Evaluation of the CALPUFF model performance for the estimation of the urban ecosystem CO2 flux

Land-use and land-cover changes due to urban expansion is recognized as one of the crucial factors affecting carbon dioxide emissions. In this study, the land conversion effects on soil CO2 fluxes associated with temperate re-established grasslands within the Forest Botanical Garden found on an anthropogenic landform were investigated. The present work analyses the capabilities and requirements of the CALPUFF Lagrangian puff air quality modelling system to simulate the spatial distribution of ecosystem respired CO2 in the urban domain. The results are validated against the available observations of CO2 fluxes in the urban environment using the closed-chamber method with the measurements of the stable carbon isotope ratio (delta C-13) of daytime soil-respired CO2. The isotope mass balance partitioning approach was applied to distinguish biogenic portions of CO2 from the admixture of atmospheric air. The spatial and temporal amplitude of the simulated CO2 concentrations from the CALPUFF model showed considerable agreement with the tracer measurements of the biogenic CO2 component in the near-ground air (0.25 m). In most cases, however, the CALPUFF predictions of ecosystem-derived CO2 showed a general tendency toward considerable underestimation of real concentration levels. Such discrepancies are related to the difficulties associated with the optimization of biospheric CO2 flux and uptake from ecosystems by means of local scale modelling. The modelled results implied that the CALPUFF performance in the dispersion simulation of CO2 concentrations within the urban ecosystems is very sensitive to the initial meteorological conditions, grid resolution, measurement timescale, and the calculated gas flux rate from soils. A significant negative correlation was found between hourly values of the average modelled CO2 and observed wind speed during the entire study campaign (r =-0.58 and rho =-0.82 for Pearson and Spearman statistics, respectively, p < 0.05). Moreover, analysis of the impact of the deposition parameters on changes in the atmospheric concentration of carbon dioxide indicated significant dependency of the temporal CO2 distribution patterns on the precipitation-based events. According to the obtained estimates, the wet deposition rate during rain events was approximately two orders higher than the average dry deposition flux. Overall, the present case study indicates that the CALPUFF model has a rather acceptable predictive ability. A better agreement of model predictions and all field measurements, however, require further studies of CO2 exchange between the ecosystem and atmosphere and understanding where they need to be improved.