As an example, PPM indicates "parts for every million". If you find out which the concentration of NO2text NO _2NO2​ is equivalent to 1 ppm, it implies that if you take a "sample" of air and divide it into a million parts, one of them will consist of NO2text NO https://marioiwhtb.theobloggers.com/43092436/ppm-calculator-no-further-a-mystery