By way of example, PPM means "sections per million". If you find out which the focus of NO2text NO _2NO2​ is equal to 1 ppm, it signifies that if you take a "sample" of air and divide it into 1,000,000 sections, one of these will consist of NO2text NO _2NO2​. https://online-ppm-calculator16047.blogminds.com/5-simple-statements-about-ppm-calculator-explained-33669403