Barrow
Mauna Loa
Samoa
South PoleE. G. Dutton
Barrow
Mauna Loa
Samoa
South Pole
1977-92
Measurements of direct solar irradiance have been carried out routinely since 1977 at each of the four baseline atmospheric monitoring stations operated by the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL). The four stations are located at Barrow, Alaska, U.S.A. (71° N); Mauna Loa, Hawaii, U.S.A. (19° N); Samoa, U.S. Territory (14° S); and the South Pole (90° S). The sites were chosen for their global distribution and their relatively great distance from significant anthropogenic sources of gaseous and particulate pollutants. Monitoring is conducted by means of pyrheliometers and wideband Schott glass cutoff filters. Although not as accurate or precise as monochromatic techniques, the wideband pyrheliometric method is stable and has provided a long-term aerosol optical depth record from all four NOAA/CMDL baseline monitoring sites. The measured values are compared with the results of solar irradiance calculations [using the spectral one-layer radiative transfer model of Bird and Riordan (1986)] to derive aerosol optical depth (AOD), defined as the aerosol component of the exponent in the exponential decrease in solar beam intensity as the beam passes through the atmosphere. The resulting data set is presented as mean AOD for the wavelength range 0.35-0.69 µm. Individual AOD values are expressed as monthly average anomalies (i.e., deviations) in relation to a baseline comprised of all AOD values from the nonvolcanic years at a given site, with mean seasonal variations removed. Over the range of the data set, these AOD anomalies are essentially proportional to the radiation loss due to aerosol backscattering to space. An approximate estimate of the climatic impact of an aerosol-generating event (such as an explosive volcanic eruption) can therefore be obtained by integrating the area under the AOD anomaly curve over the period of the event. The measurement accuracy of the computed AOD anomalies has been estimated at ±0.04 with an overall precision of less than ±0.02, based on comparisons with AODs derived by sunphotometry (Dutton and Christy 1992).
NOAA/CMDL aerosol optical depth monitoring sites
CITE AS: Dutton, E.G. 1994. Aerosol optical depth measurements from four
NOAA/CMDL monitoring sites. In Trends: A Compendium of Data on Global Change.
ORNL/CDIAC-65. Carbon Dioxide Information Analysis Center, Oak Ridge National
Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
Date created 12/06/96 (jaw)
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