The lower atmosphere's major gases (N2, O2, and inert Ar) are well-mixed (homogeneously distributed) up to the turbopause, unlike variable trace gases such as H2O, CO2, or N2O.
Below the turbopause (~100 km), turbulent mixing keeps the bulk atmospheric gases N2, O2, and Ar in constant, homogeneous proportion throughout the lower atmosphere, since...
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