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Nov 4 (Reuters) - Air Transport Services Group said on Monday it was being acquired by investment firm Stonepeak in a deal valued at $3.1 billion, including debt, sending the aircraft lessor's shares up 22.9% before the bell.

Stonepeak will pay $22.5 per share for the company, representing a 29.3% upside to Air Transport's closing price on Friday.

Reuters reported on Sunday that Stonepeak was in advanced talks to take over Air Transport, whose primary business involves acquiring used passenger aircraft and converting them to freighters to lease to its customers.

As of June 30, Air Transport had 114 freighter aircraft in service, with the significant majority being Boeing's 767 model.

It also operates freighters for Amazon.com's air cargo network. The online shopping giant holds warrants to purchase shares in the lessor.

(Reporting by Abhinav Parmar in Bengaluru; Editing by Pooja Desai)

((Abhinav.Parmar@thomsonreuters.com;))