In other words, attenuation in air will also increase according to the ISL
attenuation should be proportional to the mass of air between the point of interest and the source. however, at the smaller scales of measurement you were referring to in this instance, on the earth, the inverse square law will also be significant in reducing the flux density the further you are from the source. Both effects count.
Of course, the difference between 60k and 40k is miniscule compared to the distance from Earth to the Sun, but given that a finite amount of radiation arrives at the limits of atmosphere, it will still decay from that level according to the ISL, as well as be the action of the atmosphere?
thats always true, however in this case that is very much a second order effect, compared to the atmospheric attenuation (because the delta distance from upper to lower atmosphere is small compared to the orbital radius of the earth about the sun).