ARM TR-056

> Cimel Sunphotometer (CSPHOT) Handbook
>
> January 2005
>
> M. Rainwater
>
> L. Gregory
>
> 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 1](#general-overview)

[2. Contacts 1](#contacts)

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

[4. Near-Real-Time Data Plots 1](#near-real-time-data-plots)

[5. Data Description and Examples 1](#data-description-and-examples)

[6. Data Quality 5](#data-quality)

[7. Instrument Details 6](#instrument-details)

**Figures**

[Figure 1. 2](#__RefHeading___Toc94603668)

[Figure 2. 2](#__RefHeading___Toc94603669)

[Figure 3. 3](#__RefHeading___Toc94603670)

[Figure 4. 7](#__RefHeading___Toc94603671)

[Figure 5. 7](#__RefHeading___Toc94603672)

**Tables**

[Table 1. 9](#__RefHeading___Toc94603673)

# General Overview

The Cimel sunphotometer (CSPHOT) is a multi-channel, automatic
sun-and-sky scanning radiometer that measures the direct solar
irradiance and sky radiance at the Earth's surface. Measurements are
taken at pre-determined discrete wavelengths in the visible and near-IR
parts of the spectrum to determine atmospheric transmission and
scattering properties. This instrument is weather-proof and requires
little maintenance during periods of adverse weather conditions. It
takes measurements only during daylight hours (sun above horizon).

# Contacts

## Mentor

Instrument hardware for ARM owned systems:\
Mike Rainwater\
SGP Cart Site Operations\
Phone: 580-388-4053\
E-mail: <Miker@ops.sgp.arm.gov>

Data issues:\
Laurie Gregory\
Brookhaven National Laboratory\
Upton, NY 11973-5000\
Phone: 631-344-2266\
E-mail: <gregory@bnl.gov>

## Instrument Developer

This section is not applicable to this instrument.

# Deployment Locations and History 

A Cimel sunphotometer was successfully installed for long-term
operations at the SGP site in March 1998. Another CSPHOT has been
deployed at Nauru.

As of 2004-10-01, ARM owns and operates two CSPHOTs in the field, one at
SGP CF, one at TWP C2 (Nauru), and two other spare units. AERONET also
obtains data from CSPHOTs owned by other agencies, one at TWP C3
(Darwin) and one at NSA C1 (Barrow).

# Near-Real-Time Data Plots

This section is not applicable to this instrument.

# Data Description and Examples

[Current and recent data quicklooks](http://aeronet.gsfc.nasa.gov/) --
Go to Real Time Data (Level 1).

AOT and PW as a function of time are depicted in the following Figures 1
and 2 as a function of time for one day. Also shown in Figure 3, are
results of almucantar scan, size distribution derived from almucantar
data using Nakajima's method (Nakajima et al. 1983) and phase function
derived from the size distribution using the same method.

![](media/image1.png){width="3.6131944444444444in" height="3.55625in"}

[]{#__RefHeading___Toc94603668 .anchor}Figure 1.

![](media/image2.png){width="3.404861111111111in"
height="3.4930555555555554in"}

[]{#__RefHeading___Toc94603669 .anchor}Figure 2.

![](media/image3.png){width="5.860416666666667in"
height="5.434027777777778in"}

[]{#__RefHeading___Toc94603670 .anchor}Figure 3.

## Data File Contents

### Primary Variables and Expected Uncertainty

Atmospheric transmission is measured from the direct-normal solar
irradiance measurement (sunphotometric operation). Vertical aerosol
optical thickness is derived to an accuracy of ± 0.02 - 0.04 at an
airmass of 2. Water vapor column abundance is measured to an accuracy of
± 10%. Aerosol size distribution in the 0.1 to 3 micron size range is
derived from the sky radiance measurements (radiometric operation) using
radiative transfer algorithms. Phase function is obtained from the size
distribution. The accuracy of the latter derived products is unknown.

#### Definition of Uncertainty

The primary quantities measured by the CSPHOT are as follows: the
aerosol optical thickness, water vapor column abundance, aerosol size
distribution, and the aerosol phase function. Calibration introduces a
major uncertainty in the determination of the above quantities,
especially the optical thickness and the water column abundance. It is
expected that the AOT can be determined to an accuracy of ± 0.01 and
water vapor column abundance to an accuracy of ± 10%. The accuracy in
the determination of the size distribution and phase function is not yet
assessed. In fact, inversion radiative transfer algorithms, such as
those used here to invert the aureole brightness (Nakajima et al. 1983),
provide non-unique size distribution for observations in the visible and
near-IR. It is debatable whether size distribution beyond about 3 micron
is reliable at all. It is recommended that the size distribution data be
used for qualitative assessment of the nature of the aerosol size
distribution.

The data will be supplied to the ARM database as external data and will
have passed all quality checks. The issue of primary concern is the
presence of clouds as the clouds may distort the data. Clouds will be
removed by inspection of triplet measurement and by eliminating all data
showing an Angstrom exponent of less than 0.5.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

This section is not applicable to this instrument.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

The data are processed and quality controlled by AERONET at NASA. ARM
reingests these data as external data, see [external data stream
description](http://www.arm.gov/xds/static/csphot.stm).

## Frequently Asked Questions

This section is not applicable to this instrument.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality 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 ARM's data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

-   **QC frequency**: Daily and weekly checks

-   **QC delay**: Not specified

-   **QC type**: Comparisons with normal standards; reviews of the
    symmetry of sky radiance scans (to check for automatic gain
    performances); comparisons with MFRSR data; water vapor column
    abundance comparisons with estimates derived from BBSS and MWR
    observations

-   **Inputs**: Transmitted signal from GOES satellite at NASA Wallops
    Island Facility

-   **Outputs**: Data flags in headers

-   **Reference**: None.

After the CSPHOT is installed at the SGP central facility in early 1998,
data will be collected via GOES satellite at the National Aeronautics
and Space Administration (NASA) Wallops Island Facility. Information on
the transmitted signal will be inspected for signal strength,
transmission time, and time interval between transmissions.

Daily and weekly checks will be routinely made on the data. The Cimel
clock time, Cimel battery power, GOES Transmission Module (GTM) clock
time, and its battery power will be checked. Daily and weekly checks of
measured Aerosol Optical Thickness (AOT) in different channels will be
made to isolate inadequately performing channels. Symmetry of sky
radiance scans (almucantar and principle plane scans) will be inspected
to check automatic gain performance. After such data quality control
procedures are applied and data are flagged appropriately, the data will
be made available to ARM data users as external data

Calibrations are performed once a year either at the mountain-top sites
or by comparisons at NASA Goddard Space Flight Center (GSFC) with other
sensors that have been at such sites. Once a month, Langley analyses
will be carried out to infer sensitivities, which will be compared to
those derived from the annual calibrations. In addition, the AOT data
will compared with those derived from MFRSRs, and water vapor column
abundance using a modified Langley approach will be compared with
estimates derived from BBSS and MWR observations.

## Data Assessments by Site Scientist/Data Quality Office

Quality control is provided by AERONET during processing and retrieval
of geophysical quantities.

## 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
CART 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 in order to fill unmet
measurement needs, but instead 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 the [VAPs
and QMEs](http://www.arm.gov/data/vaps.stm) web page.

For the CSPHOT, size distribution and phase function can be considered
as value-added products. The AOT comparison with MFRSR derived values
and the PW comparison with radiosonde and micro-wave radiometer values
are QMEs.

# Instrument Details

## Detailed Description

### List of Components

The CSPHOT consists of (1) the instrument itself (made up of the sensor
head and scanning motors and robot arm); (2) a control box (provides
software for controlling predetermined scanning and sampling strategies,
for acquiring data, and for formatting remote satellite-based
transmissions and batteries); and (3) a radio antenna (for transmitting
data to a GOES satellite). The instrument is made by Cimel Electronique
of France; the GOES transmitter module (GTM) and antenna are made by
Vitel Corporation (USA). Figure 4 presents a schematic of a CSPHOT in a
mounted configuration.

### System Configuration and Measurement Methods

The following paragraphs describe the components that make up the system
(see Figure 5).

**Instrument (Figure 5)**

The instrument consists of the main stem containing the azimuth motor,
on the top of the motor is an attached a robot arm consisting of the
zenith motor on one side and the sensor head on the other side. The
collimators are attached to the sensor head. Inside the sensor head are
two silicon detectors, one for each of the collimators. A filter wheel
is placed in between the collimator windows and the detectors, inside
the sensor head. The wheel consists of eight narrowband interference
filters (at 340, 378, 440, 499, 613, 870, 940 and 1020 nm) mounted along
the circumference. The two collimators have the same field of view (1.2
degree) but differ in the size of apertures. They are physically part of
a single unit that is attached to the sensor head by a long screw
tightened down to prevent light and water leakage. The larger aperture
collimator\--10 times as large as the sun-viewing collimator\--provides
the necessary dynamic range to observe the sky. Three cables (a thick
cable from the sensor head to the control box, and two battery power
cables\--one each to the motors) are attached to the instrument. The
main stem is connected to a base plate consisting of mounting holes to
ensure the instrument is mounted on a level surface.

Schematic of CSPOT Systems and Mounting Configuration

![](media/image4.png){width="2.486111111111111in"
height="3.473611111111111in"}

[]{#__RefHeading___Toc94603671 .anchor}Figure 4.

CSPOT Instrument Parts and Operation

![](media/image5.png){width="3.609722222222222in"
height="3.2555555555555555in"}

[]{#__RefHeading___Toc94603672 .anchor}Figure 5.

**Control boxes**

The Cimel control box consists of a control module\--a
rectangular-shaped white box\--that actually controls the scan and
measurement sequence of CSPHOT. It has an internal battery that services
only the software portion of the instrument. The box also stores data
that can be queried and transferred upon demand by the GOES Transmission
Module (GTM). The GTM is also housed in the control box and physically
consists of two parts: the GTM control box that schedules and formats
data for transmission, and an amplifier box that boosts the signal for
transmittal through the antenna. The control box also includes a battery
pack that supplies power to the CSPHOT control box, the GTM. A separate
battery charger provides the necessary power from the input mains.

**Antenna**

The antenna is connected to the GTM amplifier through a 6-foot,
heavy-duty coaxial cable. The antenna points South - South-East at the
CART site to enable transmission to the GOES East satellite. The antenna
transmits data every hour at a predetermined time and takes up to a
maximum of 2 minutes. The frequency for transmission is 401.76 MHz. The
password code and duration and time of transmission are all set in the
GTM control box.

**Location (Siting)**

The instrument is located at a height of about 5 feet from the surface
to minimize accidental obstruction of the field of view of the
collimators. The location should allow an unobstructed view of the sky
above 5 degrees of elevation, especially in the general region of sunset
and sunrise at the CART site.

**Weather-Related Details**

All the cables interconnecting the above components pass through
rainproof silicone seals in their respective boxes. A wet sensor,
connected to the instrument control box, effectively shuts down the
scanning by the sensor head during precipitation. The fail-safe pointing
for the sensor head is the "down" position with the collimators pointing
down toward the base.

**Measurement Method**

Cimel operates automatically without operator assistance as with other
ARM instruments. It measures direct solar irradiance by first pointing
the collimator toward the approximate position of the sun (provided it
is aligned properly) based on an in-built program that takes into
account the time of the year and the coordinates of the location that
are input to the Cimel control box prior to operation. A 4-quadrant
detector then positions the sun at the center of the fields of view of
the collimators by using a feedback control loop. The filter wheel
rotates in front of the detector to obtain a measurement sequence. A
sequence takes about 10 seconds. In order to discriminate against the
presence of thin cirrus clouds, which may be non-uniform, three
measurement sequences are performed (called as a triplet), lasting about
35 seconds. During a data analysis procedure, the measured voltages are
compared to eliminate non-uniform scenes. Almucantar sky radiance is
obtained by scanning the sky at the solar zenith angle but different
azimuth angles to obtain the angular variation of skylight in 4 filters.
Solar principal plane sky measurement is obtained by scanning the sky in
a plane containing the sun and the instrument and normal to the surface.
Data are taken more frequently near the sun since the intensity varies
rapidly in the solar aureole. The sky brightness data is inverted by
radiative transfer routines to derive aerosol size distribution and
phase function.

**Data Acquisition**

Direct solar irradiance data are obtained in all the filters every 0.5
airmasses above an airmass of 2, and every 15 minutes otherwise. Solar
almucantar data (sky radiance at constant solar zenith angle) are
obtained twice daily when weather permits at a solar zenith angle of
approximately 60 degrees in 4 channels - 440, 499, 613, 870 nm. Solar
principal plane sky radiance is obtained 4 times daily in the same
channels as for the almucantar. All the data are stored in memory in the
Cimel control box. At predetermined times, a minute before the hourly
GOES transmission, the GTM acquires and formats the data for the
transmission. The data are transmitted to a computer at NASA's Wallops
Island Facility from where the data are downloaded to computers at
Goddard Space Flight Center. Since the ARM Cimels are also part of
Aeronet network (Holben et al. 1997), the data are automatically
analyzed for the required parameters.

### Specifications

AOT is measured at 440, 499, 670, 870, and 1019 nm, typically.
Additional wavelengths are at 339 and 380 nm where some ozone absorption
is present. The primary purpose of 940 nm filter band is to measure
column water vapor. All filters have a narrow bandwidth of about 10 nm.
The following table summarizes the instrument characteristics and
observational specifications.

[]{#__RefHeading___Toc94603673 .anchor}Table 1. Instrument
Characteristics and Observational Specifications

  ------------------------ ----------------------------------------------
  Detector:                 Silicon

  Number of Filters:        5 to 8

  Number of collimators:    2

  Field of View/            1.2. One has 10 times the aperture of the
  Aperture:                other.

  Solar Scanning:           4 quadrant detector.

  Sky scanning:             Azimuth and Zenith motors.

  Frequency of Sun          0.5 airmass intervals for airmass \> 2. 
  acquisition:             Otherwise, 15 minutes apart.

  Frequency of              At airmass of 4 and 2. 4 times daily.
  Almucantar:              
  ------------------------ ----------------------------------------------

## Theory of Operation

Direct Normal Solar Irradiance E (W/m\^2) at the surface at a given
wavelength is given by Bouguer's law,

E=(E_0/R\^2)exp(-m\*tau)

where E_0 is the extra-terrestrial solar irradiance at a distance of 1
Astronomical Unit (AU), R is the sun-earth distance in AU at the time of
measurement, m is the airmass and tau is the total vertical optical
thickness. If the instrument is voltage V for irradiance E, the above
equation can be written as,

V=(V_0/R\^2) exp (-m\*tau) \...\...\...\...\...(1)

V_0 on the right hand side of this equation is the calibration
coefficient obtained by measuring voltage V as a function of airmass m
and extrapolating the resulting curve to zero airmass. The total optical
thickness tau is made up of molecular (Rayleigh) attenuation, trace gas
attenuation (such as ozone) and of course aerosol attenuation. By
appropriately estimating the molecular contributions, aerosol optical
thickness can be estimated. The accuracy in the estimation depends on
the uncertainty in V and uncertainty in V_0. The former is a function of
the precision of the silicon detector which is very high and therefore
can be neglected. Accuracy in the calibration coefficient determination
will be discussed in the calibration section.

For the 940 nm channel Bouguer's law is not valid since the exponential
attenuation law is valid strictly for monochromatic radiation. For a
band attenuation, preferential attenuation at the line centers results
quickly in saturation and further passage through the atmosphere follows
exponential attenuation law only in the pressure broadened line wings
where the attenuation is weaker. Transmission can be modeled as a two
parameter expression (Halthore et al. 1997) thus:

T=exp(-a\*w\^b)

where T is the transmission in the band, a and b are constants that are
determined by modeling with a radiative transfer equation and convolving
the result with the shape of the 940 nm band, for a given atmosphere.
Halthore et al. (1997) showed that by choosing a narrow 940 nm band
width, sensitivity to atmosphere can be removed resulting in a equation
of type,

V=(V_0/R\^2)\* exp(-m\*tau)\* exp(-a\*w\^b) \...\...\...\...\...\...(2)

for any atmosphere. The optical thickness tau here comprises of Rayleigh
and aerosol components which are estimated independently (by
interpolation in the case of aerosol). w the water column abundance, = m
\* PW, where PW, the perceptible water, is obtained once the calibration
constant is known. Calibration for the 940 nm channel is accomplished in
a manner similar to the previous case and is described below.

## Calibration

### Theory

Calibration for the non-water absorbing channels is performed by the
Langley method. Taking logarithms on both sides of the equation (1) and
separating the total optical thickness to its components, we get,

lnV + m tau_R + m\* tau_oz = ln (V_0/ R\^2) - m \* tau_aer

Measurements of voltage for each channel are taken frequently during
periods of maximum airmass change, that is sunrise and sunset, above
zenith angle of 60 degrees. A practical upper limit for airmass is
usually 6 or 7, determined by the inability to account for refractive
index effects in the atmosphere. With the Rayleigh, and any gaseous
absorption accounted for in each channel, the left hand side of the
above equation is plotted against airmass m to yield a straight line if
the aerosol optical thickness tau_aer remains constant during the time
of measurement. The intercept is the required calibration coefficient Vo
and the slope gives the aerosol optical thickness during calibration.
Langley plots are best performed when aerosol is both constant and low
during the time of measurement, conditions that are best achieved at
mountain sites during morning when the convective activity has not yet
built up.

For the 940 nm channel, calibration is performed by the modified-Langley
method. This is best illustrated by taking logarithms on both side of
equation (2) to obtain,

ln V + m\* tau = ln (V_0 / R\^2) - a \* PW\^b \* m\^b

where the column abundance along the path, w, is written in terms of the
product of the perceptible water which is the water vapor vertical
column abundance and airmass m. Thus, a plot of the left hand side
versus m\^b will yield a straight line if in addition to the aerosol
optical thickness (contained in the ordinate), PW also remains constant.
AOT is estimated for the 940 nm channel by interpolation between 870 and
1020 nm channels. Here a and b are constants that are determined
(Halthore et al. 1997) by running a radiative transfer model such as
MODTRAN-3 and convolving its output with the measured filter function of
the 940-nm band. Narrow band widths yield values of a and b that are
independent of the type of atmosphere present.

### Procedures

This section is not applicable to this instrument.

### History

Any change in the calibration coefficients will be documented and the
calibration event and the method will be listed in one file which will
be part of the database. Uncertainties in the estimates will be given
with their effect on the derived parameters of AOT and PW, and the size
distribution.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

This section is not applicable to this instrument.

### Software Documentation

This section is not applicable to this instrument.

### Additional Documentation

This section is not applicable to this instrument.

## Glossary

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

## Acronyms

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

## Citable References

Halthore, R.N., T.F. Eck, B.N. Holben, and B.L. Markham. 1997.
Sunphotometric Measurements of Atmospheric Water Vapor Column Abundance
in the 940-nm Band. *J. Geophys. Res.*, 102, 4343-4352.

Holben, B.N., T.F. Eck, I. Slutsker, D. Tanre, J.P. Buis, A. Setzer, E.
Vermote, J. Reagan, Y. Kaufman, T. Nakajima, F. Lavenu, and I.
Jankowiak. 1997. Automatic Sun and Sky Scanning Radiometer System for
Network Aerosol Monitoring. Accepted for publication in *Rem. Sens. of
Environ*.

Nakajima, T., M. Tanaka, and T. Yamauchi. 1983. Retrieval of the optical
properties of aerosols from aureole and extinction data. *Appl. Opt.,*
22, 2951 - 2959.