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

Abstract

NOx 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 NOx 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 NOx 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 NOx transformation processes under specific seasonal and emission conditions. While these results improve the understanding of pNO3- formation and may aid in refining regional NOx 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.