Optical and particulate properties of dust aerosols in El Paso, Texas

Abstract

We characterized optical properties (absorption, scattering, and backscattering coefficients, Ångström exponents, single scattering albedo) of aerosols during dust events at an urban site in El Paso, Texas using in-situ, real-time measurements. Simultaneous measurements of particulate matter (PM₁₀, PM_2.5) and wind data from an adjacent Texas Commission on Environmental Quality Continuous Ambient Monitoring Station are used in this study. Data from both sites was obtained at 5-min resolution. Twenty-seven dust events (15 synoptic, 12 mesoscale) occurred during 24 days from April–June 2021. HYSPLIT and STILT back-trajectories indicated many dust events (∼46 %) originated from the west-southwest (favoring transport across dust source areas in the Chihuahuan Desert). Mean absorption Ångström exponent values of El Paso dust (1.19 and 1.15 for synoptic and mesoscale events) were lower than published values for “pure” dust at other locations (generally accepted as greater than 2). Single scattering albedo values averaged 0.94 during synoptic and mesoscale dust events, confirming the scattering nature of mineral dust in the region. PM₁₀ and PM_2.5 concentrations averaged 191 and 41 μg/m³ during dust event periods versus 28 and 8.7 μg/m³ during non-event (background) periods. Absorption / scattering Ångström exponent ratios of dust event aerosols were representative of large particles-black carbon mixture rather than a “pure” dust, suggesting mixing with urban aerosols, while many observations during non-event periods fall into the large particles-black carbon mixture and large particles-low absorption categories, suggesting that dust is a constant key component of the aerosols in El Paso. The 5-min time resolution revealed the evolution of complex aerosol events (e.g., brown carbon followed by dust, multiple dust events within one day, fireworks, advection of low-absorption gypsum dust from White Sands. New Mexico), emphasizing the importance of detecting and quantifying high intensity, short-duration events “lost” in 24-h or hourly air quality data.