"Atmospheric oxygen" refers to the oxygen that is present in the Earth's atmosphere. It is the oxygen that we breathe and is crucial for the survival of living organisms, including humans and animals. Full definition
How else could one explain how the high level of atmospheric oxygen and vast coal and oil reserves. (green.blogs.nytimes.com)
At the same time, the plants boosted atmospheric oxygen levels, thus paving the way for the evolution of complex animals. (scientificamerican.com)
The results of this comprehensive study suggest that atmospheric oxygen during most of the past 220 million years was considerably lower than today's 21 percent. (sciencedaily.com)