As an example, PPM implies "elements per million". If you find out that the focus of NO2text NO _2NO2​ is equal to one ppm, it signifies that if you are taking a "sample" of air and divide it into a million pieces, one of them will include NO2text NO _2NO2​. https://online-ppm-calculator06047.dm-blog.com/36703092/a-simple-key-for-online-ppm-calculator-unveiled