Work

Evaluating NO x fate and organic nitrate chemistry from α -pinene oxidation using stable oxygen and nitrogen isotopes

Year: 2025

Type: article

Abstract: . The chemical interaction between nitrogen oxides (NOx = NO + NO2) and α-pinene plays a critical role in air quality and climate. However, uncertainties remain regarding their coupling in NOx loss, renoxification, and oxidation chemistry. To address these gaps, we conducted controlled chamber experiments, analyzing nitric acid (HNO3), NO2, and particulate nitrate (pNO3) for their oxygen and nitrogen stable isotope variations (Δ17O, δ18O, and δ15N). A strong linear relationship between δ18O and Δ17O across experiments revealed contributions of oxygen from ozone (O3) and atmospheric oxygen (O2) in forming reactive radicals. The δ15N values followed the order δ15N(pNO3) < NO2 < HNO3, reflecting isotope fractionation during NOx oxidation. A new chemical mechanism accurately predicted aerosol precursor decay and simulated Δ17O and δ15N values. Simulations showed NOx photochemical cycling and pNO3 formation, primarily from organic nitrate, with Δ17O(NO2) simulations achieving a root mean square error (RMSE) of 1.7 ‰. Improved δ15N(NO2) and pNO3 simulations used a nitrogen isotope fractionation factor (15α) of 0.997 for NO2 + OH reactions. However, modeling Δ17O and δ15N of HNO3 proved challenging, likely due to sampling artifacts. This study provides insights into Δ17O transfer dynamics, nitrogen isotope fractionation, and the role of NOx-BVOC chemistry in air quality, highlighting the potential of Δ17O and δ15N as tools for evaluating complex atmospheric processes.

Source: Atmospheric chemistry and physics

DOI: 10.5194/acp-25-10707-2025