35 ILCS 200/10-90
In counties with 200,000 or more inhabitants, in addition to valuation as otherwise permitted by law, upon the filing of an application under Section 10-95 by the person liable for the taxes on that property, which is used for airport purposes and has been so used for the 3 years immediately preceding the year when the assessment is made shall be valued on the basis of 33 1/3% of its fair cash value, based upon the price it would bring at a fair, voluntary sale for use by the buyer for airport purposes.
Property is considered used for airport purposes under this Section if it is devoted primarily to the operation of an airport or restricted landing field approved by the Department of Transportation in accordance with the Illinois Aeronautics Act and is open to the public except as restricted by the Department of Transportation or the Illinois Aeronautics Act.
35 ILCS 200/10-90