CHANGES IN DEPOSITED DUST CHEMISTRY IN THE SOUTHWESTERN U.S., 1984-2010

Deposited dust collected in passive marble traps in southern Nevada and southern California from 1984 through 2010 exhibits long-term trends in dust chemistry that may represent anthropogenic changes due to population increase and emissions. Carbonate dust flux has decreased from an average value of about 1.5 g/cm²/yr to less than 0.1 g/cm²/yr, in contrast to sites in the eastern Colorado Plateau where no significant decrease was observed since 1999 when collections began in that area. Notably, the strongest decreases in carbonate content (from 20-35% to 0) and flux (from 2-4 g/cm²/yr to 0) have occurred at sites on limestone substrates located near Las Vegas. Elemental analyses on selected samples (not all samples are large enough to analyze) and NADP precipitation data from sites in the region show no trend in Ca content, suggesting that the observed decrease is in CO₃ content. We hypothesize that this change is occurring in the dust traps, where samples accumulate over periods from 6 months to one year, and during this time are episodically submerged by rainwater in the pans. The observed trend suggests that rainwater acidity may be increasing in the study area, possibly as a result of increasing atmospheric CO₂ and (or) acidic pollutants, and particularly in the area proximal to Las Vegas.

Urban growth in the Las Vegas metro area has been extremely rapid during the study period; population has nearly quadrupled, as has the number of annual visitors. Elemental dust analyses for two sites in the central Mojave Desert, 120 km south-southwest and upwind of Las Vegas, either show no change or gradual decreases in anthropogenically influenced elements such as Cu, Ni, and Pb since 2000, whereas sites that are on the western edge of and 70 km north of Las Vegas exhibit increases in the same elements since 1991. Although rainwater pH data are lacking, we hypothesize that the decrease in CO₃ in deposited dust may reflect anthropogenic changes to the atmosphere.