QUIZ YOURSELF ON THE 12 TYPES OF VERB TENSES!
How to use ppm in a sentence
Prior to the Industrial Revolution of the late 19th and early 20th centuries, the carbon dioxide level was about 280 ppm.
Today global atmospheric carbon dioxide concentrations are at 395 parts per million (ppm).
Atmospheric carbon-dioxide levels have risen 38% in the industrial era, from 280 to 385 parts per million (ppm).