AEROSOL IN THE ARID SOUTHWESTERN UNITED-STATES - MEASUREMENTS OF MASS LOADING, VOLATILITY, SIZE DISTRIBUTION, ABSORPTION CHARACTERISTICS, BLACK CARBON CONTENT, AND VERTICAL STRUCTURE TO 7 KM ABOVE SEA-LEVEL

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 almost-equal-to 10 mug m-3 in the fall rainy season (August-October) to almost-equal-to 100 mug m-3 during the spring windy season (February-June). The mass concentration frequency of occurrence of both total aerosol and BC are well approximated lognormal probability distributions. Geometric mean masses of aerosol range from 16 to 32 mug m and are smaller for mountain sites than for basin sites; geometric mean BC mass concentrations at a single basin site are almost-equal-to 0.15 mug 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 almost-equal-to 0.84, although during dusty conditions occurring in spring or early summer, non-BC aerosol absorption contributes up to almost-equal-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.