A review of “online” air quality forecast models used by NOAA

This paper will review “online”, or “fully-coupled” air quality forecast models used by NOAA. We will describe both the NCAR/Penn State Mesoscale Model Version 5 (MM5) and the Weather Research and Forecasting (WRF) model as they are coupled to fully online chemistry. Both models may also be used in an “offline” mode. In online models, the chemistry is integrated simultaneously with the meteorology. Interaction and transport of meteorological, chemical, and aerosol species are calculated using the same numerical advection schemes and physical parameterizations with no need to interpolate in time and/or space. These models now include many atmospheric chemistry routines covering biogenic emissions, deposition, photolysis, and chemical mechanisms. In addition, atmospheric aerosol routines have been added, as well as a feedback from the chemistry to the meteorology. We will give an overview of some of the most important choices in the chemical and meteorological modules, and show how the air quality forecast models perform using some of these modules. In addition, we will summarize some of the differences seen when comparing online versus offline simulations using these models.