Such as, PPM usually means "parts per million". If you find out that the concentration of NO2text NO _2NO2​ is equivalent to 1 ppm, it means that if you take a "sample" of air and divide it into one million areas, one of these will encompass NO2textual content NO _2NO2​. https://fernandoxocoz.tribunablog.com/about-online-ppm-calculator-51055486