Considering Ecological Formulations for Estimating Deposition Velocity in Air Quality Models

A dry deposition modeling approach that includes vegetation-atmosphere interactions through photosynthesis/carbon assimilation relationships was recently developed. In this approach, gas deposition velocity (Vd) is calculated using an electrical resistance-analog approach in a coupled soil-vegetation-atmosphere transfer (SVAT) model. For this, a photosynthesis-based model is dynamically coupled to an atmospheric model with prognostic soil hydrology and surface energy balance. The effective surface resistance (composed of aerodynamic, boundary layer, and canopy-based resistances) is calculated for a realistic and fully interactive estimation of gaseous deposition velocity over natural surfaces. Results from the coupled model studies to estimate observed deposition velocity estimates for ozone over agricultural fields showed good agreement. The same model was tested for its ability to simulate ammonia Vd near an animal agricultural facility. The scheme did not reproduce the bidirectional exchange and had a much smaller range as compared to observations. The ecological scheme was modified to include a simple ammonia compensation point formulation and the model results were much closer to the observations. Study results indicate that ecological approaches with default parameterization and biophysical are convenient and effective approaches for developing Vd estimates in air quality models.