By way of example, PPM implies "pieces per million". If you find out which the concentration of NO2text NO _2NO2​ is equal to one ppm, it ensures that if you take a "sample" of air and divide it into one million pieces, one of these will include NO2text NO _2NO2​. https://josueiddbh.bloguetechno.com/new-step-by-step-map-for-free-ppm-calculator-71989096