Chemical Compositions of Rocky Mountain Dust-on-Snow

In mid-February, 2006, a dust cloud from windstorms in Arizona, Utah, and western Colorado deposited dust on snow throughout the majority of the Colorado Rockies, creating a 0.5–2.0 cm layer of red snow that remained visible throughout the year. Rhoades et al. (2010) examined the chemical compositions of dust-event snow layers from south-central Colorado up to Southern Wyoming, comparing the dust-laden layer to snow layers 25–35 cm below and above the layer from the dust event. They also compared dust-layer samples in different regions, at different elevations, and during different seasons. Rhoades et al. concluded from their analyses that dust-event snowpack had a pH 1.5 units higher, on average, than snow before and after the event. The dust-event snow also had 100-times the capacity to neutralize acids, and 10-times the amount of calcium, as well as completely different elemental distributions than non-dust layers. As of yet, no chemical changes of the stream or lake waters have been observed in relation to the dust, nor have other snow layers been affected. However, the possible consequences of the dust in changing the quality of soil—especially between upper and lower elevations—and the earlier seasonal snowmelt could influence the biodiversity of animal and plant life in the region. In providing the chemical compositions of dust-event snow, this study lays the foundation for further research on the possible implications of dust-laden snow on alpine ecological cycles.—Elise Wanger
Rhoades, C., Elder, K., Greene, E., 2010. The influence of an extensive dust event on snow chemistry in the southern Rocky Mountains. Arctic, Antarctic, and Alpine Research 42, 98–105.

Dust regularly travels to the Rockies from neighboring regions in Colorado, Utah, Wyoming, and occasionally even Arizona, making dust-on-snow events a long recognized phenomenon. Geology studies have long confirmed that cations from dust deposits (Ca2+, Mg2+, Na+ and K+) elevate the pH and the pH buffering capacity. The pH specifies the concentration of hydrogen ions (H+) in a solution, with a higher pH designating a lower concentration. Since H+ concentrations tend to be incredibly small, the pH takes the negative log of the moles per liter, or molarity (M). For example, the H+ concentration of pure water is 10-7 M, or 0.0000007 M, making the pH –log (10-7) = – (-7) = 7. Thus every unit difference in pH means a 10-fold difference in H+ concentrations. Acid ions (NO3, Cl, SO42-), carbonates (HCO3), and bicarbonates (CO32-) will often raise the pH by bonding with H+ ions, while cations will lower the pH by bonding with acid ions and reducing acid ion availability for H+: which is why cation-laden dust elevates pH.
Dust events in the Rockies have also enhanced the nutrient availability of soils. Inputs from the Colorado Plateaus double the phosphorus content of surface soil, as well as adding carbonates, calcium, magnesium, potassium, sodium, and various micronutrients (chemicals that biota need only in trace amounts, like zinc). The calcium (Ca2+) concentration of Colorado lakes that receive dust inputs is higher in comparison to California lakes with no dust and similar geology. The addition of fine-textured clay particles from dust also increases the water- and nutrient-holding capacity of the soil, which has a huge impact on the fertility of the earth in regions that would otherwise be barren rock.
Given the myriad benefits, Alpine ecosystems have depended on regular deposits of dust to maintain biodiversity and vitality for millennia. Yet not until about five years ago had dust-on-snow made enough of a visible impact to draw the attention of researchers to investigate the possible ecological consequences of increased dust, and the source of such amplified dust storms. The cause of recent major dust events remains uncertain, although most scientists attest to combinations of agricultural practices such as ranching, that kick up dust; the ten year drought from the mid-nineties to mid-2000’s breaking up the cyptobiotic mat keeping soil intact; and oil drilling in southeast Arizona. Whatever the cause, increased amounts of dust on snow have created major changes, especially pertaining to the rate of snowmelt. Snow has a very high albedo factor, meaning that it’s incredibly reflective. Pristine snow reflects up to 98% of the sun’s radiation back into the atmosphere, keeping the snow cool and the ice-pack thick. When the snow is darkened by dust, however, the darker color absorbs more UV rays, reducing the albedo factor to reflecting only 50–60% of the radiation and making the snow melt faster. And since water is denser than snow, the snow melts from the bottom out, so that the red-dust layer remains on top until everything beneath has melted away. After the 2006 dust event, snow melted 18–35 days ahead of the usual pattern, and late-season avalanches became more prevalent with the softer, weakened snowpack. Such a change in seasonal melting patterns changes the length and period of the growing season for high-alpine plants, and the fluctuation of water-levels. Faster snowmelt means that rivers run higher and faster earlier, and lower and slower later. In late August, mountain rivers rely on that last reservoir of snowmelt to keep flowing, which depends on a gradual rate of melting throughout the summer months.
Rhoades et al. examined the molecular compositions of dust-laden and dust-free snow, as well as wetfall precipitation before, after, and during the dust event to help future research shed light on the possible chemical consequences of dust events on soil and water content. They took snowpack and wetfall precipitation samples at 17 sites throughout the Colorado Front Range and Southern Wyoming. The 30-kilometer radius that received the greatest impact from the dust storm—about 85-km west of Denver, in the heart of the Fraser Experimental Forest (FEF)—was most extensively studied, and in the geographic center of their range of sites. Rhoades et al. compared samples of snow 25–35 cm below and above the dust-layer to dust-layer snow from all sites, as well as pre- and post-dust precipitation to precipitation during the dust event. They then compared the data in the 30-kilometer radius to the FEF precipitation and snowpack records from the past 17 years.
The concentration of anions and cations were analyzed using ion chromatography and an 18-minute isocratic method. Chromatography separates the components in a solution—in this case, water—by moving the solution through a medium in which different materials will move at different rates. In ion chromatography, the medium is a gel matrix with polar functional groups. The solution gets pushed through the gel by adding concentrations of similarly charged ions that repel the ions in the solution, forcing them to move forward. Sometimes the volume of charged ions necessary to push the solution ions forward is enough information to identify the element, but usually the separated ions get identified through conductivity or UV/visible light absorbance. The isocratic method is a form of ion chromatography in which the mobile phase—when charged ions are being added—is constant for the respective time of the process, in this case 18 minutes.
Using these techniques, Rhoades et al. discovered that dust event snowfall precipitation around the Fraser Experimental Forest (FEF) had 35, 9, 16 and 5-fold higher levels of Ca2+, Mg2+, Na+, and K+, respectively, than the pre- or post-event snowfalls. In other words, the two-day dust event from February 14–15 deposited as much calcium as half the annual average, and provided as much Mg2+, Na+, and K+  as during a typical snow-free month, when soil-derived cations typically peak in concentration.
The electrical conductivity (EC) in dust-free wetfall precipitation from FEF averaged 3.6 mS cm-1, while dust-event snowfall had an EC of 26.3 mS cm-1. Conductivity increases with carbonate minerals, dissolved salts, and dissolved ions. Distilled water, for instance, has almost no conductivity (and correspondingly a neutral pH of 7). EC is measured by two metal electrodes exactly 1 cm apart in the water (which is why the units are in micro-Seimens per centimeter), from which a constant voltage creates an electric current indicative of the ion content. The raw data of the electric current then gets standardized to a temperature of 25°C—since warmer water tends to be more conductive—and thus the current flow in amperes (I) gets converted to Seimens (S).
FEF snowfall typically lacks any acid neutralizing capacity (ANC), and therefore can’t buffer an influx of acids that would lower pH. Buffers are any molecules that attenuate the change in pH an influx of strong acids or bases would typically cause, either by providing a balance of weak acids and bases that will offset the contribution of new ions, or by bonding with them and neutralizing their charge. During the dust event, snowfall had an ANC of 198.7 meqL-1. The ANC is calculated as the difference between strong bases (Ca2+, Mg2+, Na+, K+, NH4+) and strong acid anions (NO3, Cl, SO42-), including bicarbonate and carbonate (HCO3 and CO32-), which are most responsible for neutralizing acids. Because of this influx of acid neutralization, pH in the dust event snowfall increased from 5.4 to 8.2. This conclusion is reiterated in the snowpack cores as well, in which pre- and post-dust snow for all sites had an average 16-fold higher concentration of H+ than dust-layer snow.
To the researchers’ surprise, these differences of the dust-laden snow in composition did not change the composition of the non-dust layers neither before nor after the event. Nor did the influx of bases and carbonates change the composition of rivers, snowmelt streams or glacial lakes in chemical compositions or pH. The dust-layer snow did differentiate by elevation, however, and Rhoades et al. suggest further research to see if snowmelt and soil near the treeline has changed more that that from lower elevations. Treeline snowpack (3350 m) had a pH 0.6 units higher both in the dust-layer and post-event snow, as well as 40-fold higher ANC and double the calcium than the lowest sample sites tested (2750 m). This is most likely due to trees acting as a barrier to dust reaching the ground in forested areas, while high elevations are more exposed. Whether this elevation difference has ecological consequences, especially in regards to snowmelt rates, demands further research.
Larger dust storms have continued to dominate the western landscape since the 2006 event. Twelve dust-on-snow events from 20082009 caused record streamflow rates earlier in the season, with snowmelt 4050 days faster than the non-dust predicted rate. The 20092010 season estimated eight dust events, with the effects yet to be published, but will certainly reflect the same patterns. As dust flux continues to escalate, non-profits and research groups are beginning to measure the impact of dust-on-snow, and the National Foundation of Science (NFS) has been funding researchers to continue investigating the source of the dust and the dust’s influence on alpine ecology. The data from Rhoades et al. will help propel the conclusions of future findings, and hopefully shed light on these new patterns.
Other sources:
Dybas, C. Dust-on-snow: On spring winds, something wicked this way comes. The National Science Foundation [database online]. Arlington, Virginia, 2010. 

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