The Earth's atmosphere uses the three cell model which defines the movement of air from high to low density areas. Hadley, Ferrel and Polar cells all work in varying capacities as part of this model. The Hadley cell transports air from the equator to the tropics, creating rainforests and deserts. The Polar cell involves descending cold air that meets rising air masses, forming the Ferrel cell. Limited mixing occurs between hemispheres and cells, but within a cell, mixing happens more easily. This three cell model does have implication for pollutant gradients and temporal scales, affecting local, regional, and global atmospheric chemistry.
For us to be able to understand the variations in environmental chemistry of the atmosphere, we must always place them within the context of the physical systems in which they operate.
A simple starting point for this is the simplified three-cell convection model of the earth’s atmosphere. This gives us an idealized structure of general air movement and its characteristics at a global scale - all driven by the simple principles of air moving from areas of high density to areas of lower density.
If we start from the equator, where solar insolation is at its greatest, we see warm, wet air rising rapidly - much of the equator, after all, lies on the ocean. As the rising air cools, it condenses and renders these regions with very high levels of precipitation - the location, of course, of much of the world’s rainforests.
The air mass cannot move vertically beyond the thermal boundaries in the atmosphere and so is forced north, in the northern hemisphere, and south, in the southern hemisphere, increasingly further into higher latitudes. On this journey, the now relatively dry air cools, increases in density, and so begins to descend back to earth, creating bands of high pressure roughly coinciding with 30 degrees north and south of the equator. This is, of course, where we find the great deserts of the earth, such as the Sahara. This air movement from equator to tropics defines the Hadley cell.
At the other end of the globe, in the two polar regions, we see cold and dense air descend and move away from the poles. As it decreases latitude it gradually warms, as levels of insolation increase, and begins to take on more moisture. At approximately 60 degrees latitude it meets the air masses from the south; air rises and so we define the Polar cell.
Between the two we see the Ferrel cell, driven by the thermally direct Polar and Hadley cells either side, it moves like a cog between two gear wheels to the north and south of it.
There is, of course, much more complexity to this global scale system, but for our needs it serves to highlight a very important point: mixing of air between hemispheres and between cells is relatively limited, as they are to a large extent isolated from each other by the cell structure. But mixing within a cell can happen relatively easily. This is enhanced in the east-west mixing within a cell by the various winds, such as the Trade winds or Westerlies, that are deflected from north and south by the coriolis force caused by the spinning of the earth. It can take only 1-2 weeks for air to mix laterally across the planet within a cell, but 1-2 months to mix between cells, or more than 12 months to mix between hemispheres.
When we consider that the large majority of the global population, and the major global polluters, are located within the same atmospheric band, this equates to huge global gradients of short-lived pollutants.
The temporal scales are important here for a number of reasons. Short-lived radicals have limited temporal or geographical range, but they cause local problems in areas of emission.
Moderately long-lived species, like CO or SO2 can move over 10s or 100s of kilometers over months to impact areas they were not emitted from.
The long-lived species, such as some of the halogenated compounds, have the potential to mix globally and also vertically into the stratosphere causing long-term changes to atmospheric chemistry.