Simulating Complex CO₂ Background Conditions for Indianapolis, IN, With a Simple Ecosystem CO₂ Flux Model

We evaluated the ability of a simple ecosystem carbon dioxide (CO₂) flux model, the Vegetation Photosynthesis and Respiration Model (VPRM), to capture complex CO₂ background conditions observed in Indianapolis, IN. Using simulated biogenic CO₂ fluxes and mole fraction tower influence functions, we estimated biogenic CO₂ mole fractions at three background towers in the Indianapolis Flux Experiment (INFLUX) network from April 2017 to March 2020. The model captures afternoon average CO₂ 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 CO₂ 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 CO₂ fluxes at agricultural eddy covariance flux sites well, indicating that the model is able to capture rural CO₂ fluxes in our domain in addition to capturing differential impacts of the fluxes on CO₂ mole fractions. VPRM can be effectively used in CO₂ 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 CO₂ flux models in cities with similar monitoring networks.