I don't recall there being very many options. Its not something that you can easily control.

Your best bet would be to upgrade farmers to biochemists. Or what ever they were called. The improved version is supposed to be able to reduce pollution by a few % points. I think this was affected by one of their stats.

I think there was special project or 2 that could reduce pollution, but I don't know what they are. I think 1 might have been the baratak grove.

You could try destroying factory districts. Building those raises pollution and destroying them removes the pollution they produced.

I believe that there are artifacts that could reduce pollution, but they are rare. Atmosphere cleanser?

There were some random events that reduce pollution, but most were in response to high pollution and may negatively affect manufacturing.