Subgrid‐scale treatment for major point sources in an Eulerian model: A sensitivity study on the European Tracer Experiment (ETEX) and Chernobyl cases

Abstract

We investigate the plume‐in‐grid method for a subgrid‐scale treatment of major point sources in the passive case. This method consists in an online coupling of a Gaussian puff model and an Eulerian model, which better represents the point emissions without significantly increasing the computational burden. In this paper, the plume‐in‐grid model implemented on the Polyphemus air quality modeling system is described, with an emphasis on the parameterizations available for the Gaussian dispersion, and on the coupling with the Eulerian model. The study evaluates the model for passive tracers at continental scale with the European Tracer Experiment (ETEX) and the Chernobyl case. The aim is to (1) estimate the model sensitivity to the local‐scale parameterizations and (2) bring insights on the spatial and temporal scales that are relevant in the use of a plume‐in‐grid model. It is found that the plume‐in‐grid treatment improves the vertical diffusion at local scale, thus reducing the bias, especially at the closest stations. Doury's Gaussian parameterization and a column injection method give the best results. There is a strong sensitivity of the results to the injection time and the grid resolution. The “best” injection time actually depends on the resolution but is difficult to determine a priori. The plume‐in‐grid method is also found to improve the results at fine resolutions more than with coarse grids by compensating the Eulerian tendency to overpredict the concentrations at these resolutions.