For instance, PPM means "sections per million". If you find out which the concentration of NO2text NO _2NO2​ is equal to one ppm, it ensures that if you are taking a "sample" of air and divide it into a million parts, one of these will include NO2text NO _2NO2​. Sections https://ppm-calculator07272.uzblog.net/free-ppm-calculator-an-overview-50346021