Near-surface and lower tropospheric aerosol characteristics have been determined at several remote sites near Orogrande, New Mexico, using a variety of methods and sensors including quartz fiber filter samplers, hi-vol samplers, ground-based and aircraft-mounted light-scattering aerosol counters, an aerosol counter equipped with a heated inlet, and an aethalometer (a device that measures aerosol absorption). The results of these measurements, which have been taken sporadically over the last 15 years but with a higher density of measurements during 1989-1991, suggest that regardless of season, aerosol consists of two modes: a submicron fraction composed primarily of ammonium/acid sulfates and elemental black carbon (BC) and a supermicron fraction composed mainly of quartz and clay minerals of soil origin. Aerosol mass concentrations determined from weekly or biweekly samples are dominated by wind-derived aerosol of soil origin and typically range from • 10/xg m -3 in the fall rainy season (August-October) to • 100/xg m -3 during the spring windy season (February-June). The mass concentration frequency of occurrence of both total aerosol and BC are well approximated by -3 lognormal probability distributions. Geometric mean masses of aerosol range from 16 to 32/xg m and are smaller for mountain sites than for basin sites; geometric mean BC mass concentrations at a single basin site are •0.15 /xg m -3. Aerosol loadings decrease slightly from 1975-1977 to 1990 measurement periods, whereas BC concentrations, within statistical error, remain unchanged. However, a statistically significant increase in BC did occur from 1990 to 1991. The fraction of (visible spectrum) aerosol absorption attributable to BC has a yearly mean value of •0.84, although during dusty conditions occurring in spring or early summer, non-BC aerosol absorption contributes up to •30% of the total. Sulfate and BC aerosol concentrations are sometimes highly correlated, suggesting they have a common source and may be internally mixed. Limited aircraft measurements in the lowest few kilometers of the troposphere reveal a well-mixed aerosol for a neutral atmospheric condition, and a significant decrease in aerosol concentration with altitude for a stable atmospheric condition. INTRODUCTION Previous measurements of the physiochemical properties of natural aerosol in the arid southwestern United States have been directed toward determination of aerosol composition [Hoidale and Blanco, 1969; Lindberg, 1975; Reagan et al., 1984; P. J. Sheridan et al., unpublished manuscript, 1992] and size distribution [Twomey, 1976; Patterson and Gillette, 1977a; Reagan et al., 1980], characteristic morphologies [Gillette and Walker, 1977], average refractive index [Grams et al., 1974; Herman et al., 1975; Lindberg and Gillespie, 1977], effect on visibility [Patterson et al., 1976; Patterson, 1977; Patterson and Gillette, 1977b], vertical structure [Spinhirne et al., 1980; Schwiesow et al., 1981; Reagan et al., 1984], backscatter to extinction ratios [Reagan et al., 1988], w...