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

Abstract

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...

Airborne measurements of aerosol extinction in the lower and middle troposphere over Wyoming, USA

“…Bimodal size distributions adequately describe observed SWYVIS size distributions, and display slight broadening of the coarse mode with increasing altitude. In the accumulation mode, median particle diameters and geometric standard deviations (Tables 4 and 5) are very similar to those reported by Kim et al (1993a) and Pinnick et al (1993). For the coarse mode, however, both inter-and intrastudy comparisons of the diameters show slightly more variability, though modal standard deviations are comparable.…”

“…Examples in the marine environment include NARE-96 (Cowling et al, 1998), ACE-1 (Bates et al, 1998), ACE-2 (Collins et al, 2000a;, and INDOEX (de Reuss et al, 2001). Over coastal and continental locations, following earlier work of Rosen et al (1992), Kim et al (1993a), Gunter et al (1993), Pinnick et al (1993) and Gunter (1994), more elaborate airborne data sets have been acquired in the Arctic (Hegg et al, 1996), in the US mid-Atlantic coastal region (Hegg et al, 1997;Hartley et al, 2000), and over the Los Angeles Basin (Collins et al, 2000b), among others.…”

“…However, while satellite instruments offer the advantage of extended coverage in time and space, calibration of the retrieval products is still necessary and ideally requires three-dimensional in situ measurements for appropriate comparisons. Aircraft observations afford the principal advantage of yielding detailed information on aerosol properties with height, albeit within short time periods over geographic areas of limited extent, thereby enabling the question of inhomogeneities of aerosol properties in the vertical to be addressed (Kim et al, 1993a;Pinnick et al, 1993;Raga and Jonas, 1995;Clarke et al, 1996;Sheridan and Ogren, 1999). Airborne observations that are sufficiently comprehensive to inter-relate the optical, physical, and chemical properties of particulates have been carried out relatively infrequently.…”

Effects of mixing on extinction by carbonaceous particles

“…Ch)•lek and Hallet, 1992] that when soot is near the surfaces of droplets, its absorption may be enhanced. Calculations [Pinnick et al, 1993;Fuller, 1995a, b] have shown that this is certainly the case for soot located at selected positions on or in a droplet, with enhancements factors as high as about 20, but these same calculations also show locations where the refractive properties of the droplets actually shield the soot from incident light. Averages of absorption over positions near the surface of a host droplet [Fuller, 1995b] indicate that for txm-sized water droplets, there is very little net enhancement as long as the absorber's position is (as one would expect) unrestricted.…”

Characteristics of the Receptor for the Biosphere Model

“…The proportion of small particles (< 10 µm) for most activities was less than 10% and generally was lower for dustier activities. Another study of natural aerosols in the arid southwestern United States concluded that near-surface aerosol is comprised to two modes: a wind-derived supermicron component which is likely soil-derived and local in origin and a submicron component that is likely a product of long-range atmospheric transport (Pinnick et al 1993 [160312] [160312]), the airborne particles originating from the local soils range in size from about 0.1 µm to about 100 µm.…”