BOREAS RSS-11 Ground Network of Sunphotometer Measurements

Summary

The BOREAS RSS-11 team operated a network of five automated (Cimel) and
two hand-held (Miami) solar radiometers from 1994 to 1996 during the
BOREAS field campaigns. The data provide aerosol optical depth
measurements, size distribution, phase function, and column water vapor
amounts over points in northern Saskatchewan and Manitoba, Canada. The
data are useful for the correction of remotely sensed aircraft and
satellite images. The data are provided in tabular ASCII files.

Table of Contents

1\. Data Set Overview

2\. Investigator(s)

3\. Theory of Measurements

4\. Equipment

5\. Data Acquisition Methods

6\. Observations

7\. Data Description

8\. Data Organization

9\. Data Manipulations

10\. Errors

11\. Notes

12\. Application of the Data Set

13\. Future Modifications and Plans

14\. Software

15\. Data Access

16\. Output Products and Availability

17\. References

18\. Glossary of Terms

19\. List of Acronyms

20\. Document Information

1\. Data Set Overview

1.1 Data Set Identification

BOREAS RSS-11 Ground Network of Sunphotometer Measurements

1.2 Data Set Introduction

A ground-based sunphotometer network was installed within the BOReal
Ecosystem-Atmosphere Study (BOREAS) study areas to characterize the size
and distribution of atmospheric aerosols and column water vapor amount
and to aid in the correction of aircraft and satellite imagery.
Automated sunphotometer data are available fairly continuously from
early 1994 through 1996 for two sites in the Northern Study Area (NSA)
and two in the Southern Study Area (SSA). Data from a fifth automated
instrument located at the Flin Flon airport at the Manitoba/Saskatchewan
border; this instrument was moved in 1996 to Paddockwood, SK, in the
SSA. Measurements made by two additional hand-held instruments, operated
during the 1994-96 time period at each of the study areas, are also a
part of this data set. A local observer made the hand-held measurements
near noon when the Sun was not obscured by any clouds.

1.3 Objective/Purpose

The purpose was to establish a data set of aerosol, water vapor, and
ozone atmospheric properties in the BOREAS region for a 3-year period
for use by BOREAS in atmospheric correction of remotely sensed data.
Another goal was to establish an aerosol climatology of the region by
specifying, for example, the annual cycle of aerosol loading across the
region.

1.4 Summary of Parameters

Parameters:

Direct solar irradiance (340, 380, 440, 500, 670, 870, 1020nm)

Sky radiance (440, 670, 870, 1020nm)

Aerosol optical thickness

Column water vapor amount

1.5 Discussion

A network of five automated (Cimel) and two hand held (Miami) solar
radiometers were operated during the 1994-1996 time period for BOREAS to
provide aerosol optical depth measurements, size distribution, phase
function, and column water vapor amounts. The purpose was to establish a
data set of aerosol, water vapor and ozone atmospheric properties in the
BOREAS region for a three year period for use by BOREAS in atmospheric
correction of remotely sensed data.

1.6 Related Data Sets

BOREAS RSS-03 Reflectance Measured from a Helicopter-Mounted Barnes MMR

BOREAS RSS-12 Airborne Tracking Sunphotometer Measurements

BOREAS RSS-12 Automated Ground Sun Photometer Measurements in the SSA

BOREAS RSS-18 Ground Sunphotometer Measurements in the SSA

An additional data set collected and used in comparison was
sunphotometer measurements made by Norman O'Neill (RSS-19) at the Canada
Centre for Remote Sensing (CCRS). These data were not submitted to the
BOREAS Information System (BORIS).

2\. Investigators

2.1 Investigators Name and Title

Brian L. Markham

Physical Scientist

2.2 Title of Investigation

Characterization of Atmospheric Optical Properties for BOREAS

2.3 Contact Information

Contact 1

\-\-\-\-\-\-\-\--

Brian L. Markham

NASA/GSFC

Greenbelt, MD

\(301\) 286-5240

\(301\) 286-0239/1757 (fax)

Brian.L.Markham.1@gsfc.nasa.gov

Contact 2

\-\-\-\-\-\-\-\--

Joel Schafer

NASA/GSFC

Greenbelt, MD

\(301\) 286-4547

\(301\) 286-0239 (fax)

Joel.S.Schafer.1@gsfc.nasa.gov

Contact 3

\-\-\-\-\-\-\-\--

Jaime Nickeson

BORIS RSS Team Representative

NASA/GSFC

Greenbelt, MD

\(301\) 286-3373

\(301\) 286-0239 (fax)

Jaime.Nickeson@gsfc.nasa.gov

3\. Theory of Measurements

Sunphotometers measure direct surface irradiance. Because direct solar
irradiance depends on the atmospheric transmission, the components
affecting this transmission such as aerosol optical thickness (AOT),
water vapor column abundance, ozone absorption, and molecular scattering
can either be estimated or bemeasured.

Optical thickness is calculated from spectral extinction of direct beam
radiation at each wavelength based on Beer\'s Law:

![](media/image1.wmf){width="2.4305555555555554in"
height="0.4027777777777778in"}

where V = digital voltage measured at wavelength 

Vo = extraterrestrial voltage

m = optical airmass

 = total optical thickness

R = relative Earth-Sun distance

Attenuation due to Rayleigh scatter, and absorption by ozone and gaseous
pollutants is estimated and removed to isolate the AOT ().

The dependence of transmission, Tw, on water column abundance can be
written as the following \[Halthore et al., 1992, Bruegge et al, 1992\]:

![](media/image2.emf){width="1.9722222222222223in"
height="0.5277777777777778in"}

where W is the vertical column abundance (cm), and the constants a and b
depend on the wavelength position, width and shape of the filter
function, and atmospheric conditions.

Sky radiance measurements are inverted with radiative transfer equations
\[Nakajima et al., 1983\] to provide aerosol properties of size
distribution and phase function over the particle size range of 0.1 to 5
µm.

4\. Equipment

4.1 Sensor/Instrument Description

4.1.1 Collection Environment

The automated sunphotometers operated during non-winter months according
to their diurnal schedule (section 4.1) when there was no precipitation
and the atmosphere is clear enough to allow the instrument to focus on
the Sun with a four-quadrant detector (AOT roughly \<6.0). Precipitation
activates an externally mounted wet sensor that causes the instrument to
remain in the parked (nadir) position.

4.1.2 Source/Platform

All instruments were mounted on the roof of a building or hut, with a
clear view of the sky above 5 degrees elevation angle.

The handheld instruments were held by the human observer.

4.1.3 Source/Platform Mission Objectives

The purpose of the building roof was to cover the building and provided
a convenient place to mount the sunphotometers.

The objectives of the human observer during the measurement times was to
obtain good measurements and varied at others.

4.1.4 Key Variables

Direct solar irradiance

Sky radiance

4.1.5 Principles of Operation

Eight interference filters are located in a filter wheel that is rotated
by a direct drive stepping motor. A thermistor measures the temperature
of the detector, allowing compensation for any temperature dependence in
the silicon detector.

The sensor head is pointed by stepping azimuth and zenith motors with a
precision of 0.05 degrees. A microprocessor computes the position of the
sun based on time, latitude, and longitude and directs the sensor head
to within approximately 1 degree of the Sun. Then, a four-quadrant
detector tracks the Sun precisely prior to a programmed measurement
sequence.

4.1.6 Sensor/Instrument Measurement Geometry

The Cimel automatic sunphotometers take direct solar irradiance and sky
radiance measurements with a 1.2-degree FOV.

4.1.7 Manufacturer of Sensor/Instrument

The automatic sunphotometers were made by:

Cimel Electronique

5 Cite de Phalsbourg

75011 Paris, France

The hand-held sunphotometers were made by:

University of Miami

Dept. of Meteorology

Coral Gables, FL 33124

4.2 Calibration

A number of automatic sunphotometers from the Goddard Space Flight
Center (GSFC) were regularly taken to the Mauna Loa Observatory in
Hawaii, where the Langley method of calibration \[Shaw, 1983\] was used
for absolute calibration. Instruments used for BOREAS were calibrated by
intercomparison with reference instruments using measurements conducted
on the top of a building at GSFC on clear days with low aerosol loading.
On occasion, in-field Langley calibrations were performed.

For the 940-nm channel that includes water absorption, calibration was
performed using the following procedure.

At 940 nm, the measured digital voltage is:

![](media/image3.emf){width="3.125in" height="0.4305555555555556in"}

where t is the extinction due to Rayleigh and aerosol scatter and water
vapor absorption, and Tw, again, is water vapor transmission.

Combining this relation with that of Section 3 describing

![](media/image4.wmf){width="1.5694444444444444in"
height="0.3888888888888889in"} yields:

![](media/image5.wmf){width="4.44375in" height="0.4305555555555556in"}

Plotting the left-hand side of the above equation against mb gives a
straight line with the desired y-intercept ln(Vo) and slope Wb
\[Halthore et al 1997\]. This modified Langley method was used only for
calibrating the water vapor band.

Hand-held instruments are intercalibrated at the beginning and end of
the season with collocated automatic instruments on clear days of low
aerosol.

4.2.1 Specifications

The automatic instruments used were Cimel CE-318s which, during 1994,
had 10-nm bandpass filters in the visible and near-infrared with center
wavelengths at 340, 380, 440, 670, 870, 940, and 1020 nm, with an
additional 50-nm bandpass filter centered at 940 nm. The two 940-nm
channels were to be used for column water vapor abundance determination.
The wider bandpass filter proved unnecessary for determinating water
vapor column abundance, so it was replaced with a 10-nm bandpass filter
centered at 500 nm in spring of 1995. In addition to measuring solar
irradiance with a field of view (FOV) of 1.2 degrees, these instruments
measure the sky radiance in four spectral bands (440, 670, 870, and 1020
nm) along the solar principal plane (i.e., at constant azimuth angle,
with varied solar zenith angles) up to nine times a day and along the
solar almucantar (i.e., at constant solar zenith angle, with varied
azimuth angles) up to six times a day.

The two hand-held sunphotometers have four channels (500, 670, 870, and
940 nm). They are capable of only the direct solar measurements and
require manual data entry. They have a peak hold feature that allows
them to record the highest voltage response when pointed in the general
direction of the Sun.

4.2.1.1 Tolerance

If the aerosol conditions were considered to be constant during the
Langley procedures at Mauna Loa, then deviation of measurements from the
linear regression line gives an indication of the sunphotometer
precision. Triplet variability from three reference instruments deployed
at Mauna Loa was calculated based on 168, 264, and 288 observations,
respectively. For all wavelengths, the variability of a triplet was
always less than 1%, and generally about 0.3%, suggesting that
uncertainty due to instrument precision is minimal \[Holben et al.,
1996\].

4.2.2 Frequency of Calibration

The instruments were intercompared at GSFC on clear days with reference
Cimels calibrated by Langley or modified Langley techniques at Mauna
Loa. This process was done before and after each field campaign, and the
reference instrument (#13) was taken to Mauna Loa every 6 months.
Occasionally, filter changes during deployment or short-term
contamination by spider webs in the collimator warranted the use of
in-field Langley calibrations.

4.2.3 Other Calibration Information

The instrument referred to below as #13 is the reference instrument used
for BOREAS.

1994 Cimel Sun data are calibrated as follows:

The ratios of the post season to preseason calibrations follow for each
instrument in the order. The bracketed numbers are for instrument #31
and #32

1020 870 670 440 340\[380\] 380\[340\]

#11 NSA-YJP \-- interpolated between the May cross comparison with #13
and the November cross comparison with #13 (with the exception of the
June 26 through July 4 \-- a spider web incident that was calibrated
with onsite Langley techniques for 26-Jun-1994).

0.99233306 1.00224738 0.99804205 1.0118189 0.97430213 1.01370528

#12 Flin Flon\-- interpolated between the May cross comparison with #13
and the November cross comparison with #13.

0.96776048 0.97619891 0.98253527 0.95644371 0.89236603 0.90532343

#31 YJP-SSA \-- interpolated between the May cross comparison with #13
and November cross comparison with #13.

0.99090514 0.99927597 0.98348781 0.99160097 0.99534901 0.94812591

#32 Waskesiu \-- used May cross comparison with #13 for full BOREAS
season (with exception of 21-25 July-1994 spider web incident calibrated
with onsite Langley techniques) \-- the November cross comparison with
#13 was not valid for BOREAS because of an apparent change in the
instrument during its return.

#6 Thompson Zoo/Prince Albert \-- used May cross comparison with #13
until 09-Jun-1994; used cross comparison with #13 in July until
11-Jul-1994; interpolated between 11-July-1994 cross comparison with #13
and 12-Oct-1994 cross comparison with #13.

1.00103256 0.98735627 0.96687931 0.74577641 1.02234866 mid season

1.01655301 1.016745 0.99002756 0.74729656 1.05768801 post season

1995 Cimel sunphotometer data are calibrated as follows:

The time and date are given for each zero airmass voltage (Vo) and for
each wavelength and the seasonal change in filter response is given as
well.

#6 Flin Flon

Wavelength 1020 870 670 500

DATE: 08-May-95 12517.458 12897.622 13899.506 13655.055

TIME: 22:32:37

DATE: 05-Nov-95 12582.104 12927.327 13833.724 13723.94

TIME: 15:58:58

Post/preseason 1.005 1.002 0.995 1.005

#11 Waskesiu

Wavelength 1020 870 670 500

DATE: 07-May-95 12885.336 9326.284 14941.283 18483.47

TIME: 14:41:54

DATE: 29-Dec-95 13404.445 9648.732 15809.717 18588.241

TIME: 17:31:53

Post/preseason 1.04 1.035 1.058 1.006

#12 Thompson

Wavelength 1020 870 670 500

DATE: 08-May-95 13327 13223 12849 17887

TIME: 15:49:24

DATE: 07-Dec-95 13291 13418 13149 17854

TIME: 16:37:45

Post/preseason 0.997 1.015 1.023 0.998

#31 YJP-SSA

Wavelength 1020 870 670 500

DATE: 07-May-95 13329.603 13645.573 14900.62 18606.043

TIME: 15:49:23

DATE: 29-Dec-95 13620.063 14025.419 15261.078 18132.432

TIME: 17:31:58

Post/preseason 1.022 1.028 1.024 0.975

#35 YJP-NSA

Wavelength 1020 870 670 500

DATE: 08-May-95 19516.054 14028.61 22462.976 9513.898

TIME: 22:21:34

DATE: 07-Jun-95 19819.766 14270.977 22896.422 9808.112

TIME: 11:12:32

DATE: 02-Dec-95 19953.95 14316.853 22858.584 9665.12

TIME: 16:34:38

Post/preseason 1.022 1.021 1.018 1.016

#6 Flin Flon

Wavelength 440 380 340 940

DATE: 08-May-95 18147.869 19241.212 19757.94 22399.36

TIME: 22:32:37

DATE: 05-Nov-95 18344.145 20505.336 18284.42 20831.22

TIME: 15:58:58

Post/preseason 1.108 1.066 0.925 0.93

#11 Waskesiu

Wavelength 440 380 340 940

DATE: 07-May-95 18296.37 21366.285 33970.048 21292.029

TIME: 14:41:54

DATE: 29-Dec-95 19135.821 22185.209 30718.195 21590.383

TIME: 17:31:53

Post/preseason 1.046 1.038 0.904 1.014

#12 Thompson

Wavelength 440 380 340 940

DATE: 08-May-95 16964 24463 32590 29583.1

TIME: 15:49:24

DATE: 07-Dec-95 17465 25282 31927 28451.3

TIME: 16:37:45

Post/preseason 1.03 1.033 0.98 0.962

#31 YJP-SSA

Wavelength 440 380 340 940

DATE: 07-May-95 16585.035 17619.795 32936.416 23163.098

TIME: 15:49:23

DATE: 29-Dec-95 15980.247 18374.366 28336.05 22616.378

TIME: 17:31:58

Post/preseason 0.964 1.043 0.86 0.976

#35 YJP-NSA

Wavelength 440 380 340 940

DATE: 08-May-95 18035.776 6789.077 33650.941 32735.451

TIME: 22:21:34

DATE: 07-Jun-95 17950.115 7319.054 34008.235 32404.751

TIME: 11:12:32

DATE: 02-Dec-95 18170.429 7149.50 32357.469 31599.466

TIME: 16:33:38

Post/preseason 1.007 1.053 0.962 0.965

1996: Cimel sunphotometer data are calibrated as follows:

The time and date are given for each zero airmass voltage (Vo) and for
each wavelength, and the seasonal change in filter response is given as
well, where appropriate.

#12 Thompson

Wavelength 1020 870 670 500

DATE: 27-Apr-96 13420.234 13491.316 13087.821 18244.657

TIME: 20:05:38

DATE: 04-Nov-96 13557.683 13635.545 12996.507 18142.348

TIME: 17:32:28

Post/preseason 1.01 1.011 0.993 0.994

#12 Thompson

Wavelength 440 380 340 940

DATE: 27-Apr-96 17474.941 25292.378 30613.31 29172.311

TIME: 20:05:38

DATE: 04-Nov-96 17271.154 24877.615 30012.10 28021.078

TIME: 17:32:28

Post/preseason 0.988 0.984 0.98 0.961

#10 Paddockwood

Wavelength 1020 870 670 500

DATE: 31-Mar-96 12449.513 8361.421 11602.164 15602.52

TIME: 15:59:22

#10 Paddockwood

Wavelength 440 380 340 940

DATE: 31-Mar-96 15192.358 20776.75 26675.68 20253.18

TIME: 15:59:22

#31 YJP SOUTH

Wavelength 1020 870 670 500

DATE: 29-Dec-95 13620.063 14025.419 15261.078 18132.432

TIME: 17:31:58

#31 YJP SOUTH

Wavelength 440 380 340 940

DATE: 29-Dec-95 15980.247 18374.366 28336.050 22616.378

TIME: 17:31:58

#35 YJP NORTH

Wavelength 1020 870 670 500

DATE: 24-Apr-96 19600.860 14273.589 22715.018 9523.237

TIME: 15:20:45

DATE: 20-Dec-96 19295.777 13997.458 22520.035 9453.443

TIME: 18:32:59

Post/preseason 0.984 0.981 0.991 0.993

#35 YJP NORTH

Wavelength 440 380 340 940

DATE: 24-Apr-96 18016.307 6759.045 30243.207 31001.937

TIME: 15:20:45

DATE: 20-Dec-96 18160.719 6866.885 29525.022 29548.983

TIME: 18:32:59

Post/preseason 1.008 0.976 1.016 0.953

#11 Waskesiu

Wavelength 1020 870 670 500

DATE: 24-Apr-96 13622.427 9649.122 15575.813 18969.570

TIME: 16:13:30

DATE: 02-Dec-96 13602.240 9565.654 15565.203 18980.755

TIME: 19:10:59

Post/preseason 0.999 0.991 0.999 1.001

#11 Waskesiu

Wavelength 440 380 340 940

DATE: 24-Apr-96 19495.370 29884.542 22098.695 21705.495

TIME: 16:13:30

DATE: 02-Dec-96 19322.585 28941.093 21926.102 21078.678

TIME: 19:10:59

Post/preseason 0.991 0.992 0.968 0.971

5\. Data Acquisition Methods

A preprogrammed sequence of measurements is taken by these instruments
starting at an airmass of 7 in the morning and ending at an airmass of 7
in the evening. During the large airmass periods, direct Sun
measurements are made at 0.25 airmass intervals; at smaller airmasses,
the interval between measurements is typically 15 minutes. The
almucantar measurements are taken at 0.5-degree intervals near the Sun
(within 6 degrees) and increase from 2 to 10 degree intervals away from
the solar position. The data are collected and transmitted via the
Geostationary Environmental Satellite (GOES) at 1-hour intervals to a
computer at Wallops Island Flight Facility \[Holben et al., 1996\].

The hourly transmitted radiometer data stream includes date, time,
temperature, battery voltage, wet sensor status, and time of
transmission as well as several levels of identification numbers. The
Vitel transmitter adds a time stamp at the time of transmission, as does
the Wallops receiving station. The transmitter also checks for parity
errors and signal strength of the transmission. After data are
downloaded from the central receiving station, a status report and a
troubleshooting report are automatically generated and e-mailed to
appropriate system and instrument managers. The status report provides a
comprehensive assessment of the operation of the radiometer and the
Vitel transmitter for the data transmitted with the current download.
Network managers then have sufficient information to assess the
operation of individual stations.

Within Demonstrat, a package of user-friendly UNIX software, the raw
data (voltages) are converted to AOT or precipitable water using the
relevant instrument-specific calibration coefficients.

6\. Observations

6.1 Data Notes

The data sets are generally complete from May to October or September
with some exceptions noted below.

1994:

#11 YJP-NSA 26-Jun - 04-Jul collimator obstruction

#32 Waskesiu, SK 21-Jul - 25-Jul collimator obstruction

1995:

#11 Waskesiu, SK 01-Aug - 22-Aug collimator obstruction

#31 YJP-SSA 04-Sep - 07 Sep

#31 YJP-SSA 12-Sep - 22-Sep

#31 YJP-SSA 01-Jun - 01-Jul removed because of local forest fire

1996:

#10 Paddockwood 18-Apr - 27-Apr \* Installed in February \*

02-Sep - 14-Sep

#11 Waskesiu, SK 01-May - 12-May

04-Sep - 30-Sep

#31 YJP-SSA 23-Sep - 02-Oct

15-May - 29-May

04-Sep - 08-Sep (440-nm unusable)

12-Sep - 23-Sep (440-nm unusable)

#35 YJP-NSA 24-Sep - 03-Oct

The hand-held records are generally complete for the whole year
depending on availability of the observer.

6.2 Field Notes

None given.

7\. Data Description

7.1 Spatial Characteristics

7.1.1 Spatial Coverage

Deployment Sites Latitude Longitude UTM Easting UTM Northing

1994:

Flin Flon, MB 54.67777N 101.67843W 327315.2 6062229

NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9

NSA-Thompson Airport 55.78344N 97.83359W 573151.1 6182593.1

SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5

Prince Albert Airport 53.20004N 105.68383W 454320.6 5894742.2

SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3

1995:

SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5

NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9

SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3

Flin Flon, MB 54.67777N 101.67843W 327315.2 6062229

NSA-Thompson Zoo 55.75N 97.8867W 571137 6178837

1996:

SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5

NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9

SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3

SSA-Paddockwood School 53.50951N 105.56697W 462400 5929100

NSA-Thompson Zoo 55.75N 97.8867W 571137 6178837

In addition, a hand-held sunphotometer was operated at Lake Waskesiu and
Thompson during each year.

7.1.2 Spatial Coverage Map

Not available.

7.1.3 Spatial Resolution

Each sunphotometer takes measurements that are generally representative
of the local area under quiescent conditions. During forest fire
episodes, however, the spatial resolution depends on the nature of smoke
dispersion.

7.1.4 Projection

Not Applicable

7.1.5 Grid Description

Not Applicable

7.2 Temporal Characteristics

7.2.1 Temporal Coverage

Automatic sunphotometer data are acquired at 0.25 or 0.5 airmass
intervals from an airmass of 7 up to 2. Air mass is approximately equal
to 1/cos(zenith angle). Then, measurements are made every 15 minutes
until an afternoon airmass of 2, where time intervals are again dictated
by airmass fractions.

7.2.2 Temporal Coverage Map

The following are the operation periods for the Cimel automatic
sunphotometers during the 1994, 1995, and 1996 BOREAS campaigns:

1994 OPERATION PERIODS

Flin Flon, MB 19-May - 13-Sep

NSA-YJP 18-May - 26-Jun, 4-Jul - 01-Nov

SSA-YJP 23-May - 13-Oct

Waskesiu, SK 25-May - 21-Jul, 25-Jul - 13-Oct

Thompson, MB 08-Jun - 13-Jun, 27-Jul - 10-Sep

Prince Albert, SK 17-May - 06-Jun, 20-Jul - 26-Jul, 12-Sep -18-Sep

1995 OPERATION PERIODS

Flin Flon, MB 18-May - 07-Sep

NSA-YJP 16-May - 03-Nov

SSA-YJP 25-May - 01-Jun, 01-Jul - 12-Sep, 22-Sep - 21-Nov

Waskesiu, SK 24-May - 01-Aug, 22-Aug - 04-Nov

Thompson, MB 15-May - 29-Oct

1996 OPERATION PERIODS

Paddockwood, SK 27-Feb - 07-Nov

NSA-YJP 14-May - 24-Sep, 30-Sep - 09-Oct

SSA-YJP 10-May - 14-May, 28-May - 20-Sep, 01-Oct - 23-Oct

Waskesiu, SK 08-May - 14-May, 23-May - 04-Sep, 29-Sep - 27-Oct

Thompson, MB 14-May - 09-Sep

The following are the peration periods for the hand-held sunphotometers
during 1994, 1995, and 1996 BOREAS campaigns. Gaps may indicate extended
cloudy conditions, instrument problems, or operator unavailability.

1994 OPERATION PERIODS

Waskesiu, SK 04-Jan - 03-Mar, 22-Mar - 26-Sep, 10-Oct - 22-Dec

Thompson, MB 01-Jan - 29-Jun, 01-Aug - 21-Oct

1995 OPERATION PERIODS

Waskesiu, SK 01-Jan - 11-Apr, 04-May - 02-Sep, 21-Sep - 08-Oct

Thompson, MB 04-Feb - 20-Dec

1996 OPERATION PERIODS

Waskesiu, SK 03-Jan - 31-Dec

Thompson, MB 03-Jan - 31-Dec

7.2.3 Temporal Resolution

Cimels operated approximately every 15 minutes from morning to evening.
Hand-held sunphotometers were operated near noon only.

7.3 Data Characteristics

Data characteristics are defined in the companion data definition file
(r11sunpd.def).

7.4 Sample Data Record

Sample data format shown in the companion data definition file
(r11sunpd.def).

8\. Data Organization

8.1 Data Granularity

All of the RSS-11 Ground Network of Sunphotometer Measurements are
contained in one dataset.

8.2 Data Format(s)

The data files contain numerical and character fields of varying length
separated by commas. The character fields are enclosed with a single
apostrophe marks. There are no spaces between the fields. Sample data
records are shown in the companion data definition files (r11sunpd.def).

9\. Data Manipulations

9.1 Formulae

[Equation]{.underline} [Used for]{.underline}

![](media/image6.wmf){width="2.0694444444444446in"
height="0.3472222222222222in"} Calculation of optical depth

![](media/image7.wmf){width="1.4166666666666667in"
height="0.3888888888888889in"} Calculation of precipitable water

![](media/image8.wmf){width="3.0708333333333333in"
height="0.3194444444444444in"} Calibration of water vapor channel

![](media/image9.emf){width="1.3194444444444444in"
height="0.5416666666666666in"} Cloud-screening procedure

9.1.1 Derivation Techniques and Algorithms

Optical thickness is calculated from spectral extinction of direct beam
radiation at each wavelength based on Beer\'s Law:

![](media/image10.emf){width="2.1666666666666665in"
height="0.4444444444444444in"}

where V= digital voltage measured at wavelength 

m = optical airmass

 = total optical thickness

R = relative Earth-Sun distance

Attenuation due to Rayleigh scatter and absorption by ozone and gaseous
pollutants is estimated and removed to isolate the AOT ().

The dependence of transmission, Tw, on water column abundance can be
written as the following \[Halthore et al., 1992, Bruegge et al.,
1992\]:

![](media/image2.emf){width="1.9722222222222223in"
height="0.5277777777777778in"}

where W is the vertical column abundance (cm), and the constants a and b
depend on the wavelength position, width and shape of the filter
function, and atmospheric conditions.

Sky radiance measurements are inverted with radiative transfer equations
\[Nakajima et al., 1983\] to provide aerosol properties of size
distribution and phase function over the particle size range of 0.1 to 5
µm.

9.2 Data Processing Sequence

9.2.1 Processing Steps

The automatic radiometers acquire data regardless of sky conditions,
except for rain, and thus require cloud-screening procedures. Three
quality control schemes are considered to reject data obtained under
marginal conditions. The Cimels perform three scan sequences spaced 30
seconds apart, and thus acquire three AOT measurements at each
wavelength. If any of these triplets exhibit a coefficient of variation
of more than 12%, the data derived from all channels are rejected.
Secondly, data exhibiting an increasing or nearly flat AOT with
wavelength between 440 nm and 870 nm are considered cloud-contaminated.
Therefore, data with an Angstrom coefficient, a, of less than or equal
to zero are removed. The Angstrom coefficient is calculated by:

![](media/image11.emf){width="1.9027777777777777in"
height="0.7777777777777778in"} for  at 440 nm and 870 nm, where  is
wavelength.

Finally, the remaining data are plotted along with the direct Sun
observations acquired during almucantar measurements exhibiting high
azimuthal symmetry about the solar plane, which are expected to
represent cloudless conditions. A regression line through these
almucantar observations is plotted as well. The abscissa shows the AOT
at 440 nm, while the ordinate depicts the ratio of this AOT to the
Angstrom coefficient described above. The core of the remaining data set
is found to follow the trend of the almucantar regression, and outliers
are removed by subjectively drawn polygons.

Hand-held data are cloud-screened in a similar, but less rigorous,
manner because the manual instruments do not take triplet measurements.
The same plotting technique is used with axes of x: \[500nm AOT\] and y:
\[500nm / a (500/870)\], although no almucantar measurements are
available to plot simultaneously.

9.2.2 Processing Changes

None.

9.3 Calculations

None.

9.3.1 Special Corrections/Adjustments

None.

9.3.2 Calculated Variables

Water vapor, AOT.

9.4 Graphs and Plots

None.

10\. Errors

10.1 Sources of Error

Error Result

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Tracking misalignment Overestimation of AOT and/or reduced
reproducibility

Filter degradation Generally underestimation of AOT

Detector response Over- or underestimation of AOT

Temperature stability Over- or underestimation of AOT at 1020 nm

Collimator obstruction Overestimation of AOT

Failure to find peak Overestimation of AOT

response (hand-held)

10.2 Quality Assessment

All data have been cloud-screened and checked for almucantar symmetry.

10.2.1 Data Validation by Source

Pre/postdeployment intercomparison was made with reference instruments,
pre/postdeployment filter calculations were made and agreement of
proximal instruments was verified

10.2.2 Confidence Level/Accuracy Judgement

Data are generally good, with an absolute AOT accuracy of typically
±0.02 for the automatic sunphotometers and ±0.04 for the hand-held
sunphotometers. Precipitable water measurements have agreed within 10%
of radiosonde measurements \[Halthore et al., 1997\].

10.2.3 Measurement Error for Parameters and Variables

The Cimels make three direct Sun measurements at each wavelength in a
30-second scan sequence. This group of three measurements is referred to
as a triplet. The coefficient of variability of the triplets for three
reference instruments deployed at Mauna Loa was calculated based on 168,
264, and 288 observations, respectively. For all wavelengths, the
variability of a triplet was always less than 1%, and generally about
0.3%, giving an estimate of the instrument\'s reproducibility \[Holben
et al., 1996\]. The calibration coefficients, Vo are usually determined
by averaging the y-intercepts from three to seven Langley plots at Mauna
Loa Observatory. The averaged Vo values from all calibration sessions at
Mauna Loa have a coefficient of variability of \~0.25% to 0.5% for the
visible and near-infrared wavelengths, \~0.5% to 2% for the ultraviolet,
and \~1.0% to 3.0% for the water vapor channel \[Holben et al., 1996\].
The overall accuracy of AOT measurements is expected to be in the range
of 0.02 at an airmass of 1.0. The hand-held sunphotometers were
calibrated by intercomparison with colocated Cimels. The estimated level
of uncertainty for these instruments is greater (0.04) because of less
frequent recalibration. For the sky radiance measurements, calibration
was performed at the NASA GSFC Calibration Facility using a calibrated
integrating sphere to an accuracy of +/- 5%. For the 940-nm channel that
includes water absorption, calibration was performed using a variant of
the modified Langley method as described in Halthore et al. \[1997\].
The method used is similar to that described elsewhere; for instance,
Bruegge et al., 1992b, and Halthore et al., 1992b. Column amounts of
precipitable water derived from sunphotometer measurements at BOREAS
have compared favorably with radiosonde observations \[Halthore et al.,
1997\] to within +/- 10%.

10.2.4 Additional Quality Assessments

None given

10.2.5 Data Verification by Data Center

BORIS personnel have reviewed portions of the actual parameter values
and generated plots for use in visually spotting any anomalous values.

11\. Notes

11.1 Limitations of the Data

None given.

11.2 Known Problems with the Data

Most data are good, though occasionally the AOT at 1020 nm will exceed
that at 870 nm, or the AOT at 500 nm will exceed that at 440 nm for very
low aerosol conditions (AOT 500 \< 0.04), when sensitivity to aerosol is
minimal and calibration errors are greatly accentuated. Most of the
apparent errors are on the order of 0.01 in AOT, so it is typically a
minor concern. On most instruments, the 340-and 380-nm channels are less
reliable than the longer wavelengths, likely because of the nature of
the filter design, and at times, they yield AOT values that are lower
for 340 nm than 380 nm.

Triplet variation is larger for instrument #31, which is currently
suspected to result from a 4-quadrant detector that is misaligned or
otherwise functioning inadequately. Similar problems in other
instruments at GSFC, not involved with BOREAS are being investigated.

11.3 Usage Guidance

None given.

11.4 Other Relevant Information

None given.

12.0 Application of the Data Set

The AOT data can be used as input for standard radiative transfer
programs such as LOW-resolution Radiative Transfer Code (LOWTRAN-7),
MODerate Resolution Radiative Transfer Code (MODTRAN), or the Second
Simulation of the Satellite Signal in the Solar Spectrum (6S) to perform
atmospheric correction of remotely sensed data.

13\. Future Modifications and Plans

Possible fully objective cloud-screening routine.

14\. Software

14.1 Software Description

\"Demonstrat\" is a package of user-friendly UNIX software developed at
GSFC used for data analysis and can be found at spamer.gsfc.nasa.gov.

Within Demonstrat, the raw data (voltages) are converted to AOT or
precipitable water using the relevant instrument-specific calibration
coefficients.

14.2 Software Access

Guest accounts are available and accessible via X-term window. Contact
persons:

Brent Holben (Brent.N.Holben.1@gsfc.nasa.gov) or Ilya Slutsker
(Ilya.Slutsker.1@gsfc.nasa.gov).

15\. Data Access

15.1 Contact Information

Ms. Beth Nelson

BOREAS Data Manager

NASA GSFC

Greenbelt, MD

\(301\) 286-4005

\(301\) 286-0239 (fax)

beth@ltpmail.gsfc.nasa.gov

15.2 Data Center Identification

See Section 15.1.

15.3 Procedures for Obtaining Data

Users may place requests by telephone, electronic mail, or fax.

15.4 Data Center Status/Plans

The RSS-11 sunphotometer data are available from the Earth Observing
System Data and Information System (EOSDIS) Oak Ridge National
Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS
contact at ORNL is:

ORNL DAAC User Services

Oak Ridge National Laboratory

Oak Ridge, TN

\(423\) 241-3952

ornldaac@ornl.gov

ornl@eos.nasa.gov

16\. Output Products and Availability

16.1 Tape Products

None.

16.2 Film Products

None.

16.3 Other Products

The data are available in tabular ASCII files.

17\. References

17.1 Platform/Sensor/Instrument/Data Processing Documentation

None.

17.2 Journal Articles and Study Reports

Ahern, F.J., R.P. Gauthier, P.M. Teillet, J. Sirois, G. Fedosejevs, and
D. Lorente, 1991, Investigation of continental aerosols with
high-spectral-resolution solar-extinction measurements, Appl. Optics,
30, 5276-5287.

Bruegge, C.T., J.E. Conel, R.O. Green, J.S. Margolis, R.G. Holm, and G.
Toon, 1992, Water vapor column abundance retrievals during FIFE, JGR,
97(D19), 18759-18768.

Bruegge, C.T., R.N. Halthore, B. L. Markham, M. Spanner, and R. Wrigley,
1992, Aerosol optical depth retrievals over the Konza Prairie, JGR,
97(d19), 18743-18758.

d\'Almeida, G.A., and P. Koepke, 1998, An approach to a global optical
aerosol climatology, in Aerosols and Climate, edited by P.V. Hobbs and
M.P. McCormick, Deepak Publishing, Hampton, VA.

Halthore, R.N., B.L. Markham, R.A. Ferrare, and T.O. Aro, 1992, Aerosol
optical properties over the midcontinental United States, JGR, 97(D17),
18769-18778.

Halthore, R.N., T.F. Eck, B.N. Holben and Brian Markham, 1997,
Sunphotometric measurements of atmospheric water vapor column abundance
in the 940-nm band, JGR.

Hansen, J.E. and A.A. Lacis, 1990, Sun and dust versus greenhouse gases:
An assessment of their relative roles in global climate change, Nature,
345, 713-719.

Hegg, D.A., P.V. Hobbs, R.J. Ferek, and A.P. Waggoner, 1995,
Measurements of some aerosol properties relevant to radiative forcing on
the east coast of the United States, J. Appl. Meteor., 34, 2306-2315.

Holben, B.N., T.F. Eck and R.S. Fraser, 1991b, Temporal and spatial
variability of aerosol optical depth in the Sahel region in relation to
vegetation remote sensing, Int. J. Remote Sens., 12, 1147-1163.

Holben, B.N., T.F. Eck, I. Slutsker, D. Tanré, J.P. Buis, A. Setzer, E.
Vermote, J.A. Reagan, Y.J. Kaufman, T. Nakajima, F. Lavenu and I.
Jankowiak, Automatic sun and sky scanning radiometer system for network
aerosol monitoring, Rem. Sens. Environ. (in press)

Holben, B.N., Y.J. Kaufman, A. Setzer, D. Tanré, and D.E. Ward, 1991a,
Optical properties of aerosol from biomass burning in the tropics,
BASE-A, in Global Biomass Burning, the MIT Press, Cambridge, MA,
403-411.

Kaufman, Y.J., A. Setzer, D. Ward, D. Tanré, B.N. Holben, P. Menzel,
M.C. Pereira, and R. Rasmussen, 1992, Biomass burning airborne and
spaceborne experiment in the Amazonas (BASE-A), J. Geophys. Res.,
97(D13), 14581-14599.

Kaufman, Y.J., B.N. Holben, D. Tanré, and D.E. Ward, 1994, Remote
sensing of biomass burning in the Amazon, Remote Sensing Reviews, 10,
51-90.

Lenoble, J., 1993, Atmospheric Radiative Transfer, Deepak Publishing:
Hampton, VA, pp 532.

Markham, B.L., J.S. Schafer, B.N. Holben, and R.N.Halthore, 1997,
\"Atmospheric aerosol and water vapor characteristics over north central
Canada during BOREAS\", JGR (in press)

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

Reagan J.A., K.J. Thome, and B.M. Herman, 1992, A simple instrument and
technique for measuring columnar water vapor via Near-IR differential
solar transmission measurements, IEEE Trans. Geosci. Remote Sensing, 30,
825-831.

Sellers, P.and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study:
Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94).

Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study:
Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96).

Sellers, P., F. Hall and K.F. Huemmrich. 1996. Boreal
Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC
94).

Sellers, P., F. Hall and K.F. Huemmrich. 1997. Boreal
Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC
96).

Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den
Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D.
Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere
study (BOREAS): an overview and early results from the 1994 field year.
Bulletin of the American Meteorological Society. 76(9):1549-1577.

Sellers, P.and F. Hall. 1997. BOREAS Overview Paper. JGR Special Issue
(in press).

Shaw, G.E., 1983, Sun photometry, Bull. Am. Meteorol. Soc., 64, 4-10.

Thome, K.J., B.M. Herman and J.A. Reagan, 1992, Determination of
precipitable water from solar transmission, J. Appl. Met., 31, 157-165.

Twomey, S., 1991, Aerosols, clouds and radiation, Atmos. Environ.,
25(A), 2435-2442.

Ward, D.E., R.A. Susott, J.B. Kauffman, R.E. Babbit, D.L. Cummings,
B.Dias, B.N. Holben,Y.J. Kaufman, R.A. Rasmussen and A.W. Setzer, 1992,
Smoke and fire characteristics for cerrado and deforestation burns in
Brazil: BASE-B experiment, J. Geophys. Res., 97(D13), 14601-14619.

18\. Glossary of Terms

Aerosol optical depth - a dimensionless measure of the extinction of the
direct solar beam due to scattering and absorption by atmospheric
particulates.

Air mass - approximately equal to 1/cos(solar zenith angle)

Almucantar - sunphotometer procedure that measures the sky radiance in
four spectral bands (440, 670, 870, and 1020 nm) at constant solar
zenith angle, with varied azimuth angles ranging from 0.25-degree
intervals near the solar azimuth position to 10-degree intervals far
from the solar azimuth position.

Asymmetry parameter - average cosine of the scattering angle weighted by
the phase function.

Phase function - a dimensionless measure of the relative scattering
intensity of a particle as a function of angle relative to the original
propagation direction.

Rayleigh scatter - scatter of solar energy by gaseous molecules that is
highly predictable for a given atmospheric pressure.

19\. List of Acronyms

6S - Second Simulation of the Satellite Signal in the Solar Spectrum

AOT - Aerosol Optical Thickness

ASCII - American Standard Code for Information Interchange

BOREAS - BOReal Ecosystem-Atmosphere Study

BORIS - BOReas Information System

CCRS - Canada Centre for Remote Sensing

DAAC - Distributed Active Archive Center

EOS - Earth Observing Satellite

EOSDIS - EOS Data and Information System

FOV - Field of View

GOES - Geostationary Operational Environmental Satellite

GSFC - Goddard Space Flight Center

LOWTRAN - LOW resolution radiative TRANsfer code

MODTRAN - MODerate resolution radiative TRANsfer code

NASA - National Aeronautics and Space Administration

NSA - Northern Study Area

ORNL - Oak Ridge National Laboratory

PANP - Prince Albert National Park

SSA - Southern Study Area

URL - Uniform Resource Locator

YJP - Young Jack Pine

20\. Document Information

20.1 Document Revision Date

Created: 20-Jun-1996

Revised: 19-Mar-1998

20.2 Document Review Dates

BORIS Review: 19-Sep-1997

Science Review: 16-Oct-1997

20.3 Document ID

20.4 Citation

If this data set is referenced by another investigator, please
acknowledge the paper by Markham, et al (in press), listed in Section
17.

20.5 Document Curator

20.6 Document URL

Keywords:

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Aerosol Optical Thickness

Water Vapor

Volume Distribution

Phase Function