The exponential decrease of atmospheric humidity with height is a well-established physical principle of the Earth's troposphere, corresponding to the exponential decrease of pressure and temperature with altitude. No citation is needed for this foundational atmospheric observation.
The claim states that atmospheric humidity decreases exponentially with height above the Earth's surface. This is a standard, fundamental principle of atmospheric science dictated by the exponential drop in pressure and air density (scale height). While paper [2] discusses scale height datasets for tropospheric parameters, the exponential decline of atmospheric water vapor/humidity with altitude is a basic, textbook physical property of the atmosphere that does not require citation as a contested empirical finding.