How Can Smog in Metropolitan Cities like Delhi & Beijing Be Reduced?
The first thing to do is to stop putting pollution in the air, particularly very small particulate matter that are produced by combustion.

For the Beijing Olympic Games of 2008, the Chinese government forced the polluting plants and factories in the vicinity of Beijing and nearby areas to stop operating for weeks prior to the games and during the games to ensure the air in Beijing was clear and had minimal pollution during the duration of the games. The measure was successful.

Burning wood, animal dung, coal, petroleum products (particularly diesel and heavy oil) produce soot and other small particulate matter which leads to poor air quality and smog.

This is an easy solution that results in clean air. The difficulty with this is that it is detrimental to the economy in the short term. For a longer term solution, energy sources need to be changed to ones that do not cause air pollution.