RII Track-4:NSF: Assessment of Multiscale Air Quality Model (CMAQ) Representation of Spatiotemporal Atmospheric Nitrate Chemical Production in New England

Brown University, Providence RI

Investigators

Abstract

Nitrate is a key component of the atmosphere that derives from precursor nitrogen oxide emissions that have important implications for air quality, acid deposition, and climate. Nitrogen oxide emissions are dominated by anthropogenic activities, typically associated with fossil-fuel combustion, which has dramatically declined across the US over the last several decades due to effective regulations. Yet, atmospheric deposition of nitrate remains a significant environmental stressor. Atmospheric chemistry transport models, which are commonly utilized to help guide policy, have difficulties reproducing observed atmospheric nitrate concentrations and deposition, resulting in unclear guidelines to improve existing Clean Air Act Secondary Standards. This disagreement may be related to complicated production mechanisms of atmospheric nitrate, requiring a deeper understanding of nitrate atmospheric chemistry to better predict the atmospheric burden of nitrogen deposition. In this project, the PI will partner with scientists from the US EPA Research Triangle Park to evaluate and improve upon a commonly utilized atmospheric chemistry and transport model’s representation of atmospheric nitrate formation utilizing a novel oxygen isotope tracer technique (Δ17O). Using a three-dimensional atmospheric chemistry and transport model, this project will enhance the PI’s research expertise and contribute to the training and learning of a graduate student. This project will establish new and long-lasting collaborations between the PI and host scientists. Project outcomes will include establishing the first atmospheric chemistry and transport model at Brown University that will be utilized in future research projects by the PI and other collaborators at Brown, enhancing Rhode Island's research capabilities. The oxygen stable isotopic composition (Δ17O) has been proven to provide quantitative observational constraints on the production mechanisms of atmospheric nitrate. This is because key atmospheric oxidants have distinctive Δ17O signatures (or "fingerprints") incorporated in the product nitrate derived from the oxidation of precursor nitrogen oxide emissions. This work will evaluate the chemical representation of spatiotemporal atmospheric nitrate production in New England from 2005-2017 using the Community Multiscale Air Quality Model (CMAQ). Model simulations will be conducted using a high-resolution US domain, and atmospheric nitrate formation pathways will be tagged and used to calculate the oxygen stable isotopic composition that quantitatively expresses nitrate production mechanisms. The model output will be used to compare with detailed spatiotemporal Δ17O observations of atmospheric nitrate in New England, leveraging detailed Δ17O data from prior NSF support to test and improve upon atmospheric chemistry model representation of nitrate chemistry in New England. The goals of this project include evaluating the chemical representation of the spatiotemporal formation of atmospheric nitrate and differences between nitric acid and particulate nitrate across the northeastern US during a period of significant reductions in nitrogen oxide emissions. Ultimately, this project will lead to a better understanding of the role of emission reductions on the atmospheric cycle of nitrogen and its connection to atmospheric composition, air quality, and acid deposition. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.