Sonoma County residents were slapped with 108 of the 346 warning letters issued during the Spare the Air season that ended Wednesday.

That was the most of any jurisdiction in the nine-county Bay Area.

When the Bay Area Air Quality Management District catches someone violating the rule, a warning letter is issued. A second instance will result in a notice of violation and a $400 fine.

Two Sonoma County residents were fined $400 for a second violation, said Aaron Richardson, a district spokesman.

Overall, the agency issued 15 Spare the Air bans, the most in five years. The high number of alerts, during which home wood fires are prohibited, was brought on by dry, cold, stagnant weather that trapped air pollution throughout the Bay Area.

Also, there were 10 days that exceeded the national air quality health standard and another nine days that nearly exceeded the standard.

Some 409 complaints for possible Spare the Air violations were issued in Sonoma County. There were 3,777 for the entire Bay Area, significantly more than the 1,453 complaints in 2008-2009; 2,355 in 2009-2010; and 1,373 a year ago.

Still officials said there is less wood-burning taking place.

"It was a tough winter for air quality and public health, but things could have been much worse," Jack Broadbent, the district's executive officer, said in a statement. "Wood burning overall is decreasing, but we still see unhealthy levels in certain neighborhoods throughout the Bay Area."

Air pollution from wood smoke in the Bay Area has decreased 15 percent in the past five years based on new air quality monitoring data, officials said.

The agency said that wood smoke is the largest source of Bay Area wintertime air pollution and contains carbon monoxide and particulate matter. The latter is linked to respiratory illnesses such as asthma, bronchitis and lung disease.

Though the season of Spare the Air alerts has ended, the wood-burning rule requires year-round that residents who burn in a fireplace or outdoor fire pit burn dry, seasoned firewood rather than garbage, leaves or other material that causes excessive smoke. Those who violate this provision can still be ticketed throughout the year.