Measurements of aerosol optical depth and diffuse-to-direct irradiance ratios in the Northeastern United States
- Pacific Northwest Lab., Richland, WA (United States)
- State Univ. of New York, Albany, NY (United States); and others
Simultaneous observations of total and diffuse irradiance on a horizontal surface in six narrowband filtered detectors and one broadband shortwave detector have been made since late 1991 at a nine-site network of multi-filter rotating shadowband radiometers. From these measurements, the direct normal irradiance values are calculated. These data are then used to calculate the outside-the-atmosphere direct irradiance (lo) and total optical depth using the Langley method of regressing the natural logarithm of the direct irradiance against air mass for cloud-free conditions. Frequent determinations of lo allow tracking of changes in lo caused by soiling and filter degradation. The daily average total optical depth is calculated in two ways: (1) from the slope of the Langley regression line and (2) from 30-minute averages calculated from the Beer-Lambert-Bougeur law using the median lo for that day. Finally, aerosol optical depths for five wavelengths (the other narrowband wavelength is used to estimate water vapor) are obtained by subtracting Rayleigh scattering and Chappuis ozone absorption optical depths from the total optical depths. The aerosol pattern at each site is consistent with an annual cycle superimposed on a decaying aerosol loading associated with the Mt. Pinatubo eruption. Moreover, the wavelength dependence of the aerosol pattern shows seasonal changes in the aerosol size distribution. The irradiance data are also used to calculate the diffuse-to-direct irradiance ratio, a quantity which in theory is related to the aerosol optical depth and surface albedo. A radiative transfer model based on the adjoint method, combined with a nonlinear least squares method. is used to estimate aerosol optical depth and surface albedo from the observed diffuse-to-direct ratios. The aerosol optical depths are in good agreement with those calculated from the direct beam data and the surface albedos are in accord with other observations.
- OSTI ID:
- 166554
- Report Number(s):
- CONF-9510126-; TRN: 95:008181-0045
- Resource Relation:
- Conference: Annual meeting of the American Association for Aerosol Research, Pittsburgh, PA (United States), 9-13 Oct 1995; Other Information: PBD: 1995; Related Information: Is Part Of American Association for Aerosol Research (AAAR) `95; PB: 464 p.
- Country of Publication:
- United States
- Language:
- English
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