Can a Simple Ecosystem CO2 Flux Model Simulate Complex CO2 Background Conditions for Indianapolis, IN?

In this study, we evaluate the ability of a simple ecosystem CO₂ flux model, the Vegetation Photosynthesis and Respiration Model (VPRM), to capture the complex CO₂ background conditions observed in Indianapolis, IN. Using simulated biogenic CO₂ fluxes in conjunction with mole fraction tower influence functions, we estimated biogenic CO₂ mole fractions at three background towers in the Indianapolis Flux Experiment (INFLUX) network for three years (April 2017 to March 2020). From the simulated biogenic CO₂ mole fractions, we estimated CO₂ enhancements for two towers compared to the third tower. We compared simulated and observed afternoon average CO₂ enhancements at daily, monthly, and seasonal time scales for both towers. Additionally, we compared simulated and observed average daily cycles of CO₂ fluxes during the growing season at agricultural flux observation sites surrounding Indianapolis. At a daily time scale, the model-observation residuals for CO₂ enhancements were on the same order of magnitude as the observed enhancements themselves; therefore, the model could not capture the observed day-to-day variations of afternoon average CO₂ enhancements. However, monthly mean model-observation residuals rarely differed significantly from zero, indicating that the model can capture afternoon average CO₂ enhancements at a monthly time scale with no seasonal bias. Comparing modeled and observed afternoon average enhancements smoothed using a 31-day running median and averaged to create a 365-day composite year, we found that the model captures the general seasonal trends and timing of the observed enhancements during the growing season. When compared to the average observed daily cycles of CO₂ fluxes during the growing season at corn and soybean sites, the modeled fluxes often capture the timing of the observed daily trends in fluxes, but the model is 8-50% weaker than the observed peak afternoon CO₂ drawdown and 18-70% weaker than the observed peak in nighttime respiration. The results of this study indicate that the simple and computationally inexpensive VPRM can be effectively used in urban 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; these results are thus encouraging for the use of VPRM in other urban settings.