Alaska Airlines will add four more converted Boeing 737-800 freighters to its fleet, raising to nine the number of all-cargo aircraft the company operates.

The additional freighters will be dedicated to serving Alaska and Hawai’i, not any specific route, Tim Thompson, the company’s director of public relations, said in an email July 22.

Alaska operates a weekly cargo flight into Petersburg on Wednesdays, using a Boeing 737-700, which is slightly smaller than the 800 series…