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- Author or Editor: John A. Ogren x
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Abstract
The uncertainties of integral aerosol properties calculated using aerosol size distributions retrieved from multiwavelength observations of aerosol optical depth have been determined for a variety of typical atmospheric aerosol size distributions and refractive indices. The results suggest that more information about the aerosol composition, as well as more information about the sizes that are less efficient, in the optical sense, is needed to improve the shape of the retrieved size distributions. All the calculations in this paper assume spherical homogeneous particles. The sensitivity results refer to these conditions. The moments of the retrieved size distributions are systematically underestimated and errors can be as large as −82%, −30%, and −35% for the total number of particles, the total surface, and the total volume, respectively. The errors in the mass scattering efficiency, the effective radius, and the total volume depend very much on whether the actual volume size distribution is monomodal or bimodal. For a known refractive index, the total scattering coefficient, the hemispherical backscattering coefficient, and the extinction coefficient, as well as the hemispheric backscattering to total scattering ratio and the asymmetry factor, are obtained with absolute values for the average errors less than 4%. Similar behavior was expected for cases with uncertainty in the refractive index, especially for parameters defined by the ratio of two integral properties. However, it turns out that the hemispheric backscattering coefficient and the hemispheric backscattering to total scattering ratio were poorly retrieved, reaching errors of 29% in several cases, while the asymmetry factor was very well recovered with absolute values of the average errors always under 7%. When the wavelength dependence of the refractive index is included, the retrieved size distribution is very unrealistic, with average errors in the hemispheric backscattering coefficients and the hemispheric backscattering to total scattering ratio around 30% at some wavelengths. However, even in this case the errors in the retrieved asymmetry factor stay under 8%. Thus, for spherical and homogeneous particles, the spectral optical depth data can be used to determine the asymmetry factor with little sensitivity to the assumptions in the calculations. Furthermore, the retrieved size distribution can be used as an intermediate step to extrapolate one set of optical properties from another set of optical properties.
Abstract
The uncertainties of integral aerosol properties calculated using aerosol size distributions retrieved from multiwavelength observations of aerosol optical depth have been determined for a variety of typical atmospheric aerosol size distributions and refractive indices. The results suggest that more information about the aerosol composition, as well as more information about the sizes that are less efficient, in the optical sense, is needed to improve the shape of the retrieved size distributions. All the calculations in this paper assume spherical homogeneous particles. The sensitivity results refer to these conditions. The moments of the retrieved size distributions are systematically underestimated and errors can be as large as −82%, −30%, and −35% for the total number of particles, the total surface, and the total volume, respectively. The errors in the mass scattering efficiency, the effective radius, and the total volume depend very much on whether the actual volume size distribution is monomodal or bimodal. For a known refractive index, the total scattering coefficient, the hemispherical backscattering coefficient, and the extinction coefficient, as well as the hemispheric backscattering to total scattering ratio and the asymmetry factor, are obtained with absolute values for the average errors less than 4%. Similar behavior was expected for cases with uncertainty in the refractive index, especially for parameters defined by the ratio of two integral properties. However, it turns out that the hemispheric backscattering coefficient and the hemispheric backscattering to total scattering ratio were poorly retrieved, reaching errors of 29% in several cases, while the asymmetry factor was very well recovered with absolute values of the average errors always under 7%. When the wavelength dependence of the refractive index is included, the retrieved size distribution is very unrealistic, with average errors in the hemispheric backscattering coefficients and the hemispheric backscattering to total scattering ratio around 30% at some wavelengths. However, even in this case the errors in the retrieved asymmetry factor stay under 8%. Thus, for spherical and homogeneous particles, the spectral optical depth data can be used to determine the asymmetry factor with little sensitivity to the assumptions in the calculations. Furthermore, the retrieved size distribution can be used as an intermediate step to extrapolate one set of optical properties from another set of optical properties.
Abstract
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Abstract
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 (σ ap) is 10 times larger and the mean aerosol scattering coefficient (σ sp) is 5 times larger at the anthropogenically influenced site at BND than at BRW. The aerosol optical properties of single scattering albedo (ω o) 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 ω o 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 [σ ap, ω o, b, ΔF/δ, and Ångström exponent (å)] and the amount of aerosol present (measured by σ sp) 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.
Abstract
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 (σ ap) is 10 times larger and the mean aerosol scattering coefficient (σ sp) is 5 times larger at the anthropogenically influenced site at BND than at BRW. The aerosol optical properties of single scattering albedo (ω o) 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 ω o 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 [σ ap, ω o, b, ΔF/δ, and Ångström exponent (å)] and the amount of aerosol present (measured by σ sp) 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.
Abstract
Tropospheric aerosols are calculated to cause global-scale changes in the earth's heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy—the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40–400 km and temporal scales of 2–48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or “airmass” scale (where the aerosol is often assumed to be essentially homogeneous except for plumes from point sources) and below the scales that are readily resolved by chemical transport models. The present study focuses on documenting this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.
Abstract
Tropospheric aerosols are calculated to cause global-scale changes in the earth's heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy—the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40–400 km and temporal scales of 2–48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or “airmass” scale (where the aerosol is often assumed to be essentially homogeneous except for plumes from point sources) and below the scales that are readily resolved by chemical transport models. The present study focuses on documenting this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.
