Not anymore.

Our air may never become as bad as theirs once was. But right now, in many ways, ours is worse than theirs.

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The reason? During the 1990s, as Southern California's air grew dramatically cleaner, the Valley's remained about the same – or grew smoggier.

This is a tale of two cities.

Azusa lies in the San Gabriel Valley northeast of downtown Los Angeles, where coastal breezes sweep the emissions of a megalopolis into a steep mountain range and sunshine cooks them into ozone.

Clovis lies northeast of Fresno's vehicle-dominated sprawl. Prevailing winds swirl the urban area's emissions over the flatlands abutting the Sierra foothills, where they, too, undergo a sun-driven transformation.

As the 1990s began, Azusa held a firm grip on its dubious distinction as one of the nation's smoggiest places. Its average daily one-hour ozone peak in summer 1991 was a choking 112 parts per billion. On many days, readings exceeded 120 ppb, the one-hour federal health standard.

"When I used to drive east of downtown L.A. on a smoggy afternoon, my eyes would water, the air pollution was so strong and thick," says Tim Carmichael, executive director of the Santa Monica-based Coalition for Clean Air.

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Back then, the daily ozone peak in Clovis was far lower, averaging 75 ppb. But levels in Azusa were beginning to drop. In Clovis, they were rising – and they haven't stopped.

Sometime in the mid-1990s, the two cities switched places. And since, Clovis has had higher average ozone readings than Azusa. Clovis still isn't as bad as Azusa in 1991, but it's growing worse, and it has become one of California's leading ozone hot spots.

This past summer, Clovis had an average daily one-hour ozone peak of 81 ppb; Azusa had 69 ppb. The same is true for eight-hour ozone readings, which measure longer-term health risks. Clovis averaged 70 ppb; Azusa had 53 ppb. Ozone levels in Azusa exceeded 120 ppb on seven days this summer; in Clovis, they passed that mark on 14 days.

Not every Valley monitoring station is showing the same rise in ozone levels that Clovis has experienced. Most are remaining steady, or even dropping some. But none has dropped as rapidly as Azusa and other Southern California stations.

The contrasting trends of Azusa and Clovis sum up the challenge that faces the Valley's air pollution regulators. The strategies that worked in Southern California, turning the nation's air pollution capital into its greatest smog-fighting success story, haven't had the same impact in the Valley. And the reasons are a mystery.

"I'm not really sure we know why we see an increase in Clovis," says Evan Shipp, chief meteorologist for the San Joaquin Valley Air Pollution Control District.

The district's computer models have trouble simulating what's happening in Clovis, Shipp says. No matter how much analysts tinker with the mix of sources, weather and other inputs, they can't coax the model to produce ozone levels as high as the ones measured by the district's monitor, located in Letterman Park on Villa Avenue south of Bullard Avenue.

"It's going to be pretty difficult to figure out what the problem is with the tools we have now," Shipp says. "We can only guess."

Shipp's counterpart in the South Coast Air Quality Management District, Joe Cassmassi, has an easier time discussing Azusa's trend.

In summers past, sea breezes would blow across the coastal basin from the west and south, sweeping the urban area's emissions into the San Gabriel Valley and east toward San Bernardino. Along the way, the emissions baked in the sun, producing ozone in the classic smog-forming reaction.

For years, the reaction peaked right about when the pollutant cloud reached the San Gabriel Valley. Cities such as Azusa, Pasadena and Glendora regularly posted the nation's highest ozone levels.

"Everything kind of came together right there," Cassmassi says.

What changed in the 1990s? The cumulative impact of ever-greater pollution controls – aided by a more forgiving climate and topography – removed hydrocarbons (a type of reactive organic gas) and nitrogen oxides from the air. That slowed the photochemical smog reaction.

"Places like Pasadena, Azusa and Glendora have actually improved rapidly because the smog doesn't form there anymore. It's still cooking as it moves through there."

Today, the region's highest ozone levels typically occur later and farther east, near San Bernardino. But even there, the peaks are lower than they used to be. Ozone levels are down all over Southern California.

And they're down the most where they used to be the worst, in the San Gabriel Valley.

Says Cassmassi: "It was a pleasant surprise that it cleaned up as fast as it did."