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), particulate nitrate (pNO), and organic nitrate (RONO), 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 formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (ΔO) to quantitatively depict ozone (O) involvement in precursor nitrogen oxides photochemical cycling and HNO formation. This framework has been integrated into the US EPA Community Multiscale Air Quality (CMAQ) modeling system to facilitate a comprehensive assessment of NO oxidation and HNO formation. In application across the northeastern US, the model ΔO compares well with recently conducted diurnal ΔO(NO) and spatiotemporal ΔO(HNO) observations, with a root mean square error between model and observations of 2.6 ‰ for ΔO(HNO). The model indicates the major formation pathways of annual HNO production within the northeastern US are NO+OH (46 %), NO hydrolysis (34 %), and organic nitrate hydrolysis (12 %). This model can evaluate NO chemistry in CMAQ in future air quality and deposition studies involving reactive nitrogen.