Implementing the trajectory-grid transport algorithm in an air quality model

Eulerian-based air quality models encounter a serious numerical problem in solving the advection equation. In addition, mass conservation is often violated when meteorological model output is used as input to air quality models. The trajectory-grid algorithm handles the advection and eddy-diffusion in the Lagrangian and Eulerian framework, respectively. It is very accurate and can be used to trivially solve the advection equation for molar mixing ratios to address (but not correct) the mass conservation issue. We incorporated the algorithm into the state-of-the-science comprehensive air quality model with extensions (CAMx). Applications of the model reveal the inaccuracy of the commonly used Bott advection scheme, and the subsequent compensating errors of the model. The results clearly call for a more reliable description of eddy diffusivity and emissions inventory in order to truly improve the reliability and predictive capability of air quality models.