Brewer Dobson circulation takes Aerosols up to between 25-30 km's therefore Volcanic Aerosols cannot explain upper Stratospheric (40-50km) cooling.
Note cooling of 4 degC over 16 years at 50 km.
Ozone depletion can explain upper Stratosphere cooling, ironically this means accepting that Humans can have significant affect on the Atmosphere via the
reduction of a Greenhouse gas.
Increased Tropospheric CO2 can also explain all levels of Stratospheric cooling and was predicted by Ramanthan in 1968, there is no reason why both CO2 and 03 do not have an effect.
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Another possible effect of a expanding, cooling outer atmosphere or increasing Methane emissions, Noctilucent Clouds, which have been increasing.