Such as, PPM indicates "elements per million". If you discover out the concentration of NO2textual content NO _2NO2​ is equal to 1 ppm, it means that if you take a "sample" of air and divide it into one million sections, one of them will consist of NO2textual content NO _2NO2​. https://ppmcalculator70246.targetblogs.com/37146599/the-basic-principles-of-free-ppm-calculator