UV-RSS - PMOD/WRC

DDR(380nm)
Direct-to-diffuse UV solar irradiance ratio for a UV rotating shadowband spectroradiometer
(UV-RSS) and a UV multi-filter rotating shadowband radiometer (UV-MFRSR)
K. Lantz, P. Disterhoft, and P. Kiedron
Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA
J. Slusser
Natural Research Ecology Laboratory, Colorado State University, Fort Collins, Colorado, USA
Abstract. The purpose of the work is to compare
radiative transfer model calculations (TUV) with the
results from the UV-Rotating Shadowband
Spectroradiometer (UV-RSS) to estimate direct-to-diffuse
solar irradiance ratios (DDR) that are used to evaluate the
possibility of retrieving aerosol single scattering albedo
(SSA) under a variety of atmospheric conditions: large
and small aerosol loading, large and small surface albedo.
Results from the parameterization of the diffuse and direct
irradiance are used to correct global horizontal irradiance
for the effects of non-Lambertian angular response of
instruments such as Brewer spectrophotometers within the
NOAA-EPA UV Network and the U111 spectroradiometer
at Table Mountain.
Introduction
The purpose of the work is to compare radiative
transfer model calculations (TUV) with the results from
the UV-Rotating Shadowband Spectroradiometer
(UV-RSS) to estimate direct-to-diffuse solar irradiance
ratios (DDR). The direct to diffuse ratio is a powerful
tool as an indicator of atmospheric conditions and can
provide information on aerosol loading, changing surface
albedo, and cloud conditions [Kiedron et al., 2005]. The
direct to diffuse solar irradiance ratios are used to evaluate
the possibility of retrieving aerosol single scattering albedo
(SSA) under a variety of atmospheric conditions: large
and small aerosol loading, large and small surface albedo
[Krotkov et al., 2005; Petters et al., 2003]. As an
example, results from the parameterization of the diffuse
and direct irradiance are used to correct global horizontal
irradiance for the effects of non-Lambertian angular
response of instruments such as Brewer
spectrophotometers within the NOAA-EPA UV Network at
Table Mountain. The impact of the corrections will be
evaluated in the context of accuracy of erythemal dose
from those instruments.
NOAA-EPA Brewer Network
The new NOAA-EPA Brewer Network (NEUBrew)
consists of six sites across the continental United States
and includes Bondville, IL, Fort Peck, MT, Raleigh, NC,
Mountain Research Station, CO, Boulder, CO, and
Houston, TX. The network locations were chosen because
of their proximity to aerosol and cloud instrumentation
sites associated with programs at the National Oceanic and
Atmospheric Administration’s (NOAA) SURFRAD
Program, the USDA UV Monitoring and Research
Program [Bigelow et al, 1998], and several U.S.
universities. Table 1 lists the sites, location, and
elevation.
Table 1. NEU-Brew site locations
http://esrl.noaa.gov/gmd/grad/neubrew/
Site Latitude, Longitude Elevation
Bondville, IL 40.040, 88.360 213 m
Ft Peck, MT 40.310, 105.100 634 m
MRS, CO 40.031, 105.533 2896 m
Raleigh, NC 35.720, 78.680 124 m
Boulder, CO 40.125, 105.236 1689 m
Houston, TX 29.720, 95.343 18+ stories
Instrumentation and RT Model
A UV rotating shadowband spectroradiometer
(UV-RSS) measures once a minute simultaneous direct and
diffuse irradiances in the nominal wavelength range of 297
nm – 385 nm at 734 pixels with resolution varying from
0.32 nm to 0.61 nm (FWHM) [Kiedron et al., 2001].
The instrument is deployed at Table Mountain, Boulder
Colorado since June 2003. The UV-RSS is collocated
with a UV multifilter rotating shadowband radiometer
(UV-MFRSR), a high resolution (0.1 nm FWHM) USDA
U111 scanning spectroradiometer, and 3 MKIV Brewer
spectrophotometers from a new NOAA-EPA UV network.
The UV-MFRSR is part of the USDA UV monitoring
network and the instrument deployed at Table Mountain
has been set-up to measure once a minute total and diffuse
solar irradiance in nominal 2-nm channel bands in 7
channels at 300, 305, 311, 317, 325, 332, 368 nm
[Harrison et al., 1994]. The radiative transfer model used
in the analysis is the TUV radiative transfer code
developed by Madronich [1993].
Discussion of Future Work
Our work will analyze the diffuse to direct ratio
(DDR) under a variety of atmospheric conditions at the
Table Mountain Test Site, 15.0 km north of NOAA,
4
3
2
1
0
1.0
1.5
2.0
UpperEnvelope=0.34225+7.7623*exp(-0.87249*m)
2.5
Airmass - m
Figure 1. Direct to Diffuse (DDR) at 380 nm from the
UV-RSS at Table Mountain for 5412 spectra.
3.0
3.5
4.0

DDR/DDR(368nm)
Resid [%]
Mt, August 31, 2005
Table
GMT= 19.0166
Hour
58.2835¡
SEA=
1.17479
Airmass=
DDR(UVMFRSR)
3rd degree polynomial
Fit
Irradiance [W/m2/nm]
Boulder, CO.
The UV-RSS has been located at Table Mountain since
June, 2003. The UV-MFRSR 00286 has been located at
Table Mountain since July, 2005. Radiative transfer
calculations of DDR will be compared to the
measurements under a variety of atmospheric conditions.
Figure 1 shows the DDR at 380 nm from the UV-RSS for
5412 spectra from June, 2003 – July, 2006 and gives an
indication of the typical values seen under a variety of
conditions.
One application of this work will be to correct global
solar irradiance for the non-Lambertian response of the
Brewer spectrophotometers in the NEUBrew Network.
The correction of the angular response requires knowledge
of the partitioning of solar irradiance into direct and
diffuse components. All of the NEUBrew sites have are
collocated with a UV-MFRSR; however the UV-MFRSR
measures the diffuse and total solar irradiance in discrete
2-nm bands and not across the same wavelength region as
the Brewer spectrophotometer. The UV-RSS in
proximity to the UV-MFRSR at Table Mountain and in
concert with RT models provides a means of
parameterizing the DDR across the Brewer’s spectral
range. The parameterized DDR with the collocated
UV-MFRSR measurements at 368 nm can be used for the
angular response corrections to the Brewer’s total solar
irradiance.
Figure 2 shows the spectral diffuse to direct ratio
(DDR) relative to the DDR at 368 nm of the UV-RSS from
300 – 380 nm on a clear-day, August 31, 2005 at 19 UTC.
Also shown is the relative DDR for the UV-MFRSR in the
seven channels. The DDR is fitted with a 3 rd degree
polynomial with the residuals given above the main plot.
The right hand side of the plot gives the total and diffuse
solar irradiances from the spectrograph and filter
instrument.
Acknowledgments. The authors wish to thank Charles Wilson
of the Central UV Calibration Facility for UV-MFRSR
calibrations. This work was funded through a U.S.
Environmental Protection Agency Star Grant #RD833224.
References
Bigelow, D.S., J. Slusser, A. Beaubein, J. Gibson, The USDA
ultraviolet radiation monitoring program, Bull. Am. Meteorol.
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Kiedron, P., L. Harrison, J. Berndt, J. Michalsky, A. Beaubien,
Specification and Performance of UV Rotating Shadowband
Spectroradiometer, Proceedings of SPIE, Ultraviolet
Space-based Measurements, Models, and Effects, vol. 4482,
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Aerosol ultraviolet absorption experiment (2002 to 2004), part
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scattering albedo, Optical Engineering, 44(4), 2005.
Harrison, L. J. Berndt, J. Michalsky, A. Beaubien, USDA
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and operational experience, Proceedings of SPIE, Ultraviolet
Space-based Measurements, Models, and Effects, vol. 4482,
23-37, 2001.
Madronich, S., UV Radiation in the natural and perturbed
atmosphere, Environmental Effects of UV Radiation, M. Tevini,
Ed., Lewis Publisher, Boca Rotan, FL, 17-69, 1993
Petters, J., V. Saxena, J. Slusser, B. Wenny, S. Madronich,
Aerosol single scattering albedo retrieved from measurements
ofsurface irradiance and a radiative transfer model, J. Geophys.
Res., 108(D9), 4288, 2003.
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Diffuse RSS
UVMFRSR
1.2
Total RSS
UVMFRSR
DDR(RSS)
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310
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Wavelength [nm]
Figure 2. Relative Diffuse to Direct Ratio (DDR) of the UV-RSS and UV-MFRSR as a function of wavelength
on August 31, 2005 (green curve and purple squares). Right hand side of plot shows total and diffuse solar
irradiance of the UV-RSS (lines) and UV-MFRSR (squares).