Retrieval of Aerosol Microphysical Properties from MFRSR Observations
Aerosols can have significant impact on the radiative and heat balance of the Earth-atmosphere system by absorbing and scattering solar radiation (direct aerosol effect) and altering cloud optical properties and suppressing precipitation (indirect aerosol effect). However, both the sign and magnitude of the aerosol impact has proven difficult to determine due to incomplete knowledge of aerosol properties and their strong temporal and spatial variations. Reduction of these uncertainties requires an accurate global inventory of aerosol microphysical properties, such as size distribution and the refractive index. Multi-filter Rotating Shadowband Radiometers (MFRSRs) are widely deployed over the world (e.g., the surface radiation budget network). These radiometers provide measurements of the direct and the diffuse solar irradiances at six wavelengths (0.415, 0.5, 0.615, 0.673, 0.870 and 0.94 ). Currently, the direct irradiance observations are used to derive routinely spectral values of the aerosol optical depth only. We propose a simple retrieval technique that significantly extends the capability of the MFRSR to study atmospheric aerosols. In our retrieval, we assume the shape of aerosol size distribution (e.g., combination of three lognormal distributions) and the value of the real refractive index. The technique consists of three steps that compose an iterative scheme. The first step obtains the aerosol size distribution from the spectral measurements of the direct irradiance (for a given complex refractive index). To reduce the effect of ozone and water vapor contamination, we use wavelengths where ozone and water vapor weakly affect the direct irradiance (0.415 mu and 0.870 mu). The second step determines the effective value of the imaginary refractive index from the diffuse irradiance (for the aerosol size distribution determined during the first step). To reduce the effect of the surface albedo on the retrievals, we select a wavelength where the surface albedo is small (0.415 mu). The third step determines whether to stop the iterations or not. For a given iteration step, the value of the imaginary refractive index is compared with its previous value. If the relative difference exceeds the given threshold, then we repeat the first and second steps. If not, the iteration is considered to be converged. The analysis of our initial numerical results shows that accurate retrievals of aerosol characteristics can be achieved. In addition, we successfully applied the suggested technique to derive temporal variations of aerosol microphysical properties from ground-based MFRSR measurements performed in an urban region (Mexico City).
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 889063
- Report Number(s):
- PNNL-SA-42025; KP1201030; TRN: US200619%%200
- Resource Relation:
- Conference: IRS 2004: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, August 23-28, 2004, Busan, Korea, 343-346
- Country of Publication:
- United States
- Language:
- English
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