Even though this week's clouds and sprinkles may have kept pollution down briefly, ozone has made a comeback in September. Six of the first seven days had ozone problems.
But did you experience the ozone comeback at your house?
If you live in Clovis or Arvin, yes. If you live in northwest Fresno or Visalia, not so much.
Which brings me to a question I get all the time. If the San Joaquin Valley has exceeded the federal eight-hour ozone standard 68 times this summer, does that mean all those bad days happened at my house?
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The Valley is 25,000-plus square miles. It's huge. This is the largest air district in the country.
On plenty of summer days, you'll see only one or two monitors show a problem. On Sunday, Clovis and Arvin had bad days, but the rest of the Valley did not.
That still counts as a bad day for the whole Valley. Why?
Scientists and regulators have long told me that pollution drifts in this big basin. Arvin, which has few pollution sources compared to larger cities, is downwind of Bakersfield and other parts of the Valley.
When Arvin goes off -- or Parlier or Sequoia National Park or any other place -- the whole Valley is considered out of compliance.
Let's go back to the question about your house and raise another question. Which place n the Valley has been having the biggest ozone problem this summer?
So far, it has been the usual suspects: Sequoia National Park with 42 exceedances and Clovis with 41.
There's a pretty obvious reason for Clovis. It is downwind of Fresno' major freeway commutes. But Sequoia is another story.
The monitor is at Ash Mountain, the southern entrance at Highway 198. It gets pollution from the Valley. At 1,500 feet, the elevation is where ozone pollutants often hang in the air.
One other thing, vehicle exhaust in cities actually helps break up ozone when the sun goes down. That doesn't happen in Sequoia, so ozone levels often will remain higher overnight than they are in cities.