Process Analysis Study of Weekend-Weekday Differences of Ozone Calculated with the CMAQ Air Quality Model

Urban regions tend to have higher concentration of ozone on weekends than on weekdays. This "weekend effect" appears paradoxical since it is well known that the total anthropogenic emissions of NOx are typically lower on weekends. We have conducted simulations of hypothetical episodes of weekdays and of weekends, using the EPA's Community Multiscale Air Quality model (CMAQ). The episodes use identical meteorology, but different emission inputs, taking into account the different total amounts and timings of weekend NOx emissions. The SAPRC99 gas-phase mechanism is used. The domain covers the California central coast and the Central Valley, and both the San Francisco and Sacramento metropolitan areas. We utilize concentrations and process analysis variables at every timestep, for all grid cells. Process analysis quantifies the change in species concentrations due to a series of physical and chemical processes (e.g., emissions, chemical reaction, horizontal advection, etc.). Quantifying the contributions of individual processes to model predictions provides a fundamental explanation of the reasons for a model's predictions and shows the relative importance of each process. It also permits tracking of chemical species through a series of reactions or individual reactions. We compare the formation of ozone precursors (the HOx cycle) and the O3 chemistry to determine the causes of the weekend O3 excess, which are attributable to both NOx titration and an actual odd oxygen excess.