As an example, PPM usually means "sections for every million". If you find out that the concentration of NO2textual content NO _2NO2​ is equal to 1 ppm, it ensures that if you take a "sample" of air and divide it into 1,000,000 sections, one of them will encompass NO2textual content https://free-ppm-calculator15791.dm-blog.com/36708103/free-ppm-calculator-an-overview