I've tried searching the forums and the web as well, but I haven't found any information that would help me determine just what changes would come about in changing the composition of a planets atmosphere. Are there any calculations, or even an application that can help? I'm basically wanting to be close to realistic in converting things such as CO2 into O2 and what the resulting percentage changes would be as well as the affect on temperature and atmospheric pressure.

Likely such things do not exist and I may be trying to hard to stay with plausible science, but I thought I'd ask.