Formation pathways of particulate NO3− and sources of its precursor over the northwest India: Insights through dual isotopes

NO_x plays a vital role in tropospheric ozone formation, OH radical recycling, and acts as a precursor to the formation of particulate nitrate (pNO₃^-), a major reactive nitrogen species. pNO₃^- mainly forms via four pathways: oxidation of NO₂ by OH (P₁), N₂O₅ hydrolysis (P₂), reactions with VOCs (P₃), and ClO (P₄). However, studies on its sources and formation mechanisms are limited. This study uses dual isotopes (δ¹⁸O and δ¹⁵N) of pNO₃^- to explore the sources of NO_x and dominant pNO₃^- formation pathways over Patiala, a semi-urban site in the northwestern Indo-Gangetic Plain (IGP), during a large-scale paddy residue burning. Day-time δ¹⁵N and δ¹⁸O averaged −5.0 ± 2.4 ‰ and 52.1 ± 6.2 ‰, while night-time values were −0.13 ± 5.7 ‰ and 60.0 ± 8.4 ‰, respectively, reflecting enhanced nighttime partitioning due to cooler temperatures. Further, P₁ (79.6 ± 7.2 %) and P₂ (16.1 ± 7.5 %) dominated pNO₃^- formation; P₃ and P₄ were negligible (<5 %). During the study period, the major sources of NO_x were traffic exhaust (38 ± 18 %), biomass burning (29 ± 18 %), followed by emissions from coal-fired power plants (20 ± 11 %) and soil (13 ± 9 %). Our study, the first of its kind over India provide valuable insight into NO_x transformation processes under specific seasonal and emission conditions. While these results improve the understanding of pNO₃^- formation and may aid in refining regional NO_x inventories, they are representative of the particular location and time frame of sampling and may not reflect source contributions in other regions or during periods without episodic biomass burning influence.