Online versus offline air quality modeling on cloud-resolving scales

Computational advances now allow air quality forecast models to fully couple the meteorology with chemical constituents within a unified modeling system - online that allows two-way interactions. The more common approach is the offline system, which only allows one-way coupling from the meteorology - sampled at fixed time intervals - to the chemistry. To study the information loss between online and offline numerical forecasts, a next-generation nonhydrostatic air quality modeling system has been developed that can be used both offline or online. First, a control online air quality simulation is conducted and the meteorology and chemical data are saved at a 10 s time interval. Subsequently, three offline simulations are conducted with meteorological data updates at 10, 30, and 60 min time intervals. Analysis of the wind velocity power spectrum and chemical profiles indicate that the offline simulations are susceptible to large errors in the vertical mass distribution.