Hertel, O., Ellermann, T., Nielsen, O.-K., Jensen, S.S., Plejdrup, M.S., Christensen, J.H., Nygaard, J., and Willumsen, T., 2021. Clean Air in Denmark - Dedicated efforts since 1970 - Challenges, Solutions and Results, Second Edition, 48p AU/DCE, Danish EPA and State of Green, Roskilde, Denmark 2021