High winds create a dust haze around a new community, Monday, April 8, 2013 in Gilbert, Ariz. The National Weather Service has issued high wind and blowing dust advisories with wind gusts as high as 60 mph this afternoon and into the early evening. (AP Photo/Matt York)

Matt York

Once again, Southern California has blazed a trail for Arizona - this time, a dusty trail.

Most of the thick dust that blanketed Pima County three weeks ago with record-setting particle pollution probably traveled at least 300 miles from the Imperial Valley area of farm fields and deserts west of El Centro, Calif., a University of Arizona researcher says.

Giant dust plumes were covering the Imperial Valley on satellite photos on the afternoon before dust levels peaked here, said the researcher, Mike Leuthold, a lecturer in the UA's Atmospheric Sciences Department.

It's not unusual for dust to blow from Southern California to Tucson, or from Mexico into the United States, but "I've never seen dust that thick from Southeast California before," Leuthold said.

On the morning of April 9, large-particle air pollution peaked in the Green Valley-Sahuarita area south of Tucson. Levels averaged nearly three times the federal standard over 24 hours. The dust was thick enough to be rated hazardous and to be this region's worst daily score for large particles since records started being kept in 1985.

Hazardous is the worst possible rating for air pollution. In the federal air-quality index, the rating means that all residents should avoid strenuous exertion outdoors, and that people with asthma and other respiratory ailments should stay indoors.

Dust pollution, while at less extreme levels, also exceeded federal standards in South Tucson on April 8, and in South Tucson and on Tucson's north side on April 9, the Pima County Department of Environmental Quality said.

Leuthold's comments come as Pima County's Department of Environmental Quality is trying to determine if any local property owners allowed dust to leave their properties those two days, in violation of county rules. Copper mines, construction sites, vacant lots and dirt roads are potential sources, officials have said.

At this point, department officials can't say if they agree that most of the April 9 dust pollution came from elsewhere.

"It's going to take us a couple of months to go through and look at all the data," said Ursula Kramer, the DEQ's director.

The pollution was also blowing statewide those days, with Cochise, Santa Cruz, Maricopa and Yuma counties recording particulate levels above federal standards on April 8 and Santa Cruz's air topping standards the following day, the Arizona Department of Environmental Quality says. On April 8, 17 Maricopa County monitors exceeded the standards, and Yuma County recorded particulate levels at more than four times the federal standard, ADEQ spokesman Mark Shaffer said.

UA researchers used satellite imagery to trace the dust back to Southern California. Then, they used National Weather Service computer models to track the dust movements east and northeast toward Tucson, and into New Mexico.

Leuthold called the dust conditions "pretty extreme," adding that he's never seen dust that thick except in major dust storm events known as haboobs, which are generally much more local than this storm was.

"What was unusual was that enough dust remained in the atmosphere on the morning of the 9th in high concentrations and the winds were such to bring it into Tucson and Green Valley," Leuthold said.

On April 8, wind speeds in the Imperial Valley were up to 43 miles an hour in El Centro along Interstate 8, and up to 85 miles an hour on Mount Laguna, 15 miles north of the freeway.

"One picture is worth a thousand words," Leuthold said. "You can see dust normally with strong winds, but you can normally see through it. In this case, you couldn't see the Imperial Valley in the photos. You would see green on the north and green on the south, but a big brown cloud in the middle."

The common thinking at the time was that most of that dust was generated locally, but it's pretty obvious it was coming from somewhere else because wind speeds in the county were low in the Tucson area on April 9, Leuthold said. The somewhat lower dust levels of April 8 could have been generated locally, he said.

However, on both April 8 and 9, when the winds kicked up at 35 to 40 mph, the county got reports of dust arising from disturbed lots of various kinds, said county DEQ program manager Beth Gorman.

"One hundred percent of it didn't come from California," Gorman said.

The key to determining a violation is whether a landowner had taken reasonable precautions to keep dust from leaving his property, county DEQ Director Kramer said.

County officials realize that even if landowners make reasonable efforts, "during high wind events, some dust will indeed blow," she said.

State officials have determined after a preliminary review that "there is merit" to having the April 8-9 dust storms declared an exceptional event, ADEQ spokesman Shaffer said Monday. If EPA were to OK such a determination, the feds would not count the two days of higher-than-approved dust levels in Pima County's air toward possible violations, which require four days in which a single monitor records pollution above EPA standards.

"Generally speaking, the dust we experienced on April 8th and 9th was a regionwide event," Shaffer said. "The wind speeds were high enough to overcome even the most stringent of dust controls. As a result, we don't believe that it is possible to attribute the high concentrations to any single source, and we don't think there was much that could have been done to prevent that dust from blowing."

On StarNet: View photos of large dust storms in our region at azstarnet.com/gallery

Dust: Health effects

PM 10 pollution: Also known as particulates. Large particles, about one-seventh the size of a human hair, can cause or aggravate:

• Coughing, wheezing.

• Asthma.

• Lung damage, including decreased lung function and lifelong respiratory disease.

• Premature death in individuals with heart or lung diseases.

Source: Environmental Protection Agency

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.