Hemispheric Backscatter Fraction Research Articles

In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 2. Effects of mixing height on aerosol properties

The goal of this study was to determine under what meteorological conditions, if any, measurements of aerosol properties made at the Earth's surface are representative of aerosol properties measured within the column of air above the surface. Specifically, this study uses instrumentation from the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site to determine how the observed mixing height (MH) and the degree of mixing affects the vertical variation of aerosol extensive and intensive properties. The MH was estimated by applying the Heffter [1980] technique with a critical lapse rate of 0.001 K m−1 to the available radiosonde data. This estimate was then used to determine whether each vertical aerosol profile flight leg from 59 flights was within or above the atmospheric boundary layer (ABL). Although the aerosol extensive properties, such as light absorption by particles σap, light scattering by particles σsp, and total extinction by particles σext measured within the ABL were highly correlated with the surface measurements (R2 = 0.74, 0.88, and 0.88, respectively), the airborne measurements of σap were systematically lower (slope = 0.77). By contrast, the σsp and σext regression line slopes were 0.99 and 1.00. The aerosol extensive properties measured above the ABL were poorly correlated with surface values and were almost an order of magnitude lower than the simultaneous measurements made within the ABL. Aerosol intensive properties such as the single‐scattering albedo ϖo, the hemispheric backscatter fraction b, and the Ångström exponent å exhibited a similar behavior (R2 = 0.74, 0.61, and 0.55, respectively) but had regression line slopes within the ABL of 0.88, 0.75, and 0.62, respectively. There were no statistically significant correlations between the intensive aerosol properties measured above the ABL and those measured at the surface. Limiting the analysis to well‐mixed days with near‐neutral static stability did not significantly improve the results. Thus the hypothesis that the best agreement between surface measurements and those within the ABL will occur when the atmosphere is well mixed was not supported by this data set.

Mixtures of pollution, dust, sea salt, and volcanic aerosol during ACE‐Asia: Radiative properties as a function of relative humidity

The Ron Brown cruise during ACE‐Asia (March–April 2001) encountered complex aerosol that at times was dominated by marine, polluted, volcanic, and dust aerosols. Average total light scattering coefficients (σsp for Dp <10 μm, relative humidity (RH) = 19%, and λ = 550 nm) ranged from 23 (marine) to 181 Mm−1 (dust). Aerosol hygroscopicity ranged from deliquescent with hysteresis (marine frequently and polluted variably) to hygroscopic without hysteresis (volcanic) to nearly hygrophobic (dust‐dominated). Average deliquescence and crystallization RH were 77 ± 2% and 42 ± 3%, respectively. The ambient aerosol was typically on the upper branch of the hysteresis loop for marine and polluted air masses and the lower branch for dust‐dominated aerosols. Average f(RH = ambient), defined as σsp (RH = ambient)/σsp (RH = 19%), ranged from 1.25 (dust) to 2.88 (volcanic). Average h(RH ∼60%), defined as f(RH)upper branch/f(RH)lower branch, were 1.6, 1.3, 1, and 1.25 for marine, polluted, volcanic, and dust, demonstrating an importance of hysteresis to optical properties. Hemispheric backscatter fraction (b) at ambient RH ranged from 0.077 (marine) to 0.111 (dust), while single scattering albedo (ω) at ambient RH ranged from 0.94 (dust and polluted) to 0.99 (marine).

Characteristics and direct radiative effect of mid-latitude continental aerosols: the ARM case

Abstract. A multi-year field measurement analysis of the characteristics and direct radiative effect of aerosols at the Southern Great Plains (SGP) central facility of the Atmospheric Radiation Measurement (ARM) Program is presented. Inter-annual mean and standard deviation of submicrometer scattering fraction (at 550 nm) and Ångström exponent å (450 nm, 700 nm) at the mid-latitude continental site are indicative of the scattering dominance of fine mode aerosol particles, being 0.84±0.03 and 2.25±0.09, respectively. We attribute the diurnal variation of submicron aerosol concentration to coagulation, photochemistry and the evolution of the boundary layer. Precipitation does not seem to play a role in the observed afternoon maximum in aerosol concentration. Submicron aerosol mass at the site peaks in the summer (12.1±6.7mg m-3), with the summer value being twice that in the winter. Of the chemically analyzed ionic components (which exclude carbonaceous aerosols), SO4= and NH4+ constitute the dominant species at the SGP seasonally, contributing 23-30% and 9-12% of the submicron aerosol mass, respectively. Although a minor species, there is a notable rise in NO3- mass fraction in winter. We contrast the optical properties of dust and smoke haze. The single scattering albedo w0 shows the most remarkable distinction between the two aerosol constituents. We also present aircraft measurements of vertical profiles of aerosol optical properties at the site. Annually, the lowest 1.2 km contributes 70% to the column total light scattering coefficient. Column-averaged and surface annual mean values of hemispheric backscatter fraction (at 550 nm), w0 (at 550 nm) and å (450 nm, 700 nm) agree to within 5% in 2001. Aerosols produce a net cooling (most pronounced in the spring) at the ARM site

A Study of the Extinction-to-Backscatter Ratio of Marine Aerosol during the Shoreline Environment Aerosol Study*

Abstract Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which...

Optical and meteorological properties of smoke‐dominated haze at the ARM Southern Great Plains Central Facility

The optical and meteorological properties of smoke‐dominated haze at the Southern Great Plains Central Facility of the Atmospheric Radiation Measurement (ARM) Program from 1997 to 2001 are presented. Aerosol optical properties for smoke‐dominated haze vary with season at the ARM site, with an inter‐seasonal mean single scattering albedo, Ångström exponent, hemispheric backscatter fraction and hygroscopic growth factor of 0.91 ± 0.03, 2.0 ± 0.1, 0.14 ± 0.02 and 1.6 ± 0.2, respectively. In addition, the local seasonal direct radiative forcing for smoke haze ranges from −0.25 to −3.14 Wm−2. Relative to smoke‐free periods, months with enhanced smoke are characterised by reduced cloud cover, increased shortwave radiation, lower liquid water path and precipitation.

Vertical variability of aerosol properties observed at a continental site during the Lindenberg Aerosol Characterization Experiment (LACE 98)

In the summer of 1998, an aerosol column closure experiment was conducted over eastern Germany as part of the Lindenberg Aerosol Characterization Experiment (LACE 98). The experimental platforms involved were two aircraft equipped with aerosol in situ instrumentation, an airborne aerosol backscatter lidar and solar irradiance sensors (upwelling and downwelling) to measure optical and microphysical aerosol properties relevant for radiative forcing in the entire atmospheric column from the boundary layer to the tropopause region. Using Mie theory and reasonable assumptions on the particle complex refractive index, the hemispheric backscatter fraction and spectral light scattering coefficients were calculated from the measured aerosol size distribution data. A comparison between optical parameters derived from particle size distribution measurements and optical properties measured by independent techniques yields an uncertainty range of ±30% for the calculated aerosol optical parameters. Detailed information on the vertical variability of the particle size distribution, the aerosol extinction coefficient and the hemispheric backscatter fraction is given, which is applicable for a realistic representation of aerosol properties in climate models. The estimated daily averaged aerosol shortwave radiative forcing varies from −33 ± 12 to −12 ± 5 W m−2, and respective aerosol optical depths at λ = 0.710 μm range from 0.18 to 0.06.

Variability of Aerosol Optical Properties at Four North American Surface Monitoring Sites

Aerosol optical properties measured over several years at surface monitoring stations located at Bondville, Illinois (BND); Lamont, Oklahoma (SGP); Sable Island, Nova Scotia (WSA); and Barrow, Alaska (BRW), have been analyzed to determine the importance of the variability in aerosol optical properties to direct aerosol radiative forcing calculations. The amount of aerosol present is of primary importance and the aerosol optical properties are of secondary importance to direct aerosol radiative forcing calculations. The mean aerosol light absorption coefficient (σap) is 10 times larger and the mean aerosol scattering coefficient (σsp) is 5 times larger at the anthropogenically influenced site at BND than at BRW. The aerosol optical properties of single scattering albedo (ωo) and hemispheric backscatter fraction (b) have variability of approximately ± 3% and ± 8%, respectively, in mean values among the four stations. To assess the importance of the variability in ωo and b on top of the atmosphere aerosol radiative forcing calculations, the aerosol radiative forcing efficiency (ΔF/δ) is calculated. The ΔF/δ is defined as the aerosol forcing (ΔF) per unit optical depth (δ) and does not depend explicitly on the amount of aerosol present. Based on measurements at four North American stations, radiative transfer calculations that assume fixed aerosol properties can have errors of 1%–6% in the annual average forcing at the top of the atmosphere due to variations in average single scattering albedo and backscatter fraction among the sites studied. The errors increase when shorter-term variations in aerosol properties are considered; for monthly and hourly timescales, errors are expected to be greater than 8% and 15%, respectively, approximately one-third of the time. Systematic relationships exist between various aerosol optical properties [σap, ωo, b, ΔF/δ, and Ångström exponent (å)] and the amount of aerosol present (measured by σsp) that are qualitatively similar but quantitatively different among the four stations. These types of systematic relationships and the regional and temporal variations in aerosol optical properties should be considered when using “climatological” averages.

Aerosol Optical properties at Sagres, Portugal during ACE-2

Aerosol light scattering and absorption properties relevant to climate change were measured as part of the Aerosol Characterization Experiment 2 (ACE-2, 15 June to 25 July 1997) at Sagres, Portugal, a site receiving anthropogenically perturbed air masses from Europe. A controlled relative humidity (RH) nephelometry system measured the dependence of the total light scattering and backscattering coefficients by particles (σsp and σbsp , respectively) upon increasing and decreasing controlled RH, maximum particle diameter (Dp) size cut, and wavelength of scattered light (λ). An aethalometer was used to measure black carbon concentrations ([BC]) to yield estimates of light absorption by particles (σap) and single scattering albedo (ω). Parameters derived from the measurements include the dependence of σsp and σbsp on RH (f(RH)), the hemispheric backscatter fraction (b), and the Angstrom exponent (A), all as functions of λ and particle Dp size cut. During polluted periods, for Dp≤10 μm, and at λ=550 nm, means and standard deviations of aerosol parameters included σsp=75.1±30.5 Mm−1 at controlled RH=27%, f(RH=82%)=1.46±0.10 and 1.22±0.06 for σsp and σbsp , respectively, and b=0.113±0.017 and 0.094±0.015 at controlled RH=27% and 82%, respectively. Transition from “clean” to polluted periods was characterized by a mean increase in σsp and σbsp by a factor of 4 to 7, increased wavelength dependence evident from an increase in A from 0.57 to 1.48, shift from 0.32 to 0.56 of the fraction of σsp from sub-micrometer Dp particles, and suppression of f(RH) by 14 to 20%. Onset of polluted periods and aerosol hygroscopic growth each resulted in ∼1% increases in ω, and an estimated range for ω at Sagres during ACE-2 was 0.91<ω< 0.97 considering uncertainty of ±0.02, aerosol hygroscopic growth, and air mass influences. Evidence for the influence of hysteresis in f(RH) was greatest at RH=65% with mean increases of 20% during “clean” periods and 10% during polluted periods. These measurements contribute to characterizing ground-level aerosol optical properties for a site that receives “clean” and anthropogenically perturbed aerosol.

Vertical profiles of aerosol scattering and absorption measured in situ during the North Atlantic Aerosol Characterization Experiment (ACE-2)

In situ measurements of dry aerosol optical properties were made during the North Atlantic Aerosol Characterization Experiment in the vicinity of the Canary Islands. Profiles of aerosol scattering in 3 wavelengths and absorption coefficients are presented for 2 cases where North African mineral dust layers were found above a marine boundary layer (MBL) influenced by varying pollution level. The scattering and absorption coefficients showed large differences in magnitude and variability; not only between the 2 cases of mineral dust (e.g., the mean scattering in the green wavelength was 9.8×10-6±1.6×10-6 m-1 and 2.1×10-5±7.3×10-6 m-1), but also within a given layer (e.g., from less than 1×10-5 to 3.1×10-5 m-1 during the second flight). This was also true for the polluted MBL where it is likely due to origin, age and history of the air mass as well as on the mixing state of the MBL. The scattering, backscattering and absorption coefficients have been used to calculate aerosol optical properties such as hemispheric backscatter fraction, single scattering albedo, aerosol optical depth and the Ångström exponent, which are also presented. The hemispheric backscatter fraction was found to be smaller in the mineral dust layer than in the MBL, as was the wavelength dependence on scattering. The single scattering albedo of dry aerosols was found to vary between 0.73±0.12 and 0.91±0.12 in the dust layer and showed large variability within and between the dust cases. This might be due to the mixing state of the dust layer, but could also be explained by the dust having different optical properties depending on local origin within the same major source region.

Aerosol light scattering properties at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1)

Large uncertainties remain in understanding the effects of ambient aerosols on climate. As part of the First Aerosol Characterization Experiment (ACE 1, November 15 to December 15, 1995), the total light scattering and backscattering coefficients by particles (σsp and σbsp) were measured at Cape Grim, Tasmania. Such measurements were made to characterize aerosol radiative forcing relevant to climate change at a southern hemisphere site. A controlled relative humidity (RH) nephelometry system (humidograph) was used to measure the dependence of σsp and σbsp upon controlled RH, upper particle diameter (Dp), and wavelength of light (λ). Parameters derived from the measurements necessary to incorporate aerosols into climate models include the dependence of light scattering on aerosol hygroscopic growth (ƒ(RH)) and the hemispheric backscatter fraction (b), both as functions of wavelength of light. Differences in parameter values during clean marine and non‐clean‐marine time periods were greatest for σsp and σbsp for Dp ≤ 1 μm and were least for ƒ(RH) and b. During clean marine conditions, for Dp ≤ 10 μm, and at λ=550 nm, means and standard deviations of key parameters measured included ƒ(RH equal to 82%) = 1.98±0.08 for σsp, ƒ(RH equal to 82%) = 1.48±0.09 for σbsp, b = 0.16±0.01 at RH equal to 22%, and b = 0.12±0.03 at RH equal to 82%. These measurements contribute to characterizing aerosol optical properties at a remote marine site, serve as a basis of comparison to polluted sites, and may provide input to climate models.