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

NO<inf>x</inf> plays a vital role in tropospheric ozone formation, OH radical recycling, and acts as a precursor to the formation of particulate nitrate (pNO<inf>3</inf>^-), a major reactive nitrogen species. pNO<inf>3</inf>^- mainly forms via four pathways: oxidation of NO<inf>2</inf> by OH (P<inf>1</inf>), N<inf>2</inf>O<inf>5</inf> hydrolysis (P<inf>2</inf>), reactions with VOCs (P<inf>3</inf>), and ClO (P<inf>4</inf>). However, studies on its sources and formation mechanisms are limited. This study uses dual isotopes (δ¹⁸O and δ¹⁵N) of pNO<inf>3</inf>^- to explore the sources of NO<inf>x</inf> and dominant pNO<inf>3</inf>^- 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, P<inf>1</inf> (79.6 ± 7.2 %) and P<inf>2</inf> (16.1 ± 7.5 %) dominated pNO<inf>3</inf>^- formation; P<inf>3</inf> and P<inf>4</inf> were negligible (<5 %). During the study period, the major sources of NO<inf>x</inf> 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<inf>x</inf> transformation processes under specific seasonal and emission conditions. While these results improve the understanding of pNO<inf>3</inf>^- formation and may aid in refining regional NO<inf>x</inf> 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.