Insights into the sources of precursor and formation pathways of particulate NO3- during paddy-residue burning period through dual isotope proxies

Particulate nitrate (pNO₃^-) and its precursor gas nitrogen oxide (NO_x) are among the most significant reactive nitrogen species in the atmosphere. NO_x emissions over the Indian sub-continent especially the Indo-Gangetic Plain (IGP) have increased rapidly over the past decades. NO_x, an atmospheric gaseous pollutant, plays important roles in the formation of tropospheric ozone, recycling of hydroxyl radicals (OH), etc. It also serves as a precursor to pNO₃^- formation. This has significant implications for air quality, climate, and human health. Rapid accumulation of pNO₃^- can also increase PM load by aiding in secondary aerosol formations. Identification of the major sources of NO_x and the formation pathways of pNO₃^- is crucial for improving the accuracy of air quality models and effective mitigation strategies. In the atmosphere, pNO₃^- is known to form mainly via four distinct pathways: (P1) oxidation of NO₂ by OH in gas phase, (P2) hydrolysis of N₂O₅ on existing aerosols, (P3) reaction between NO₃ radicals and VOCs, and (P4) reaction of NO₃ radical and ClO. However, studies on the sources and formation pathways of pNO₃^- are limited pertaining to the Indian subcontinent as well as the globe. Dual isotopes (δ¹⁵N and δ¹⁸O) of pNO₃^- are an excellent tool to understand the formation mechanisms and sources of pNO₃^- precursor (NO_x) in the atmosphere. In this study, diurnal samples of PM_2.5 were collected over a semi-urban site (Patiala) in the IGP during a large-scale paddy residue burning period (October-November). Dual isotopes (δ¹⁵N and δ¹⁸O) of pNO₃^- along with other major ions were measured. Average δ¹⁸O and δ¹⁵N of pNO₃^- were 57.2 ± 8 ‰and -1.9 ± 5 ‰, respectively. Significant diurnal differences in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- were observed. δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- were -5.0 ± 2.4‰, 52.1 ± 6.2‰ and -0.13 ± 5.7‰, 60.0 ± 8.4‰ during day and night-time respectively. Enriched δ¹⁵N-NO₃^- during night-time was due to enhanced gas-particle partitioning owing to lower temperature. A significant negative correlation between Nitrate Oxidation Ratio (NOR), and temperature further supported the above statement. Stable isotope mixing model (MixSIAR) was used to estimate the contribution of different pathways to pNO₃^- formation and sources. The major pathways contributing to the formation of pNO₃^-  were  P1(OH)  (~ 92%) followed by P2 (N₂O₅) (~ 5%). P3 (VOCs) and P4 (ClO) had negligible contributions of ~1.3 and ~1.5% respectively. Relative contributions of P1 and P2 during day and night-time were calculated. P1 and P2 contributed to 95% and 5%, and 77% and 23% during day and night-time respectively. Presence of pNO₃^- formed via P1 during night-time could be due to the higher lifetime of pNO₃^- compared to sampling duration. Source apportionment showed biomass burning (32%) and traffic exhaust (35%) were the major contributors followed by combustion (18%) and soil emissions (15%) during the study period. Our study, first of its kind over India, is important for elucidating the formation mechanism of pNO₃^- from its precursor gas. Such studies are helpful in planning and developing mitigation strategies aiming to reduce NO_x pollution over a specific region.