Parts per what?
We're not going to worry about how Keeling's knobs, dials, and tubes worked, but... what units do you think Keeling used to measure CO2? In other words, if you took a 10 cubic meter box of atmosphere, how would you report the CO2 you measured? gallons? cubic centimeters? pounds? grams?
None of these seems quite right. You can’t use gallons or cubic centimeters because the volume of the CO2 will change, depending on how close together the molecules pack. And CO2 (like all atmospheric molecules) is very light, so pounds or grams don’t seem very useful.
Instead, think about what you know about the atmosphere – it is made of molecules of gas in constant motion, mostly nitrogen (N2) and oxygen (O2). Use the applet below to find the correct ratio of nitrogen to oxygen. Then click on the “Show me the CO2” button to see how much CO2 is present.
Wow, that’s not much CO2. In fact, for every CO2 molecule in the atmosphere, there are about 3000 other molecules (almost all of which are O2 and N2).
So, the way we measure CO2 is in “parts per million” – i.e., how many CO2 molecules are there for every million molecules in the atmosphere. By contrast, we measure O2 and N2 in parts per hundred (or percent)– about 20% O2, and 80% N2.
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