Just in case some of you have not read it yet, the Spring 1999 issue
of Communications Quarterly features a thought-provoking article by Dr
Robert Brown NM7M, on "Atmospheric Ozone". (pp.97-103)

Quoting from his conclusion: "...there can be little doubt that
atmospheric ozone plays a role in LF propagation because of its
connection to processes in the lower-D  region and the fact that ozone
can block UV radiation from reaching that altitude range at sunrise.
[...] ozone shadow has an effect, to some degree or other, every day
of the week, month or year, and requires no special solar or magnetic
activity."

73
Andre'  N4ICK