As this is for a project and it sounds like an ongoing experiment, I will try to help point you in a direction to answer your questions. You mention that vitamin C oxidizes, I assume you mean that the bell pepper will oxidize ("go bad"). It is important to distinguish between the bell pepper oxidizing and what happens to vitamin C, as the bell pepper oxidizing might not mean the vitamin C is oxidizing. Vitamin C refers to different forms of the vitamin, some of which are oxidized in their useful form. Anyway, you are onto something in that temperature is influencing the amount of vitamin C. Have you tried looking at a few different temperatures? How does the vitamin C content compare for peppers that are frozen vs ones stored in a refrigerator vs ones stored at room temperature vs ones that are heated? Our intuition tells us that refrigeration helps keep vegetables for longer than if we kept them on the counter, and that sometimes putting vegetables (which contain water) in the freezer can result in freezer burn.

Therefore, temperature definitely plays a role, and I encourage you to explore these ideas to help refine your hypothesis as to what happens at different preservation temperatures for bell peppers. Good luck!