Evaluating NOx fate and organic nitrate chemistry from α-pinene oxidation using stable oxygen and nitrogen isotopes

The oxidation of biogenic volatile organic compounds (BVOCs) such as α-pinene in the presence of nitrogen oxides (NOx= NO + NO2) initiates complex photochemical processes that produce organic nitrates (RONO2) and influence atmospheric oxidation capacity, air quality, and the fate of reactive nitrogen. However, tracking the chemical fate of RONO2 remains challenging as it includes pathways such as renoxification, aerosol partitioning, deposition, and/or hydrolysis to nitric acid (HNO3). Stable oxygen (Δ17O, δ18O) and nitrogen (δ15N) isotope measurements can provide a unique tool to probe these processes, as NOy species can exhibit distinct isotopic signatures due to characteristic oxygen-transfer dynamics and isotope fractionation. Here, we present chamber experiments of α-pinene oxidation in the presence of NOx under a range of oxidant and photochemical conditions, reporting the Δ17O, δ18O, and δ15N values of simultaneously collected NO2, HNO3, and particulate nitrate (pNO3), the latter of which derived predominantly from RONO2 in the conducted experiments. A strong linear relationship between δ18O and Δ17O across all NOy species (r=0.992; p<0.01) supports a two-endmember mixing model, in which oxygen atoms are transferred from isotopically distinct sources that include ozone (O3) with high δ18O and Δ17O as well as peroxy and hydroxyl radicals (RO2, HO2, OH) with lower values. Nitrogen isotope fractionation, quantified as the difference in δ15N values (Δδ15N), revealed consistently positive Δδ15N(HNO3–NO2) values (+28.9 ± 13.4 ‰ in daytime experiments; +22.2 ± 1.4 ‰ at night) and negative Δδ15N(pNO3–NO2) values (−13.6 ± 5.8 ‰ in daytime experiments). This reflected distinct formation pathways and isotope effects including NOx photochemical cycling, thermal dinitrogen pentoxide (N2O5)–nitrate radical (NO3)–NO2 equilibrium, and HNO3 production mechanisms. Box-model simulations based on Δ17O values as a constraint were conducted using a newly developed gas-phase mechanism, which reproduced Δ17O(NO2) and Δ17O(pNO3) (compared to simulated Δ17O(RONO2)) accurately, with an average model bias of 0.9 ± 2.4 ‰ (R2=0.98) and −1.4 ± 2.4 ‰ (R2= 0.55 and R2= 0.97 when excluding one outlier), respectively. We further empirically derived important isotopic parameters such as the Δ17O value transferred from O3 through comparison of model-simulated oxygen atom source contributions with observed Δ17O values for NO2 and pNO3 across experiments. This yielded best-fit slopes of 39.4 ± 0.6 ‰ for NOx photochemical cycling and 41.7 ± 1.2 ‰ for RONO2 formation, consistent with near-surface observations of Δ17O in the terminal oxygen atom of O3. Despite the agreement with NO2 and RONO2, accurately simulating Δ17O(HNO3) proved challenging. Sensitivity tests revealed that model biases likely stemmed from a combination of factors including background HNO3 chamber blanks affecting low-NOx experiments, missing N2O5 heterogeneous hydrolysis under nighttime conditions, and an overestimation in the Δ17O(HNO3) mass balance resulting from the NO2+ OH reaction, which was improved by adjusting the contribution from (2/3)Δ17O(NO2) to (1/2)Δ17O(NO2). These adjustments reduced the average model bias in Δ17O(HNO3) from 6.7 ± 3.3 ‰ (R2= 0.39) in the base mechanism to 1.6 ± 1.3 ‰ (R2= 0.48) in the modified mechanism. These findings demonstrate the utility of Δ17O and δ15N for disentangling nitrate formation mechanisms, while also highlighting critical gaps in our understanding of the isotope dynamics involving HNO3 formation. Future experimental work targeting isolated HNO3 pathways is essential to refine isotopic mass balance assumptions and nitrogen isotope fractionation.