Posted by Ashley on .
Atmospheric pressure is simply the weight (remember Newton’s Laws) of the atmosphere pushing down
divided by the area upon which the atmosphere is pushing. The metric units of pressure are Newtons/m2
(also called a Pascal; you might also have heard of pounds per square inch [PSI], which is the English units
equivalent).
The surface (atmospheric) pressure on Venus is around 90 bars (surface pressure on the Earth is 1 bar, by
definition). Venus’ atmosphere is 96% CO2, compared to 0.035% in the Earth’s atmosphere. How much CO2
is in the Earth’s atmosphere? To simplify this calculation, you can assume that the Earth’s atmosphere is
uniformly dense and has a height of 10 km (this is the Earth’s atmospheric scale height.). The density of air
is around 1 kg/m3.

Astronomy 
drwls,
You don't need to know the Venus data to answer this problem.
The TOTAL CO2 on Earth is
(Earth area)*10^4 m*(air density)*3.5*10^4,
assuming 3.5*10^4 is the mass fraction of CO2. That is obsolete data, and whoever provided the problem should have stated if it was a mass or volume fraction.
Actually, 3.8^10^4 is the current mole or volume fraction of CO2. The value of the present CO2 MASS fraction is 5.8*10^4
You will need the Earth's surface area to complete the problem.