Modeling the Oxygen Isotope Anomaly (Δ17O) of Reactive Nitrogen in the Community Multiscale Air Quality (CMAQ) Model: Insights into Nitrogen Oxide Chemistry in the Northeastern United States
Citation:
Walters, W., H. Pye, H. Kim, AND M. Hastings. Modeling the Oxygen Isotope Anomaly (Δ17O) of Reactive Nitrogen in the Community Multiscale Air Quality (CMAQ) Model: Insights into Nitrogen Oxide Chemistry in the Northeastern United States. ACS ES&T Air. American Chemical Society, Washington, DC, , N/A, (2024). https://doi.org/10.1021/acsestair.3c00056
Impact/Purpose:
Atmospheric chemistry converts emissions of nitrogen oxides (NOx) to various secondary species including nitrate. Once converted to nitrate, NOx can be turned into fine particle mass (PM2.5) or deposited, both of which remove NOx from the catalytic ozone cycle in the atmosphere. However, PM2.5 mass formation has implications for health and visibility while deposition of nitrate to sensitive ecosystems has other adverse outcomes. The chemical reactions responsible for converting NOx to nitrate are generally not available from routine measurements. In this work, unique oxygen isotope data provides key information that can be used to determine which chemical pathways produce nitrate in air. The three channels identified indicate a role for gas-phase oxidants (NO+OH), particle-mediated reactions (N2O5 hydrolysis), and organic compounds (organic nitrate hydrolysis). Quantifying these pathways can help constrain the CMAQ model.
Description:
Atmospheric nitrate 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 nitrate formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (Δ17O) to quantitatively depict ozone (O3) involvement in precursor nitrogen oxides (NOx = NO + NO2) photochemical cycling and atmospheric nitrate formation. This framework has been integrated into the US EPA Community Multiscale Air Quality (CMAQ) modeling system to facilitate a comprehensive assessment of NOx oxidation and atmospheric nitrate formation. In application across the northeastern U.S., the model Δ17O compares well with recently conducted diurnal Δ17O(NO2) and spatiotemporal Δ17O(HNO3) observations, with a root mean square error between model and observations of 2.6 ‰ for Δ17O(HNO3). The model indicates the major formation pathways of annual HNO3 production within the northeastern US are NO+OH (46 %), N2O5 hydrolysis (34 %), and organic nitrate hydrolysis (12 %). This model can evaluate NOx chemistry in CMAQ in future air quality and deposition studies involving reactive nitrogen.
