Cosmic rays and high energy particles can be counted with closed chambers or scintillation counters. Atmospheric absorption can be predicted from air density, so it is probably easier to monitor closer to sea level and at a few thousand feet, then predict absorption. A balloon is not really needed.
Immediate incident UV radiation is best measured directly using a semiconductor diode detector, there is no need to measure it indirectly using a chamber. It is the simplest single parameter to record.
Atmospheric chemistry will be effected by the availability of ionised molecules in the air. That ion charge density can be measured from air flow through an open chamber. An electric field separates the ions and so measures the ions as current. It is a general technique that will indicate ionised pollutants, ozone and the effects of UV and ionising radiation.
Maximum natural ozone concentration can be expected near 25km.The biggest problem with any electronics will be the typical temperature of minus 57°C found between about 5km and the maximum balloon altitude of 30km.
Above 30km the temperature gradually rises again to near zero°C at 50km, along with a very significant increase in ionisation. But the balloon will not get there.