By way of example, PPM suggests "areas for each million". If you discover out the focus of NO2textual content NO _2NO2​ is equivalent to 1 ppm, it ensures that if you're taking a "sample" of air and divide it into 1,000,000 areas, one of these will encompass NO2text NO _2NO2​. https://ppmcalculator17158.blogzag.com/80170865/free-ppm-calculator-an-overview