Modeling the Oxygen Isotope Anomaly (Δ17O) of Reactive Nitrogen in the Community Multiscale Air Quality Model: Insights into Nitrogen Oxide Chemistry in the Northeastern United States

Atmospheric nitrate, including nitric acid (HNO 3 ), particulate nitrate (pNO 3 ), and organic nitrate (RONO 2 ), is a key atmosphere component with implications for air quality, nutrient deposition, and climate. However, accurately representing atmospheric nitrate concentrations within atmospheric chemistry models is a persistent challenge. A contributing factor to this challenge is the intricate chemical transformations involving HNO 3 formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (Δ 17 O) to quantitatively depict ozone (O 3 ) involvement in precursor nitrogen oxide (NO x = NO + NO 2 ) photochemical cycling and HNO 3 formation. This framework has been integrated into the US EPA Community Multiscale Air Quality (CMAQ) modeling system to facilitate a comprehensive assessment of NO x oxidation and HNO 3 formation. In application across the northeastern US, the model Δ 17 O compares well with recently conducted diurnal Δ 17 O(NO 2 ) and spatiotemporal Δ 17 O(HNO 3 ) observations, with a root mean square error between model and observations of 2.6‰ for Δ 17 O(HNO 3 ). The model indicates the major formation pathways of annual HNO 3 production within the northeastern US are NO + OH (46%), N 2 O 5 hydrolysis (34%), and organic nitrate hydrolysis (12%), with significant seasonal variability. This model can evaluate NO x chemistry in CMAQ in future air quality and deposition studies involving reactive nitrogen.