For instance, PPM suggests "elements per million". If you find out which the focus of NO2textual content NO _2NO2​ is equivalent to one ppm, it implies that if you are taking a "sample" of air and divide it into 1,000,000 elements, one of them will encompass NO2textual content NO _2NO2​. https://free-ppm-calculator49246.angelinsblog.com/35776651/rumored-buzz-on-ppm-calculator