As an example, PPM implies "sections for every million". If you discover out which the concentration of NO2text NO _2NO2​ is equal to one ppm, it implies that if you take a "sample" of air and divide it into a million components, one of these will include NO2text NO _2NO2​. https://freeppmcalculator71356.blogitright.com/36900402/ppm-calculator-things-to-know-before-you-buy