As an example, PPM implies "parts per million". If you discover out that the focus 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 areas, one of them will include NO2text NO _2NO2​. https://online-ppm-calculator86284.goabroadblog.com/35716481/top-latest-five-online-ppm-calculator-urban-news