favored taxes over tradable permits in pollution control.

The total amount of the cap is split into allowances, each permitting a company to emit one ton of emissions. (You'd have to drive 2,400 miles, roughly the distance between New York and Las Vegas, to emit that much carbon dioxide.)

tradable permits be allowed to cut emissions more than required and ..

We will discuss emission charges and tradable discharge permits as two alternative forms of government intervention to control pollution.RationaleEconomics

Using Taxes and Tradable Permits - IMF