable isotopic signature of particulate NO₃^- serves as an excellent tool to understand its formation pathways and sources of its precursor.  Here, we have applied a dual-isotope (δ¹⁵N and δ¹⁸O) approach to understand seasonal and diurnal variations in NO₃^- formation pathways and NO_x sources over Ahmedabad, an urban megacity in western India, during winter and summer. In winter, overall particulate NO₃^- formation was driven mainly by the OH oxidation pathway (P₁, 61.9 ± 7%) and N₂O₅ hydrolysis (P₂, 24.6 ± 6%), with smaller contributions from VOC-derived (P₃, 7.6 ± 4%) and ClNO₂ pathways (P₄, 5.9 ± 3%). However, strong diurnal contrasts were evident, with P₁ accounting for 69.4 ± 5% during the day and P₂ increasing to 32.8 ± 7% at night, reflecting enhanced photochemical activity during day and nocturnal buildup of N₂O₅ under cooler, low-light conditions at night. In summer, NO_3^- formation was dominated by the OH pathway throughout the day and night (67.5 ± 7%), with no significant diurnal variability. This seasonal shift was attributed to elevated boundary layer height and enhanced atmospheric mixing, which stabilized particulate NO₃^-, which was particularly associated with stable non-volatile cations. Source apportionment of NO_x using the Bayesian model (MixSIAR) revealed no significant diurnal differences within either season; however, a distinct seasonal pattern in NO_x sources was observed. In winter, traffic was the largest contributor (46.7 ± 19%), followed by soil emissions (24.4 ± 12%), biomass burning (18.0 ± 9%), and coal-fired power plants (10.9 ± 8%). In summer, soil-related emissions increased to 38.4 ± 12% due to temperature-enhanced microbial activity and volatilization from urban waste, livestock areas, and fertilized land, while traffic remained a dominant source (40.1 ± 17%). Biomass burning and power plant contributions remained lower but persistent across both seasons. Together, these results provided the first dual-isotope-based evidence from western India showing how meteorology and emission processes jointly influence NO₃^- formation and NOx source, thus offering critical observational insight needed to improve regional nitrogen budgets and air quality mitigation strategies.