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Rocky Mountain haze - Study finds a link between drought and air quality in the western wilderness

Reno, Nevada - During summer months in the West people often head to the mountains to escape the haze regularly found in lower elevation cities and valleys. New research published in Environmental Research Letters finds the haze could be catching up.

Led by Gannet Hallar, Ph.D., a professor of atmospheric science at the Desert Research Institute (DRI) and the University of Utah, the study identifies a direct connection between the severity of ongoing drought and related wildfires in the Intermountain West with summertime air quality; particularly the concentration of aerosol particles, in remote high-elevation mountain wilderness regions.

Climate projections suggest that drought and wildfire risk will continue to increase in coming decades.

“If you take that into the future, we're going to see significant hazing of the West,” said Hallar, who conducts research at DRI’s Storm Peak Laboratory, a mountain-top observation research lab near Steamboat Springs, Colorado, at an elevation of 10,525 feet above sea level.

Haze in the air is caused by small airborne particles known as aerosols. These particles are often so small that they remain suspended in the air and don’t settle out.

Their role in modifying the climate is significant, depending on the composition of the particles. Dust particles made of mineral grains can reflect solar energy, providing a cooling effect. Aerosol particles made of organic carbon, such as soot or smoke, can absorb energy, however, warming the climate. Further, aerosol particles can trigger cloud formation, which exerts its own influence over global temperature and climate. Aerosols are an important component of the atmospheric energy balance.

Specialized instruments used in the study measured aerosol optical depth, the concentration of aerosols detected between the sensor and the sun, which indicate a crystal clear sky with maximum visibility or aerosol conditions so dense that would be difficult to see the sun at mid-day.

Figure 3

Caption: Colored icons indicate correlations between aridity (left) and fire area burned (right) in colored mountain ranges with aerosol concentrations at wilderness sampling sites.

Researchers found that decades of aerosol optical depth records consistently showed increases in during the summer months. A previous global study of aerosols illustrated that, in general, aerosol concentrations across the continental United States were thought to be decreasing except for a peak identified at one site in the western U.S.

The research team, consisting of hydrologists and atmospheric scientists, compared climate and drought records for the West to see if they could find a connection to the summer mountain haze. They found a correlation between drought and high-elevation aerosols, with a very likely explanation.

“It's the fires,” Hallar explained. “There is strong evidence that the drought is a catalyst for more wildfires and those fires are likely creating more aerosols.”

Further analysis of summer wildfire area burnt in the West showed a good correlation with aerosol optical depth and aerosol mass at the surface in the northern, central and southern Rockies.

Hallar said her team’s observations allow for comparison with climate models’ simulated effects of fires on aerosol emissions. The models use assumptions to estimate how much aerosol pollution results from an area of forest burned, which can lead to uncertainties in the conclusions.

“We're putting a moment of real data in there,” Hallar said, noting that their observational data isn’t far off from what the models predict. “That has me concerned because climate models are predicting in the future a significant increase in organic aerosol loading.”

Even more concerning is that Hallar’s measurements are mostly from wilderness areas—those preserved for their untouched natural beauty. A wilderness area’s borders, however, can’t keep the haze out.

Hallar hopes that her results highlight the importance of managing the relationship between drought, fire and haze in the West.

“We need to think more about fires in the realm of air quality,” she said.

This study was supported by the Desert Research Institute, the U.S. Department of Agriculture, and the National Oceanic and Atmospheric Administration.

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