One example is, PPM signifies "areas for every million". If you find out which the focus of NO2textual content NO _2NO2​ is equivalent to one ppm, it signifies that if you're taking a "sample" of air and divide it into 1,000,000 areas, one of these will include NO2text NO _2NO2​. https://onlineppmcalculator38269.isblog.net/the-5-second-trick-for-online-ppm-calculator-53797193