The chemical composition of the homosphere being relatively uniform due to constant turbulent mixing is a well-established textbook fact of atmospheric science that does not require citation.
The claim states that Earth's air composition is chemically homogeneous throughout the homosphere (due to turbulent mixing). This is a well-established and basic scientific fact about the atmosphere's structure (the homosphere is defined precisely by this chemical homogeneity, unlike the heterosphere above it). According to the prompt instructions, a statement that is trivially true by definition or direct scientific observation is common knowledge, meaning no citation is needed and the balance verdict should be COMMON_KNOWLEDGE.