[lwcamp]

First, a bit of data on CO2. A partial pressure of 8 kPa CO_2 will cause loss of consciousness within minutes. 5 kPa of CO_2 can cause dizziness, confusion, shortness of breath, panic attacks, and headaches. 2 kPa is mildly narcotic. 1 kPa is generally safe, but may cause drowsiness. For an 8-hour work day, the regulatory limit is around 0.5 kPa. For exposures under ten minutes, 3 kPa is considered safe, over 4 kPa can be physiologically tolerated for some time but are generally considered rapidly dangerous.

Now lets look at oxygen. Partial pressures of oxygen of around 50 kPa can cause inflammation of the lungs after several hours to about a day, with onset time decreasing as partial pressure increases. At more than 160 kPa, you start to see central nervous system toxicity as well, with several hours of exposure resulting in seizures. Low pressures are also dangerous. At partial pressures of about 16 to 15 kPa and below, you will often get altitude sickness, resulting in headaches, fatigue, and swelling. In serious cases, altitude sickness can lead to swelling of the lungs and brain which can cause death. Acclimation can help with this, but about 10 kPa of partial pressure is considered the limit of long term human habitation. 8 kPa is survivable for a few days at least, but is very taxing with digestion and sleeping becoming very difficult.

On a planet where sunlight strikes the ground, differences in absorption will lead to differences in pressure, which will in turn lead to turbulent mixing of the air - what we call weather. This means that all the gases get well-mixed and in this turbulent region the atmospheric composition will be essentially unchanged. What will change is the pressure. The ratio of carbon dioxide partial pressure to oxygen partial pressure will be near constant but the absolute values will decrease with altitude.

So we want a region where the partial pressure of oxygen is greater than 10 kPa and the partial pressure of carbon dioxide is 1 kPa. Let's assume that at some reference altitude the oxygen is at 15 kPa and the CO2 is at 0.75 kPa. This is breathable. If you descend in altitude to where the oxygen is at 20 kPa (near Earth values at sea level) the CO2 partial pressure climbs to 1 kPa. Below this altitude the CO2 starts to become toxic - but it will be quite mildly toxic unless the pressures get much higher. Still, this gives a range of altitudes in which you can breathe, and a range of altitudes below that where the CO2 starts to cause problems.

One additional complication is adiabatic heating and cooling as air rises and falls. In the above scenario, if the average temperature at our reference altitude is about 15 C (fairly comfortable if a bit warm), then where the air has a partial pressure of 20 kPa of oxygen the average temperature will be about 40 C (too warm for long term survival) and where the air has a partial pressure of 10 kPa the average temperature will be about -15 C (frozen). This is assuming dry adiabatic temperature changes - condensation will moderate it a bit, but you still have wide temperature swings. (EDIT - assuming Earth-like gravity and an Earth-like atmosphere except for about 1.5 % CO_2 concentration.)

Luke