Measurement of the seasonal and annual variability of total column aerosol in a northeastern U.S. network Page: 3 of 14
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Measurement of the Seasonal and Annual Variability of Total Column Aerosol in
a Northeastern U.S. Network
J.J. Michalsky', J.A. Schlemmer, N.R. Larsont, L.C. Harrison*,
William E. Berkheiser III*, and N.S. Laulainent
Atmospheric Sciences Research Center*
University at Albany, State University of New York
100 Fuller Road
Albany, New York 12205 USA
Pacific Northwest Laboratory'
P.O. Box 999
Richland, Washington 99352 USA
ABSTRACT
A network of multi-filter rotating shadowband radiometers has operated since late 1991 in the
northeastern United States. The data acquired are simultaneous measurements of total and diffuse
horizontal irradiances in six narrowband filtered detectors and one broadband shortwave detector.
The direct normal irradiances are calculated from these measurements.
These direct data are corrected for cosine response and used to calculate extraterrestrial
irradiance (Is) using the Langley method of regressing the natural logarithm of direct irradiance
versus air mass. With frequent determinations of I., changes in I, caused by soiling and filter
degradation, for example, can be tracked.
Using these IL's, total optical depth is calculated for every clear 30-minute period in the
record. Consequently, total optical depth may be obtained on a fair number of days throughout the
year. Using daily average total optical depth we have calculated aerosol optical depths for five
wavelengths by subtracting Rayleigh scattering optical depths and Chappuis ozone absorption optical
depths at each wavelength.
The aerosol pattern at nearly every site is an annual cycle superimposed on a decaying
stratospheric loading associated with the Mount Pinatubo volcanic eruption. An attempt is made to
remove the volcanic signal using data from another site.
INTRODUCTION
As part of the Department of Energy's Quantitative Links (QL) Program, a nine-station
radiation network was established in the northeastern and mid-western United States. A goal of the
QL program is to link global and regional climate change to changes in the mix of atmospheric trace
species. The Atmospheric Sciences Research Center (ASRC) and the Pacific Northwest Laboratory
(PNL) have been funded by this program to -study change in longwave and shortwave radiation that
occurs on time scales of minutes to years as a result of aerosol and cloud changes.
The network that was established in late 1991 is shown in figure 1. The sites were selected
to be regionally representative of an area with no significant local sources of pollution. The sites are
operated by local colleges and universities or government research laboratories who perform routine
maintenance and report, or fix, malfunctions in the instruments.
The primary instrument for shortwave radiation measurements in the network is a multi-filter
rotating shadowband radiometer (MFRSR), which measures total and diffuse horizontal irradiance,
from which direct normal irradiance is calculated.' A silicon cell pyranometer is inverted to measure
upwelling shortwave radiation, primarily for the detection of snow cover. A pyrgeometer measures
downwelling hemispherical longwave radiation. Supplementing these radiation instruments are a rain
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Michalsky, J. J.; Schlemmer, J. A.; Harrison, L. C.; Berkheiser, W. E., III; Larson, N. R. & Laulainen, N. S. Measurement of the seasonal and annual variability of total column aerosol in a northeastern U.S. network, article, September 1, 1994; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc669997/m1/3/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.