10 ppm/10 ppmv denotes 10 parts in 1,000,000 parts, or 0.001%. 100 ppm/ppmv denotes 100 parts per million, or 0.01 percent. The abbreviation 1000 ppm/ppmv stands for 1000 parts per million, or 0.1 percent. It is important to be aware of which notation is being used, as the word "million" differs in each, that is, American and European notations. In America, a million refers to a thousand millions, or 10^6; but in Europe it refers to a billion, or 10^9. The number 10 in front of the ppm symbol means that each million parts of air contains 10 particles per million parts of air. This can also be written as 100,000,000 particles per million particles. A particle is any atom or molecule that has an atomic mass greater than 5.5 u. In this case, the only elements present in the atmosphere are oxygen (16 g/mol) and nitrogen (28 g/mol). A mole of either of these gases contains 32 grams of atoms or molecules. Therefore, each mol of gas contains about 10^6 particles. In conclusion, 10 ppm means that there are 10 million particles per million particles of air, or 100,000,000 particles overall. This is such a small number that it can be written as a percentage: 0.001%.