As an example, PPM signifies "sections for each million". If you find out that 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 1,000,000 components, one of these will include NO2text NO _2NO2​. https://ppmcalculator92457.ampedpages.com/ppm-calculator-things-to-know-before-you-buy-63601584