Simulating Complex CO ₂ Background Conditions for Indianapolis, IN, With a Simple Ecosystem CO ₂ Flux Model

This is the authors' abstract. We don't add key points for this paper.

We evaluated the ability of a simple ecosystem carbon dioxide (CO 2 ) flux model, the Vegetation Photosynthesis and Respiration Model (VPRM), to capture complex CO 2 background conditions observed in Indianapolis, IN. Using simulated biogenic CO 2 fluxes and mole fraction tower influence functions, we estimated biogenic CO 2 mole fractions at three background towers in the Indianapolis Flux Experiment (INFLUX) network from April 2017 to March 2020. The model captures afternoon average CO 2 enhancements, the difference between the background towers and a common reference tower, at a monthly time scale with no significant bias, with monthly mean residuals rarely differing significantly from zero. Although not central to our application, the model could not capture day‐to‐day variations of observed afternoon average CO 2 enhancements. Random errors, when averaged over monthly to yearly time scales, were an order of magnitude smaller than typical urban enhancements. VPRM captured site‐to‐site differences in the average observed daily cycle of CO 2 fluxes at agricultural eddy covariance flux sites well, indicating that the model is able to capture rural CO 2 fluxes in our domain in addition to capturing differential impacts of the fluxes on CO 2 mole fractions. VPRM can be effectively used in CO 2 inversions to represent complex seasonal variations in background conditions observed in Indianapolis. Indianapolis, a modest‐size city surrounded by strong ecosystem fluxes, represents a rigorous test for the VPRM system. Further, this study presents an evaluation system that can be applied to assess the performance of other ecosystem CO 2 flux models in cities with similar monitoring networks.