As an example, PPM implies "pieces for each million". If you find out that the concentration of NO2text NO _2NO2​ is equal to 1 ppm, it means that if you take a "sample" of air and divide it into 1,000,000 sections, one of these will consist of NO2text NO _2NO2​. https://waylonlesfq.activablog.com/35862913/free-ppm-calculator-no-further-a-mystery