Why has the air quality worse in Pierce and Snohomish counties than King County?
Two things impact air quality: weather patterns and pollution sources. There hasn't been any significant variation in the weather pattern over the past week in the Puget Sound area. The difference is that there's been significantly more wood-burning in Pierce and Snohomish counties. The particulate matter and gases released led to unhealthful air quality in both of those counties; King county experienced moderate air quality.