For example, PPM suggests "components for every million". If you find out the focus of NO2textual content NO _2NO2​ is equivalent to one ppm, it implies that if you're taking a "sample" of air and divide it into one million parts, one of them will include NO2textual content NO _2NO2​. https://andersonwdcxt.azzablog.com/36964275/free-ppm-calculator-no-further-a-mystery