Atmospheric Gas Concentration
So how do we measure the quantity of carbon dioxide in the earth's atmosphere? An important concept in the discussion about greenhouse gases is concentration and it is the way we measure the quantity of gases in the atmosphere. Concentration is simply how much of a particular stuff is in all the stuff. Confused? Well, a simple example would be a mixture of brown and white rice grains. The brown rice concentration would be the number of brown rice grains divided by the total number of rice grains (brown grains plus white grains). Concentration can be given in a variety of units with mass and volume being most popular. A more technical definition is that concentration is calculated by the fraction of the total of a subsubstance made up of one component. Concentrations have a variety of units, but for our purposes one of the most important is parts per "something" where "something" usually is thousands, millions, billions, or trillions. The following abbreviations are used for these units:
ppm = parts per million by weight, mass, or volume
ppb = parts per billion by weight, mass, or volume
ppt = parts per trillion by weight, mass, or volume
Sometimes, there is a "v" that follows - e.g ppmv - which means parts per million by volume.