You are almost totally correct when you asked:
What are the reasons for that? Is it simply because there are more air molecules that absorb the radiation?
The Ozone Layer blocks most of UVB, but does not affect the amount UVA entering the atmosphere (as seen in the illustration below):
(Image source)
But there is a little more to it than that. From NASA's Earth Observatory's webpage What Determines How Much Ultraviolet Radiation Reaches the Earth’s Surface?, elevation's role in attenuating UVB (and by similar processes, UVA):
high elevations UV-B radiation travels through less atmosphere before it reaches the ground, and so it has fewer chances of encountering radiation-absorbing aerosols or chemical substances (such as ozone and sulfur dioxide) than it does at lower elevations.
The aerosols absorb and scatter incident UV radiation. So nearer to the surface, there is a greater concentration that the light must pass through - and as the altitude decreases, the UV radiation path is therefore increased, so the further 'down' in altitude, the more the air mass and an increased presence of aerosols absorb and scatter solar UV radiation.
In respect to the aerosols in area you have specified (the Dead Sea), according to the first link in your question, The Analysis of the Ultraviolet Radiation in the Dead Sea Basin, Israel (Kudish et al. 1997), they state
the air above the Dead Sea is characterized by a
relatively high aerosol content due to the very high salt content of the Dead Sea
Their results indicate that
The relative attenuation in the ultraviolet range as a function of wavelength, i.e.
site-specific spectral selectivity, decreases with increasing wavelength.
thus, indicating that the attenuation is also wavelength dependent.
