Don Klipstein wrote:

Right. You can put almost any (relatively clear) water in a clear
plastic bottle, put the bottle in the sun for a few hours, and,
presto, sterilized water.

Except that the main germicidal wavelngths of UV (UVC of wavelength
around 240-280 nm) are highly blocked by the ozone layer even when the
ozone layer is weakened, and these wavelengths also have a high rate
of being blocked by plastics that are transparent when thick enough
to make bottles from such plastics.

The blockage may be high, but the resulting radiation is sufficient. At
least according to all the studies I've been able to find.