About Atmospheric Extinction

Let's talk about Atmospheric Extinction today and in the past.  In UrukFSP I currently use Young's (1994) model which leads to an Air-Mass value of 40 on the horizon. Actually, there are even more recent models, but they do not seem to me to bring significant changes to the evaluation of the phenomenon. Rather, the quantification of the K coefficient is important, which enters the calculations as an evaluation of the local sky. There is a paper published in 2011 in Astronomy & Astrophysics which analyzed what we can consider perhaps the "best" current sky in the world from an astronomical point of view, in the Chilean Atacama Desert. The link to this document, publicly available, is this:

https://www.aanda.org/articles/aa/pdf/2011/03/aa15537-10.pdf 

Long story short, however, I would say that if we consider the center of the visible spectrum, Atacama could be assigned a value of K = 0.18. Perfect sky, in short.
However, in the past, and by past I mean the entire period before the Industrial Revolution of the 19th century, there was a lower presence in the atmosphere of fine dust and aerosols, which have the property of making the earth's atmosphere less clear and therefore capable of decreasing the average incidence of atmospheric extinction. At this point I preferred to be able to set K even to minimum values ​​of 0.10, in order to simulate reliable behavior in the "best skies" of the past. Additionally, the presence of dust and aerosols in today's atmosphere also has the effect of increasing refraction at low altitudes, but that's another story.