ARM TR-059

> Multi-Filter Rotating Shadowband Radiometer (MFRSR) Handbook
>
> January 2005
>
> D. Flynn
>
> G. Hodges
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 4](#general-overview)

[2. Contacts 4](#contacts)

[3. Deployment Locations and History
5](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 6](#near-real-time-data-plots)

[5. Data Description and Examples 7](#data-description-and-examples)

[6. Data Quality 10](#data-quality)

[7. Instrument Details 12](#instrument-details)

**Tables**

[1. MFRSR Status as of January 1, 2005 5](#__RefHeading___Toc95726688)

[2. Wavelengths of Narrowband (10-nm) Filters Used in the MFRSR, and
Main Trace Species Measured (Michalsky et al. 1995).
8](#__RefHeading___Toc95726689)

# General Overview

The multi-filter rotating shadowband radiometer (MFRSR) takes spectral
measurements of direct normal, diffuse horizontal and total horizontal
solar irradiances. These measurements are at nominal wavelengths of 415,
500, 615, 673, 870, and 940 nm. The measurements are made at a
user-specified time interval, usually about one minute or less. The
sampling rate for the Atmospheric Radiation Measurement (ARM) Program
MFRSRs is 20 seconds. From such measurements, one may infer the
atmosphere\'s optical depth at the wavelengths mentioned above. In turn,
these optical depths may be used to derive information about the column
abundances of ozone and water vapor (Michalsky et al. 1995), as well as
aerosol (Michalsky et al. 1994) and other atmospheric constituents.

A silicon detector is also part of the MFRSR. This detector provides a
measure of the broadband direct normal, diffuse horizontal and total
horizontal solar irradiances. A MFRSR head that is mounted to look
vertically downward can measure upwelling spectral irradiances. In the
ARM system, this instrument is called a \"multi-filter radiometer
(MFR)\". At the Southern Great Plains (SGP) there are two MFRs; one
mounted at the 10-m height and the other at 25 m. At the North Slope of
Alaska (NSA) sites, the MFRs are mounted at 10 m. MFRSR heads are also
used to measure \"normal incidence\" radiation by mounting on a solar
tracking device. These are referred to as normal incidence multi-filter
radiometers (NIMFRs) and are located at the SGP and NSA sites. Another
specialized use for the MFRSR is the narrow field of view (NFOV)
instrument located at SGP. The NFOV is a ground-based radiometer (MFRSR
head) that looks straight up. Complete instrument descriptions for the
MFR, NIMFR, and NFOV are available.

# Contacts

## Mentor

Gary Hodges

NOAA/ARL/SRRB

Boulder, CO 80305**\
**Phone: 303-497-6460\
Email: <Gary.Hodges@noaa.gov>

Donna Flynn (data analysis)\
Pacific Northwest National Laboratory

P.O. Box 999, Mailstop: K9-24

Richland, WA 99352\
Phone: 509-375-6978\
Email: <Donna.Flynn@pnl.gov>

John Schmelzer\
Pacific Northwest National Laboratory

P.O. Box 999, Mailstop: K5-08

Richland, WA 99352\
Phone: 509-375-3729

Fax: 509-375-3614\
Email: <john.schmelzer@pnl.gov>

## Instrument Developer

[Yankee Environmental Systems, Inc.](http://www.yesinc.com/)

[Airport Industrial Park](http://www.yesinc.com/)

101 Industrial Blvd.

Turners Falls, MA 01376

Phone: 413- 863-0200

# Deployment Locations and History 

[]{#__RefHeading___Toc95726688 .anchor}Table 1. MFRSR Status as of
January 1, 2005

  -------------------------- ---------------------- ---------------------
  **Site**                   **Collect Data**       **Ingest Data**

  E1 (Larned, KS)            YES                    YES

  E2 (Hillsboro, KS)         NO                     NO

  E3 (LeRoy, KS)             YES                    YES

  E4 (Plevna, KS)            YES                    YES

  E5 (Hesston, KS)           YES                    YES

  E6 (Towanda, KS)           YES                    YES

  E7 (Elk Falls, KS)         YES                    YES

  E8 (Coldwater, KS)         YES                    YES

  E9 (Ashton, KS)            YES                    YES

  E10 (Tyro, KS)             NO                     NO

  E11 (Byron, OK)            YES                    YES

  E12 (Pawhuska, OK)         YES                    YES

  E13 (Central Facility)     YES                    YES

  C1 (Central Facility)      YES                    YES

  E15 (Ringwood, OK)         YES                    YES

  E16 (Vici, OK)             YES                    YES

  E18 (Morris, OK)           YES                    YES

  E19 (El Reno, OK)          NO                     NO

  E20 (Meeker, OK)           NO                     NO

  E21 (Okmulgee, OK)         NA                     NA

  E22 (Cordell, OK)          NO                     NO

  E24 (Cyril, OK)            YES                    YES

  E25 (Seminole, OK)         NA                     NA

  E27 (Earlsboro, OK)        YES                    YES

  TWP C1 (Manus Island)      YES                    YES

  TWP C2 (Nauru Island)      YES                    YES

  TWP C3 (Darwin)            YES                    YES

  NSA C1 (Barrow, AK)        YES                    YES

  NSA C2 (Atqasuk, AK)       YES                    YES
  -------------------------- ---------------------- ---------------------

**Note about the MFRSRs at the SGP Extended Facilities (EFs)**:

Because of limited resources, efforts to maintain operability and
calibrations of MFRSRs and associated devices (MSRs, NIMFRs, and NFOVs)
are concentrated on so-called \"critical sites.\" These critical sites
are the SGP Central Facility, the Tropical Western Pacific (TWP), and
the NSA. Improvements and upgrades are quite limited for the MFRSRs at
the SGP EFs (except E13 located at the Central Facility). Data users
should use extreme caution in making assumptions about calibration
stability, continuity of data, and alignments that affect the
reliability of the total, diffuse, and direct irradiance components at
the SGP EFs.

Many MFRSRs have been in operation going on 10 or more years and are
starting to show their age. In fact, some have been removed from service
pending repair (E2, E19, E20, and E22). Primarily it is the interference
filters that are starting to show their age. ARM is currently in
negotiations with a company to manufacture replacement filters. It is
hoped this will occur in FY05. The plan is to replace the filters in
every instrument when suitable filters are procured.

An engineering change request is currently in process that will replace
the current MFRSR data logging system to Campbell Scientific loggers.
The expectation is the Campbells will be less costly to purchase and
maintain than new Yankee logger boards. Additionally, because Campbells
are universally used, not only in the ARM Program but also in general,
there will be more resources and support available for use with the
MFRSR.

An additional motivation for the logger board change is the improved
programming flexibility and sampling rate of the Campbell CR10X.
Currently, the MFRSR shadowband operation mode involves aligning the
instrument and several of its components very precisely and then
programming the shadowband position to be set for each measurement
according to solar ephemeris calculations (which are based on inputs of
time, date, and latitude/longitude). If the initial instrument
alignments are off or if they shift with time, the band position is then
also off. This results in the diffuser not being completely shaded and
consequently the measurements are erroneous. A solution to this is to
allow the band position to be monitored and corrected, if necessary,
based on real-time evaluation of the instrument measurements. By
scanning measurements near the assumed correct shading band arm
position, a jump in signal will indicate the edge of the band's proper
position. This jump corresponds to when the periphery of the diffuser is
seeing direct sun. Adjusting the band position several steps in the
appropriate direction would ensure the diffuser is completely shaded. If
the edge cannot be determined (for example, on an overcast day) the
measurement is made with the band in the position that is based on the
ephemeris calculation. This technique was developed and has successfully
been implemented by Chuck Long for a rotating shadowband radiometer
instrument he has developed. The Campbell CR10X can readily be
programmed to do this and the sampling is rapid enough to permit
scanning the measurements and taking the final measurement within a
short period of time.

# Near-Real-Time Data Plots

See the [MFRSR General
Plots](http://education.arm.gov/nsdl/visualization.shtml).

See the [MFR General
Plots](http://education.arm.gov/nsdl/visualization.shtml).

# Data Description and Examples

## Data File Contents

### Primary Variables and Expected Uncertainty

As mentioned above, the primary quantities measured by the system are
the:

1.  Direct normal solar spectral irradiance (W m^-2^ nm^-1^)

2.  Total horizontal solar spectral irradiance (W m^-2^ nm^-1^)

3.  Diffuse horizontal solar spectral irradiance (W m^-2^ nm^-1^).

These quantities are measured at wavelengths of 500, 615, 673, 870 and
940 nm. The measurements at each wavelength are made by a single
filtered detector with a nominal 10-nm full width at half maximum)
(FWHM) bandwidth.

Table 2 shows the wavelengths used in the MFRSR and the primary
atmospheric constituents measured with each wavelength channel
(Michalsky et al. 1995).

[]{#__RefHeading___Toc95726689 .anchor}Table 2. Wavelengths of
Narrowband (10-nm) Filters Used in the MFRSR, and Main Trace Species
Measured (Michalsky et al. 1995).

  ------------------------------------- ---------------------------------
  **Wavelength (nm)**                   **Trace Species**

  415                                   aerosol

  500                                   aerosol, ozone

  615                                   aerosol, ozone

  673                                   aerosol, ozone

  870                                   aerosol

  940                                   water vapor
  ------------------------------------- ---------------------------------

Ozone abundances may be inferred by measurements made by the 500-, 615-,
and 673-nm channels. These wavelengths, particularly 615 and 673 nm, are
influenced by the Chappuis band of ozone (Goody and Yung 1989). Water
vapor primarily affects the 940-nm channel. Rayleigh scattering most
strongly affects the lower wavelength channels. This phenomenon is well
understood and, therefore, can be removed from an analysis of the
optical depth measurements made at all wavelengths.

The uncalibrated silicon detector provides a measure of the:

1.  Direct normal solar broadband irradiance (counts)

2.  Total horizontal solar broadband irradiance (counts)

3.  Diffuse horizontal solar broadband irradiance (counts).

These quantities are uncalibrated and reported as counts. Note that the
actual output of the instrument is millivolts, and the use of "counts"
is historical and has been attached to the MFRSR since it was developed.
The counts are linearly scaled using broadband radiometers to be roughly
equivalent to W/m^2^. Zhou et al. (1995) discusses the proportionality
between the MFRSR broadband channels and broadband irradiances measured
by the World Meteorological Organization (WMO) first-class thermopile
radiometers.

#### Definition of Uncertainty

Note about MFRSR calibration as of May 30, 2000:

We have long suspected that the amplitude of measured irradiances from
the MFRSR is sensitive to the ambient temperature. For example, if one
examines a year-long time series of so-called \"Ios\" derived from
Langley regressions a sinusoidal pattern in the time series will be
seen. The period of this pattern is exactly one year and the peak of the
sine wave corresponds to the warmest times of the year while the valley
corresponds to the coldest times of the year.

This pattern, in itself, is not sufficient evidence to implicate
sensitivity to temperature. However, we retrieved the MFRSR head
associated with these data, and we put the head and logger board in an
\"environmental chamber\" and measured the temperature responsiveness of
the system. The responsiveness can be characterized by temperature
coefficients for each wavelength channel; these coefficients indicate
about a 1 to 2% change in MFRSR output for each 1-degree change in head
temperature. Typically, the MFRSR head temperature changes about 2
degrees over a diurnal period and about 5 degrees over the year.
Therefore, the potential for output fluctuations over the course of a
year may be as high as -+ 5%. If we consider the atmosphere to be gray,
the temperature sensitivity errors induce an equivalent broadband error
of about +- 50 w/m\^2 error (by contrast, a drifting filter may
contribute errors on the order of 500w/m\^2!!). In short, the
temperature sensitivity errors may be significant.

After measuring the temperature sensitivity we devised a way to correct
the data based on the measured temperature coefficients. When these
corrections were applied, the sinusoidal pattern in the MFRSR Ios
disappeared, and the calibration stability of the MFRSR was essentially
flat over the entire year. Furthermore, the temperature-corrected MFRSR
data was used to drive a broadband model and the output of this model
was compared with broadband measurements. In many cases, the use of
temperature-corrected MFRSR data (as opposed to the uncorrected data)
caused the agreement between the model and the broadband data to improve
significantly. (Note: these calculations are not yet finished, so we
can\'t be sure this conclusion will hold). All this evidence taken
together strongly suggests that the temperature sensitivity is real.

Independent evidence of the temperature sensitivity comes from MFRSR
data collected as part of old, defunct, non-ARM programs in which the
sinusoidal wave had been observed but not explained until now. Again, in
these old data sets the peak of the sine wave occurs during the warm
times of the year and the valley into coldest times of the year.

We (Pacific Northwest National Laboratory \[PNNL\], Yankee Environmental
Systems, and Atmospheric Sciences Research Center \[ASRC\]) don\'t know
what\'s causing this problem, and we don\'t know if the magnitude of the
problem is different from head to head. The analysis and temperature
chamber operation is costly, and we don\'t have the time or budget
needed to get to the bottom of this problem. The data can be partially
corrected by using monitoring the Ios from Langley regressions, but this
is at best, an imperfect solution.

Yankee knows about this problem, and they may try to fix it (this could
take a very long time).

Note about MFRSR calibration as of November 1, 1999:

Installation of new interference filters has significantly reduced the
calibration drift. To date, the calibrations (for MFRSRs that contain
the new filters) appear to be very stable. The new units have been
installed at the E13 and C1 sites at the SGP, and at all TWP and NSA
sites.

Note about MFRSR calibration as of March 11, 1998:

As of 11 March 1998, the calibration of the MFRSR is problematic. For
some channels (i.e., wavelengths), the calibration drifts significantly
with time. For other channels the calibration is stable but not very
accurate; in some cases, the error in the spectral fluxes may be as much
as 50%. The poor calibrations affect some of the flux measurements but
do not affect any of the optical depths derived from the MFRSR.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

-9999.0 signifies missing data.

See [MFRSR](http://science.arm.gov/tool/dod/showdod.php?Inst=mfrsr),
[MFR10M](http://science.arm.gov/tool/dod/showdod.php?Inst=mfr10m), and
[MFR25M](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=mfr25m)
Data Object Design files for ARM netCDF file header descriptions.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

![](media/image1.png){width="3.5909722222222222in"
height="5.018055555555556in"}

Figure 1.

## User Notes and Known Problems

The MFRSR used to be part of the solar and infrared observing system
(SIROS) datastream. This data-stream consisted of the MFRSR data as well
as data from the broadband radiometers. Around 1997, the SIROS
datastream was split in two: one stream became the \"Solar Infrared
Station (SIRS),\" consisting of broadband data only, while the other
became \"MFRSR,\" containing only MFRSR data. For MFRSR data including
1997 and before, you will need to retrieve SIROS data from the ARM
archive. These SIROS data contain a label that follows the pattern of
\"sgpsirosE12.a1\".

## Frequently Asked Questions

Currently there are no FAQs for this instrument.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality (DQ) health and status
results:

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by the ARM
Program\'s DQ analysts, instrument mentors, and site scientists to
monitor and diagnose DQ.

## Data Reviews by Instrument Mentor

-   **QC frequency**: Weekly

-   **QC delay**: Up to one week

-   **QC type**: Graphical plots

-   **Inputs**: Raw and Processed data

-   **Outputs**: Summary reports

-   **Reference**:

Once a week the instrument mentor downloads both the raw and processed
(NetCDF) files from the ARM Data Archive. Time series plots of the
NetCDF files for each instrument (every channel) are produced and
examined. In addition, the raw data files are processed independent of
the ARM Archive and compared to the NetCDF files from the archive for
inconsistencies. Any problems found are reported so technicians can make
the appropriate repairs. Once a month the mentor submits a report
summarizing the general health and status of all the MFRSRs.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into \"value-added\"
products or VAPs. Despite extensive instrumentation deployed at the ARM
sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not to fill unmet measurement needs,
but to improve the quality of existing measurements. In addition, when
more than one measurement is available, ARM also produces \"best
estimate\" VAPs. A special class of VAP, called a Quality Measurement
Experiment (QME), does not output geophysical parameters of scientific
interest. Rather, a QME adds value to the input datastreams by providing
for continuous assessment of the quality of the input data based on
internal consistency checks, comparisons between independent similar
measurements, or comparisons between measurement with modeled results,
and so forth. For more information see, see the [VAPs and
QMEs](http://www.arm.gov/data/vaps_all.php) web page.

An important value-added product for the MFRSR is optical depth. If the
sky is sufficiently clear, an automated Langley analysis (Harrison and
Michalsky 1994b) applied to each day\'s data provides optical depths for
all spectral channels except the 940 nm channel. (This channel requires
a modified Langley analysis \[see Michalsky et al. 1995\] that is not
done as part of the routine data processing).

# Instrument Details

## Detailed Description

### List of Components

Many of the MFRSRs deployed at the SGP site have been manufactured by
PNNL. The other MFRSRs are manufactured by Yankee Environmental Systems
(Model MFR-6).

### System Configuration and Measurement Methods

The sensors associated with each wavelength are all located in a small
canister capped by a level, upward looking, Lambertian diffuser.
Radiation that strikes the diffuser is directed towards the sensors.
These sensors consist of a single silicon photodetector for measuring
broadband radiation, and a set of interference-filter-photodiode
detectors for measuring spectral irradiances. The diffuser is shaded at
periodic intervals by a rotating shadowband. During these intervals, the
total horizontal diffuse radiation is measured for each wavelength (as
well as the broadband channel). The shadowband is then moved so the
diffuser is fully exposed to the sky. The total horizontal downward
radiation is then measured. The difference between the total horizontal
and diffuse radiation gives the downward component of the direct beam
solar irradiance. From this quantity, the direct beam solar irradiance
is easily calculated. The measurements are corrected for the cosine
response of the diffuser/sensor assembly as well as the \"excess sky\"
obscured by the shadowband during the diffuse radiation measurement. A
detailed discussion of the instrument\'s measurement methods is
contained in Harrison and Michalsky (1994a).

### Specifications

The spectral irradiances are measured at or close to, the following
wavelengths: 415, 500, 615, 673, 870 and 940 nm. These measurements are
made over a passband of nominally 10 nm. A silicon photodiode measures
broadband irradiances. These irradiances are reported in \"counts\" and
are not calibrated per se, but rather linearly scaled to broadband
instruments.

## Theory of Operation

The MFRSR is an instrument that directly measures global and diffuse
components of spectral solar irradiance. The direct normal component is
calculated from the difference of the global and diffuse measurements.
At the start of a measurement series, a global measurement is first
taken. The shadowband is then rotated from the home position and stops
in three positions before returning home. The first and third stops are
just before and after shading the diffuser. At the second stop the
diffuser is completely shaded. Measurements at the first and third stops
are used to correct the error introduced by the shadowband shading a
portion of the sky.

## Calibration

### Theory

ARM employs two methods for calibrating MFRSRs. Primarily a standard
lamp is used to produce calibration factors for the six narrowband
channels. A secondary method, often used as a check of the lamp
calibrations, is a set of Langley plots. Langley plots are used to
extrapolate the values that would be read if the instrument was at the
top of the atmosphere (TOA). The extrapolated value I~0~, representing
the irradiance at the TOA, is divided by the extraterrestrial spectrum
to give a calibration factor for each channel.

### Procedures

To perform a standard lamp calibration, the MFRSR is placed in a
calibration chamber and a lamp with a known spectral output is used. The
known spectral distribution in (W m^-2^ nm^-1^) is convolved with the
output of each channel to produce calibration factors in (V W^-1^
m^-2^). Langley plots can also be done for channels 415, 500, 615, 673
and 870 nm. (Channel 940 nm is a special case). On a clear day a Langley
plot will give a stable I~0~ for each channel when the data are
extrapolated to the top-of-atmosphere (TOA). With several I~0~s from
clear days, an average I~0~ for each channel is obtained. The average
I~0~ for each channel is divided by the extraterrestrial spectrum to
give a calibration factor. Because water vapor is highly variable, the
Langley method of calibrating is impractical for the 940-nm channel, so
only lamp calibrations are performed.

### History

A detailed calibration history of all the ARM MFRSRs is too lengthy to
be given here. To date, the instrument\'s calibration has been driven by
instrument failure; that is, when either an instrument head or board
fails, the offending component is replaced with a new one that has been
freshly calibrated. This constitutes a \"calibration\" of the
instrument. (This \"calibration\" procedure may change in the future.)

## Operation and Maintenance

### User Manual

Not available in electronic format at this time.

### Routine and Corrective Maintenance Documentation

Time series plots are monitored to ensure the shadowband is shading
properly. If it is not shading properly, the instrument is adjusted
and/or the time is set properly. Careful monitoring of the data can also
reveal interference filters that are failing.

### Software Documentation

ARM NetCDF file header descriptions may be found at
[MFRSR](http://science.arm.gov/tool/dod/showdod.php?Inst=mfrsr),
[MFR10M](http://science.arm.gov/tool/dod/showdod.php?Inst=mfr10m), and
[MFR25M](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=mfr25m)
Data Object Design Changes.

### Additional Documentation

**March 1998**: The laboratory calibration of the instruments is
problematic. By comparing the laboratory calibrations to an estimate of
the calibrations obtained by a Langley analysis, it appears that some
calibration factors can be in error by as much as 50%. The user of the
MFRSR flux data should be aware that some of these data might not be
very accurate.

**November 1999**: The lab calibrations of the MFRSR heads have
significantly improved and are now being checked before field deployment
by calibrations performed using the Langley method. This
\"pre-deployment\" checking has resulted in a significant improvement in
initial calibration accuracy; an estimate of this accuracy is plus or
minus 4% (or maybe even a little better).

The maintenance procedures seem adequate, although some of the
instruments are \"out of alignment\"; this lack of alignment can corrupt
data taken during certain times of day.

## Glossary

See the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

ARM Atmospheric Radiation Measurement (Program)

ASRC Atmospheric Sciences Research Center

EF Extended Facility

FWHM full width at half maximum

MFR multi-filter radiometer

MFRSR multi filter rotating shadowband radiometer

NFOV narrow field of view

NIMFR normal incidence multi-filter radiometer

NSA North Slope of Alaska

PNNL Pacific Northwest National Laboratory

QME Quality Measurement Experiment\
SGP Southern Great Plains

SIROS solar and infrared observing system

SIRS Solar Infrared Station

TOA top of the atmosphere\
TWP Tropical Western Pacific

VAP value-added product

WMO World Meteorological Organization

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Goody, RM and YL Yung. 1989. *Atmospheric Radiation Theoretical Basis*.
Oxford University Press, New York.

Harrison, L and J Michalsky. 1994a. "Automated multi-filter rotating
shadow-band radiometer: An instrument for optical depth and radiation
measurements." *Applied Optics* 33(22):5118-5125.

Harrison, L and J Michalsky. 1994b. "Objective algorithms for the
retrieval of optical depths from ground-based measurements." *Applied
Optics* 33(22):5126-5132.

Michalsky, J, J Schlemmer, N Larson, L Harrison, W Berkheiser III, and N
Laulainen. 1994. \"Measurement of the seasonal and annual variability of
total column aerosol in the northeastern U.S. network.\" In *Proceedings
of Aerosols and Atmospheric Optics: Radiative Balance and Visual Air
Quality, Air and Water Management Association*, Pittsburgh,
Pennsylvania, 247-258.

Michalsky, J, J Liljegren, and L Harrison. 1995. "A comparison of sun
photometer derivations of total column water vapor and ozone to standard
measures of same at the Southern Great Plains Atmospheric Radiation
Measurement site." *Journal of Geophysical Research*
100(D12):25995-26003.

Zhou, C, J Michalsky, and L Harrison. 1995. "Comparison of irradiance
measurements made with the multi-filter rotating shadowband radiometer
and first-class thermopile radiometers." *Solar Energy* 55:487-491.