Ridership on Alaska Airlines' flights to and from Santa Rosa grew 3 percent last year to nearly 214,000 passengers, a jump attributed largely to added flights last summer to Southern California.

News of the increased ridership comes as officials are preparing to break ground this year on a $53.8 million expansion at the Charles M. Schulz-Sonoma County Airport. The project, estimated to take more than a year, will lengthen runways and could result in flights by more airlines.

Part of last year's increase in ridership occurred because Alaska in June added a third flight each day to Los Angeles. The airline also ended service from Santa Rosa to Las Vegas but began a daily flight from Sonoma County to San Diego. The San Diego flights started strong with nearly 90 percent of the seats filled last summer.

"That market did very well for us," said John Stout, manager of the county-owned airport.

The third daily flight to Los Angeles ended in November but is expected to resume again this spring or summer, he said.

Alaska and airport officials last summer celebrated serving 1 million passengers in the last five years. Horizon Air, Alaska's sister carrier, began flights to Sonoma County in March 2007. The service now connects Santa Rosa with Portland, Seattle, Los Angeles and San Diego.

The average round-trip fare from Santa Rosa rose to $284 in the second quarter, an increase of 5.7 percent from a year earlier, according to the U.S. Department of Transportation. The average U.S. domestic fare for the same period increased 4.1 percent to $385.

The airport expansion originally had been planned for last year but was delayed due to federal environmental regulations. Stout said he hopes soon to meet all those rules and to put the project out to bid this winter.

Under that schedule, work would begin around May and be completed by October 2014.

As part of the project, the county will spend about $20 million for environmental mitigation work. That includes offsetting the loss of possible habitat for two endangered species: the wetland flower Burke's goldfields and the California tiger salamander.

Airport officials met with United Airlines staff last fall about the prospect of starting flights between Santa Rosa and Denver. Stout and his staff previously have talked about routes to Denver, Salt Lake City or Phoenix with Frontier, Delta and US Air.

The airlines have expressed interest, but they first need the airport's main runway expanded to a length of 6,000 feet. County business officials have said such east-west service would further boost the region's economy.

"We'll continue to keep in touch with them," Stout said of the airlines.

You can reach Staff Writer Robert Digitale at 521-5285 or robert.digitale @pressdemocrat.com