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

Abstract

Particulate nitrate (pNO3-) and its precursor gas nitrogen oxide (NOx) are among the most significant reactive nitrogen species in the atmosphere. NOx emissions over the Indian sub-continent especially the Indo-Gangetic Plain (IGP) have increased rapidly over the past decades. NOx, 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 NOx 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 (NOx) in the atmosphere. In this study, diurnal samples of PM2.5 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 NOx pollution over a specific region.