Session 4 deals with motion. It is the moving air that transports and disperses pollutants. Often, people only look at how pollution travels horizontally, and how it affects those downwind. A more recent focus has been on the transport of pollutants vertically. Though we can't see them, there are lots of important chemicals that are placed in the upper atmosphere. To truly understand the long term effects of pollutants such as CFCs and greenhouse gases, we must be able to accurately model the upward movement of air.
One concern is the assumption of hydrostatic equilibrium for many air quality models. Assuming a hydrostatic atmosphere greatly simplifies calculations and can allow quick returns (sometimes several days faster!). But are these outputs accurate? We have said that the hydrostatic equation does not hold true for small scales. Can a hydrostatic atmosphere be assumed for heavily polluted urban areas? Not if one wants accurate results. Air quality modelers are currently wrestling with methods of improving accuracy in non-hydrostatic situations without giving up speed of output.
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