On some planes flying in and out of Hartsfield-Jackson International Airport, the most precious cargo isn’t a first-class passenger. It’s actual cargo, and local leaders want more of it.

Atlanta officials pitched Hartsfield-Jackson’s strengths in air cargo to more than 4,000 industry representatives gathering this week at the Georgia World Congress Center for the International Air Cargo Association’s Air Cargo Forum, even as the industry sees the effects of a slow economy.

The city of Atlanta paid $300,000 to host the conference, part of a $1 million investment for the event co-hosted by Delta Air Lines, UPS and the Metro Atlanta Chamber.

Hartsfield-Jackson is the world’s busiest airport for passenger flights — but not for cargo, in which it ranks 33rd.

City officials say the stakes are large because attracting a weekly cargo flight generates an estimated economic impact of $5 million to $7 million.

Yet air cargo tonnage at Hartsfield-Jackson was down nearly 2.9 percent for this year through July, the most recent data available, amid weakness in world air cargo. That followed two full years of growth.

Growing air cargo has been a priority for Atlanta Mayor Kasim Reed because it generates jobs.

Hartsfield-Jackson officials on Tuesday announced Air Cargo Germany will launch service to Atlanta this month. The cargo carrier will operate two flights a week from Frankfurt to Atlanta with Boeing 747 freighters starting Oct. 11.

Separately, Hartsfield-Jackson said it is beginning to plan the addition of a fourth cargo building, a 100,000-square-foot facility that could open as early as 2014.