1. Sodium sulfite has obvious reducing ability. It is easy to be oxidized when it encounters oxidant.
2. Oxygen in the air can oxidize sodium sulfite:
3. Other unknown oxidants in the air may also oxidize sulfurous acid.
If conditions must be said, it is exposure in the air.
The rate of oxidation is not too fast.
The condition for sulphur dioxide to be oxidized by oxygen in the air is contact with catalytic substances such as dust.