A comparison of mass conservation methods for air quality models

Mass inconsistency in air quality models (AQMs) can be severe particularly over complex terrain. The vertical winds adjustment is a remedy tool that can maintain the mass conservation in AQMs. We implemented this approach in the Community Multiscale Air Quality model (CMAQ) with two different vertical advection schemes. With the first-order upwind scheme, vertical winds adjustment made mass strictly conserved, while with the Bott's scheme, a higher-order scheme, there were very small errors in the total mass because of the iterative solution for adjustment. Bott's scheme results in better ozone performance than the upwind scheme. However, both vertical winds adjustment methods introduce large trajectory deviations compared to the unadjusted wind fields generated by meteorological models. We then developed a third method with mixed use of vertical winds adjustment and concentration renormalization under different conditions. This method limits the trajectory deviations, introduces some mass conservation errors but at a lesser degree than the renormalization approach.