Would it be passable for each country to build a big enough machine to clean and replace it's dirty air?l?

Actually, instead of one large machine, people, businesses and communities are now starting to build "Green". Some organizations or commumities set new standards to how buildings must be constructed or maintained, so as to be sustainable and environmentally friendly. These buildings will take air in from outside for ventilation, air conditioning, etc., purify it for the indoor air, and repurify it before exhausting it back outdoors - thus the outdoor air becomes cleaner than it was when it was going into the building. Read more about being "Green" at the following links.

Assuming that such a piece of equipment worked, there would be no economic incentive to do it. There is no money to be made by cleaning the air.

The only practical way to get to clean the air is to dis-incentivise conduct that pollutes by making pollution more expensive than ecologically friendly practices.

Lets say if your car gets less than 15 MPG - you have to pay a $1000/ month gas guzzler fee. Heating fuel for a home can be taxed on the basis of an allowance of X gallons of fuel per month for a home of Y square feet. If you go over the allowance you pay an excess use fee of $100.month. There are many ways to disincentivise anti-social behavior which is exactly what pollution is.

All we need is a congress with the fortitude to enact the legislation. That's not likely to happen. Those congressmen who vote for such measures will simply not be reelected.

On the other hand, if pollution is made not just a socially reprehensible act but is made a felony, then we are in position to deprive pollutors of their vote. They would have no political voice and thus no way to intimidate congress into protecting their conduct.

No. Unless it is several strategically placed. There is too much air and it recycles itself around the world. Natural processes clean the air, all we need to do is help it.