THE ROLE OF DUST STORMS IN ATMOSPHERIC PARTICLE CONCENTRATIONS AT TWO SITES IN THE WESTERN U.S

Mineral aerosols are a common component of the total atmospheric particulate load (dust) in arid and semi-arid regions. The sizes of these particles can vary widely and commonly exceed PM₁₀ (particulate matter less than 10 micrometers in aerodynamic diameter). In this study, we present two continuous records of total suspended particulate (TSP) concentrations at sites in Mesa Verde (Colorado) and Canyonlands (Utah) national parks and compare those values to measurements of PM_2.5 and PM₁₀ concentrations made from nearby IMPROVE-network samplers. Average annual concentrations of TSP at Mesa Verde were 90 micrograms m^-3 in 2011 and at Canyonlands were 171 micrograms m^-3 in 2009, 113 micrograms m^-3 in 2010, and 134 micrograms m^-3 in 2011. In comparison, annual concentrations of fine (PM2.5) and coarse (2.5-10 micrometer) particulates at these sites were well below 10 micrograms m^-3 in all years. The high concentrations of TSP strongly appear associated mostly with regional dust storms and events that result in elevated concentrations of coarse particles greater than 10 micrometers in diameter. These conditions regularly occur from early spring until early fall with two-week mean TSP periodically in excess of 200 micrograms m^-3 at Canyonlands and peak concentrations in excess of 700 micrograms m^-3. Measurement of particle sizes embedded on filters indicates that the median size of all particles varies between approximately 10 micrometers in winter and 40 micrometers during the spring dust-storm season. These persistently elevated concentrations of large particles indicate that regional dust emission as dust storms and events are important determinants of air quality in this region.