Colorado scientists quantify increased dust pollution

Tracking calcium deposits shows big increase in dust deposition in the past couple of decades

Desert dust taints the snow at Loveland Pass, Colorado, speeding snowmelt and sometimes contributing to avalanche hazards. Bob Berwyn photo.
NASA satellite images can help track dust storms.

By Bob Berwyn

FRISCO — Significant dust storms the past few years have had a big impact on the timing of snowmelt in the Colorado Rockies, but scientists haven’t been able to say for certain if those events are becoming more frequent.

Now, a new study from the University of Colorado Boulder shows that the amount of dust deposition has increased, at least during the 17-year span covered by the researchers, who tracked calcium deposits to reach their conclusions. Calcium dissolved in precipitation has long been measured by the National Atmospheric Deposition Program as a way assess acid rain.

The scientists reviewed calcium deposition data from 175 NADP sites across the United States between 1994 and 2010, measuring increases in calcium deposition increased at 116 locations. The sites with the greatest increases were clustered in the Northwest, the Midwest and the Intermountain West, with Colorado, Wyoming and Utah seeing especially large increases.

“This is the first real evidence that things have changed in the last few decades,” said CU-Boulder geological sciences associate professor Jason Neff. The study showed a 300 percent to 500 percent increase in the total amount of calcium deposition, Neff said.

“That lets us make a pretty good guess as to how much the dust storms are increasing,” he said, explaining that the researchers were able to determine that the increase was linked to dust erosion because none of the other possible sources of atmospheric calcium — including industrial emissions, forest fires or ocean spray — had increased during the 17-year period studied.

Previous studies using core samples from lake-bottom sediments have helped track the history of dust storms going back a few centuries, showing there was a huge increase in dust deposition that coincided with the settlement of the West. But those core samples are not fine-grained enough to show patterns from the past few decades, Neff said, acknowledging that the 17-year study period is too short to show any statistically significant trends.

Neff said it’s important to remember that using calcium deposition as a proxy only measures part of the total dust volume.

That’s because the NADP network only measures dust that has collided with water in the atmosphere before precipitating to earth — not dust that is simply moved by the wind. And not all dust contains the same amount of calcium,” said Janice Brahney, lead author of the paper published recently in the journal Aeolian Research.

To try and get a sense of what’s causing the increase, the researchers also looked at different variables, including wind events and drought. That accounted for about half of the observed changes, but there’s still no definitive answer to what causes the rest of the increase.

“A lot of the candidate explanations  boil down to human activities like oil and gas development and off-road vehicles,” he said, explaining that it’s difficult to pinpoint the sources because there’s no systematic monitoring — although some previous studies have tried to trace the origins with chemical analyses.

Another recent effort involved a set of sampling machines that Neff described as giant vacuum cleaners, set up at different locations, including Canyonlands, Telluride and Craig, Colorado. The filters are exchanged every two weeks, giving scientists plenty of material to work with as they look for dust trends without relying on proxies.

In some cases, satellite images can help locate a general source area, but often, the windstorms start far to the southwest in the Mojave desert, then raking across the Colorado Plateau.

“By the time we see the dust, it’s from a lot of different sources,” he said.

Regardless of its exact cause, the increased transport of dust likely has implications on an ecosystem scale. Some of the key constituents of wind-transported dust, like calcium and phosphorus, are classified as nutrients, said said Janice Brahney, who led the study as a CU-Boulder doctoral student.

When the wind strips the dust from one area, it can change the fundamental soil chemistry be removing nutrients and the finer particles that can hold the most moisture; when it’s deposited in new areas — for example in the watersheds of pristine mountain lakes — it can change the chemistry of the water by increasing concentrations of nutrients, potentially leading to more algal blooms and other changes.

“And we don’t routinely monitor dust in most places, which means we don’t have a good handle on how the material is moving, when it’s moving and where it’s going, Brahney said, adding that a study scheduled for publication in a few months will show how increasing dust storms may be affecting alpine lakes in Wyoming’s Wind River Range.

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