One example is, PPM indicates "pieces for each million". If you find out which the focus of NO2text NO _2NO2​ is equal to 1 ppm, it ensures that if you are taking a "sample" of air and divide it into 1,000,000 components, one of these will encompass NO2text NO _2NO2​. https://ppm-calculator15814.blogdigy.com/how-much-you-need-to-expect-you-ll-pay-for-a-good-online-ppm-calculator-56088932