For example, PPM suggests "components per million". If you discover out the concentration of NO2textual content NO _2NO2​ is equivalent to one ppm, it implies that if you take a "sample" of air and divide it into a million parts, one of them will consist of NO2textual content NO _2NO2​. https://damienkqqqr.ltfblog.com/35495402/examine-this-report-on-online-ppm-calculator