Dr Solomon discovered that chemical reactions destroy ozone on surfaces such as the ice crystals within high-altitude 'polar stratospheric clouds' as well as on other atmospheric particles high in the atmosphere.
Usually, the prevailing south-westerly wind would take the ash cloud away from Europe and over the polar region, but a high-pressure area is bringing it over Europe, where it is sitting and barely moving as the ash spreads.
These astral connections create flux transfer events (we've got Doc Brown's attention) -- high-energy particle flows responsible for, among other things, the eerie twinkling of the polar auroras.