In other air quality news, the LA Times examines the "weekend effect":
[O]zone, the main ingredient of smog, is becoming more common in Los Angeles and many other large cities on weekends, when big trucks and other heavy polluters are least active.
Known as the "weekend effect," the phenomenon has long perplexed scientists and air pollution officials, who remain divided over why ozone is so abundant Saturdays and Sundays.
Now, some scientists, armed with new research about the weekend effect, are suggesting that environmental officials may be putting too much emphasis on the wrong pollutant because they misunderstand how smog forms in the atmosphere.
Here are Joel Schwartz and Steven Hayward with an explanation.