I have made a shiny app, the code for which is in this repository, that explains the methods used to create a PAR data product and why they are an improvement over the previous methods. Please visit: https://shiny.ethanholdahl.com/PAR-Daily-Average/
The results of this method was used as a key input for Binder, Seth, Holdahl, Ethan, Trinh, Ly, and Smith, John H. "Humanity's Fundamental Environmental Limits", Human Ecology, 2020, DOI 10.1007/s10745-020-00140-w. The code for the main model of the paper has its own repository and the data for it, including this PAR data product is hosed on Figshare.