Is the amount of oxygen in the air always the same?if yes,how?or if not, how not?
No, The amount of oxygen in the air is not same, it varies with height
Yes, and throughout history, the amount of oxygen fluctuates. Mass extinction of plants reduce oxygen.
Oxygen is a very important component in the atmosphere. It is needed by living things to breath and live. But, the pollution in the environment is drastic and a possible future climate change occur. Conventional thought is that “green automobiles,” such as hybrids and electric automobiles, are better to buy because they pollute less than traditional automobiles. However, the electricity starting place matters a lot. Power source affects environmental impact of electric vehicles and can add up to lessen oxygen in the surroundings.
You can probably ignore nicoleannB's statement without losing anything of value.
It varies. As Xidike stated, it varies with elevations above the surface and it varies with proximity to sources and sinks. It this, it behaves like almost every other atmospheric gas.
I seem to be the only one who interpreted the OP's question as 'Is the amount of oxygen in the air always the same over time?'
Here's an article on the geological history of atmospheric oxygen.
Side note: A lot of people don't realize that, biologically, oxygen is very poisonous - it burns up living organisms. A billion years of life on our planet made early life (blue green algae) so successful that accumulation of its own waste oxygen polluted the planet almost to death - the largest extinction event in Earth history.
But new lifeforms evolved to protect themselves from this poison and eventually use its chemical energy to gain an evolutionary advantage. It went on to become all the multi-celled lifeforms we see today (almost anything more complex than algae).
Humans are not the first lifeform on the planet to pollute themselves to the brink of extinction.
Again, the answer is no. As long as sources and sinks of atmospheric oxygen exist, the amount present in the Earth's atmosphere will vary from time to time and from place to place.
Let us not confuse the actual amount of oxygen in the atmosphere (reality) with some published estimate of the mean amount (some scholar's estimate of reality).
Well, that wasn't a question My links are to references that show changes in oxygen levels over the aeons.
At least we're not worse organisms than algae. Well, that's reassuring.
Historically, the amount of oxygen in the atmosphere fluctuates, but takes a large frame of time (such as a hundred million years) to show a noticable difference.
The fluctuation was anywhere from 60-80% of current levels to 120-150%, depending on plant life and volcanic activity.
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