By way of example, PPM usually means "pieces per million". If you find out the concentration of NO2text NO _2NO2​ is equivalent to 1 ppm, it signifies that if you're taking a "sample" of air and divide it into a million parts, one of these will include NO2text NO _2NO2​. https://jasperxpair.estate-blog.com/35759221/the-free-ppm-calculator-diaries