DALLAS — Southwest Airlines Co. closed its $1 billion purchase of AirTran today, securing its spot as one of the biggest U.S. airlines and extending its reach beyond U.S. borders.

For Southwest and AirTran customers, nothing much changes right away. Travelers will continue to deal with whichever airline sold them their ticket. And policies such as checked bag fees will stay the same for now. AirTran charges for checked baggage, while Southwest allows two bags for free.

Southwest, which does not fly to the Portland Jetport, announced its intention to acquire Orlando-based Airtran Holdings last year. At that time, there was speculation that the move could result in added service to Portland, and perhaps new nonstop routes from the Jetport.

Airtran operates three daily nonstop flights from the Jetport to Baltimore and flights to Orlando on Saturday and Sunday. Southwest has yet to announce how the acquisition will impact service to Portland.

Bob Jordan, the Southwest executive who will run AirTran, said Southwest probably won’t do away with bag fees or AirTran’s first-class cabins or even repaint the planes in Southwest colors until at least next year. Southwest expects to get a single operating certificate from the Federal Aviation Administration in the first quarter of 2012.

By buying AirTran Holdings Inc., Southwest gains its first presence in Atlanta and at Reagan National Airport in Washington, plus additional gates at New York’s LaGuardia Airport.

AirTran employees attending an event at Southwest’s Dallas headquarters said they believe their jobs are safe because Southwest and AirTran overlap on only a few routes, reducing the likelihood of layoffs.

Southwest has said it will end AirTran flights at Dallas-Fort Worth International because Southwest operates many more flights at nearby Dallas Love Field. Southwest CEO Gary Kelly said his airline will study AirTran’s other locations but hasn’t yet identified any for closure.

Southwest pilots hope AirTran’s strength in the Southeast and flights to Mexico and the Caribbean will allow the company to grow, providing job security.

“Southwest is about as stable as it gets in the airline business, but adding Atlanta completes our U.S. market and makes us that much more stable,” said Brad Monda, a Southwest pilot.

The deal increases Southwest’s size overnight by about a quarter, as measured by passenger traffic.

Southwest valued says the deal that closed on Monday values AirTran at $1 billion. It used cash to pay for the transaction. Southwest expects to spend about $500 million on the integration. It expects to save more than $400 million by 2013 by eliminating overlapping functions.

Southwest shares fell 3 cents to close at $11.72.