[philipkglass]

We already know a lot about making megacities smog-free. That's why London and Los Angeles had absolutely dreadful air in the mid-20th century but it's much better now. As for why Delhi hasn't followed the example of richer cities in cleaning up the air, I'd say it is mostly due to lack of funds for infrastructure and enforcement.

There is also research about reducing the amount of CO2 already in the atmosphere. Here's one of my recent favorite papers: http://www.southampton.ac.uk/assets/imported/transforms/cont...

The reason that emissions-free energy sources get discussed more than carbon dioxide reduction from the atmosphere is that reducing emissions appears to be significantly less expensive and more scaleable for dealing with the bulk of the problem. Once you bolt on the extras necessary to make coal plants low-CO2, the electricity costs more than if you'd just eliminated coal altogether, so there's no point to it. For an example, see: https://en.wikipedia.org/wiki/Kemper_Project