One example is, PPM implies "sections per million". If you discover out which the concentration of NO2text NO _2NO2​ is equivalent to 1 ppm, it means that if you're taking a "sample" of air and divide it into a million areas, one of these will consist of NO2text NO _2NO2​. https://onlineppmcalculator52973.liberty-blog.com/36845145/how-much-you-need-to-expect-you-ll-pay-for-a-good-online-ppm-calculator