Recent Advances in Satellite Measurements of Aerosol Albedo L. Franck1, A. Masterson2, K. Pistone1, K. Suski2 1Scripps Institution of Oceanography, UCSD, La Jolla, CA 2Department of Chemistry, UCSD, La Jolla, CA ABSTRACT Error in determining aerosol optical properties, and the effects of anthropogenic aerosols on planetary albedo represent a large source of uncertainty in global climate models. Aerosols directly and indirectly affect planetary albedo in a number of ways: by anisotropically scattering incoming solar radiation, and by altering cloud microphysical properties, differentially affecting their size and lifetime in the troposphere. Remote sensing techniques by satellite-based instruments are used to measure both the spatial distribution and the optical properties of aerosols. This information shows the most potential for quantifying aerosol optical properties and determining the magnitude of the cloud albedo effect. Retrieved raw data must subsequently be adapted into parameters usable by scientists, such as the single scattering albedos, through the use of retrieval algorithms. There are still limitations to these methods and uncertainties in the procured data, but both the instrumentation and the computational techniques are improving. The launch of the Aerosol Polarimetry Sensor as a part of NASA's Glory Mission in December 2008 will enhance measurement capabilities to better determine the influence of aerosols on planetary albedo. ________________________________________ 1. Introduction and Background 1.1 Aerosol Properties and Effects An aerosol is defined as a solid or liquid suspended in a gas phase. Anthropogenic and natural aerosols, such as SO2-derived sulfate particles, smoke particles from biomass burning, wind blown mineral dust, bioorganic compounds, and sea salt aerosols demonstrate a strong climatic influence in the troposphere. Aerosols are chemically heterogeneous, have unpredictable spatial distributions, and have atmospheric residence times ranging from seconds to months. As a result, they are the largest source of uncertainty in predicting climate change and in determining the terrestrial radiative balance (Quaas et al. 2008). Incoming shortwave solar radiation is directly scattered and absorbed by atmospheric aerosols, which results in an increase in albedo and a decrease in surface temperature due to a reduction in solar radiation reaching the Earth's surface. However, terrestrial longwave radiation is also absorbed by aerosols, which can then lead to surface warming (Curry and Webster 1999). The range of climatic responses is difficult to quantify and to incorporate into global climate models (IPCC 2007). In addition to these direct effects, aerosols also have indirect effects on cloud albedo, lifetime, and microphysical properties (Twomey 1977; Albrecht 1989). Aerosols serve as cloud water droplet formation sites called cloud condensation nuclei (CCN). Tropospheric aerosols increase the number of CCN at constant liquid water content (LWC), reducing the cloud droplet effective radius, increasing in the number of droplets per cloud, and increasing the cloud optical thickness (Twomey 1977). Smaller cloud droplets more effectively scatter incident solar radiation, which increases cloud albedo; this is known as the "Twomey effect" or the "first indirect effect" (Haywood and Boucher 2000). The second indirect effect results from cloud microphysical changes, which cause precipitation suppression and increased cloud lifetime (Albrecht 1989; Andreae and Rosenfeld 2008). By increasing the lifetime of clouds, aerosols increase fractional cloud cover leading to an increase in albedo. Atmospheric dimming and subsequent surface cooling can also occur due to increased cloud cover (Curry and Webster 1999). Quantifying these effects via satellite remote sensing has proven difficult; the following 1review assesses the development of technologies used to measure aerosol albedo from satellites and discusses current advances in the science. 1.2 Aerosol Optical and Physical Parameters One must accurately determine spatial and temporal estimates of aerosol optical parameters and must also reconcile the large numerical inconsistencies between different satellite and land-based measurements in order to construct an accurate model of Earth’s radiative budget. Consistent measurements of the Ångstrom exponent, the asymmetry g factor, the aerosol optical depth (AOD), and the single scattering albedo (SSA) ω are critical in evaluating the direct climatic influence of tropospheric aerosols (Liu et al. 2008). The Angstrom exponent relates the wavelength dependence of optical depth and the asymmetry parameter g, and is an integrated ratio of the angular distribution of scattered radiation. AOD quantifies the amount of incoming solar radiation scattered or absorbed by atmospheric aerosols. Single scattering albedo (SSA) ω, the ratio of scattering optical depth to total optical depth, is the most important parameter in determining the magnitude of aerosol radiative forcing (Seinfeld and Pandis 1998). Incoming solar radiation is scattered in all directions; the efficiency of scattering is dependent upon the incident wavelength and the size and type of the aerosol droplets (Charleson et al. 1992; Quaas et al. 2008). Aerosol SSA varies globally, therefore radiative balance models usually employ SSA as a derived quantity from total AOD measurements, which are intrinsically subject to their own radiometric error. Errors in these measurements are usually large due to spatial variability and error in determination of aerosol properties (Ayash et al. 2008). The usage of different model parameters such as AOD measurements from ground-based and satellite-based sources is a large source of error because measurements are made at different times and are frequently site specific. Modeling the indirect effects of aerosols on albedo also presents a challenge; models are limited in their ability to characterize the effects of heterogeneous aerosol properties on clouds already demonstrating a wide range of microphysical and physical properties (Ayash et al. 2008). Increasing the number of the measured wavelength bands in optical property measurements, and determining the angular distribution of scattered radiation will prove critical in detailing the primary interaction of aerosols
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