compostion and volume of atmosphere

During the past century, with the impact of pollution and burning of vast quantities of petroleum and coal, two questions arise.

  1. How has the composition of the atmosphere changed?
  2. What about the volume of the atmosphere - has it decreased or increased in volume? Has the barometric pressure changed?

The concentration of carbon dioxide has increased from about 280 parts per million before the industrial revolution to about 360 parts per million today. Production of carbon dioxide by burning carbon requires that oxygen be removed from the atmosphere. But since there is about a thousand times more oxygen than carbon dioxide in the atmosphere, the decrease in oxygen is probably inconsequential.

I wouldn’t expect the volume of the atmosphere to change much. For every molecule of carbon dioxide produced, one molecule of oxygen is removed from the atmosphere, and to a close approximation each molecule of gas takes up the same amount of space. But each molecule of carbon dioxide is heavier than a molecule of oxygen, so (assuming no other changes take place to the atmosphere), the mass of the atmosphere has increased slightly. My calculation has the mass of the atmosphere increasing by about 0.1 percent (one part in a thousand) because of the increase of carbon dioxide. Air pressure will have risen from, say, 30 inches of mercury to 30.03. Normal daily variation is much greater than that.

I don’t know if such an increase in average air pressure has actually been measured. Such a measurement could easily be confused by variations in water vapor and methane (both of which are lighter than air).

Okay, so maybe I’m not perfect after all. :o I left out a division step. The increase in mass of the atmosphere and of the pressure should be more like 150 parts per million, not the 1000 parts per million I stated above. Pressure would have increased from, say, 30 inches of mercury to 30.005 inches. Of course, I could be wrong again.

The atmosphere also expands and contracts according to the ~11 year solar activity cycle. The effect is strong enough to increase drag on satellites in low earth orbit.