As plants and trees grow, photosynthesis –involving the interaction of sunlight, chlorophyll in green leaves, carbon dioxide, and water–results in the net removal of carbon dioxide from the air and the release of oxygen as a by-product.  Also, moisture is released to the air through evapo-transpiration.
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As seen in this illustration, carbon dioxide from the surrounding air is absorbed by the tree and converted to oxygen which is then released back into the air during photosynthesis.