2
Holben et al.
cessing, can provide much of the ground-based validation knowledge base most importantly through inversion of
the sky radiances to derive aerosol microphysical proper-data required for future remote sensing programs and may
provide basic information necessary for improved assess- ties such as size distribution and optical properties such
as phase function (Nakajima et al., 1983; 1996; Tanre
´
etment of aerosols impact on climate forcing.
al., 1988; Shiobara et al., 1991; Kaufman et al., 1994).
This technique requires precise aureole measurements
BACKGROUND
near the solar disk and good straylight rejection. Histori-
cally these systems are rather cumbersome, not weather-The technology of ground-based atmospheric aerosol
measurements using sun photometry has changed sub- hardy, and expensive. The CIMEL and PREDE (French
and Japanese manufacturers, respectively) Sun and skystantially since Volz (1959) introduced the first handheld
analog instrument almost 4 decades ago. Modern digital scanning spectral radiometers overcome most such limi-
tations, and provide retrievals from direct Sun measure-units of laboratory quality and field hardiness can collect
data more accurately and quickly and are often inter- ments of aerosol and water vapor abundance in addition
to aerosol properties from inversion of spectral sky radi-faced with onboard processing (Schmid et al., 1997; Eh-
sani et al., 1998; Forgan, 1994; Morys et al., 1998). The ances. Since the measurements are directional and rep-
resent conditions of the total column atmosphere, theremethod used remains the same, that is a filtered detector
measures the spectral extinction of direct beam radiation are direct applications to satellite and airborne observa-
tions as well as atmospheric processes.according to the Beer–Lambert–Bouguer law:
As has been demonstrated by the shadowband net-
V
k
⫽V
0k
d
2
exp(s
k
m)
*
t
y
(1)
work and satellite remote sensing in general, prompt de-
where
livery of the data for analysis is fundamental for ob-
taining a comprehensive, continuous database, and allows
V⫽digital voltage,
assessment of the collecting instruments health and cali-
V
0
⫽extraterrestrial voltage,
bration. To achieve this goal, minimize costs and expand
m⫽optical air mass,
the coverage globally, we use the simple and inexpensive
s⫽total optical depth,
Data Collection System (DCS) operating on the geosyn-
k⫽wavelength,
chronous GOES, METEOSAT, and GMS satellites pro-
d⫽ratio of the average to the actual Earth–Sun
viding nearly global coverage in near real-time at very
distance,
little expense (NOAA/NESDIS, 1990).
t
y
⫽transmission of absorbing gases.
Finally there are the very contentious issues of pro-
The digital voltage (V) measured at wavelength (k)isa
cessing the data archive. Although the Beer–Lambert–
function of the extraterrestrial voltage (V
0
) as modified
Bouguer law is very straightforward, its implementation
by the relative Earth–Sun distance (d), and the exponent
has as many variations as there are investigators who use
of the total spectral optical depth (s
k
) and optical air
it. The central problem being agreement on the accuracy
mass (m). The total spectral optical depth is the sum of
by which the aerosol optical thickness is derived. The un-
the Rayleigh and aerosol optical depth after correction
certainties in computation of the air mass (m), the calcu-
for gaseous absorption.
lations for the Rayleigh and ozone optical depths (s
r
, s
0
),
The multifilter rotating shadowband radiometer
and water vapor expressed as total column abundance or
(MFRSR) employs a different strategy. It measures spec-
precipitable water (Pw) as well as strategies for calibra-
tral total and diffuse radiation to obtain the direct com-
tion of the instruments and monitoring the long-term
ponent from which aerosol optical thickness is computed
change in calibration all combine to preclude any glob-
using the Beer–Lambert–Bouguer law. The instrument
ally accepted processing scheme. Perhaps even more de-
nominally measures at 1-min intervals and has been
batable are the aerosol properties derived from inver-
shown to be reliable over long periods of time. The mea-
sions of the sky radiances with the radiation transfer
surements are networked to a common server by a mo-
equation. Our solutions make the raw data and calibra-
dem interface and the data processed by a common anal-
tion data available to the user and provide a basic pro-
ysis system (Harrison et al., 1994). It is widely used in
cessing package (of published, widely accepted algo-
the United States principally for the DOE ARM sites. As
rithms) with sufficient friendliness and flexibility that all
the number of measurements from the MFRSR network
data may be accessed globally through common forms of
increases, the impact of aerosol loading on the radiation
electronic communication on the internet.
balance should be more clearly understood, especially
Following is the Aerosol Robotic Network (AERO-
when taken in concert with other ground, airborne, and
NET) version of a ground-based aerosol monitoring sys-
satellite measurements.
tem that offers a standardization for a ground-based
Sky scanning spectral radiometers, that is, radiome-
regional to global scale aerosol monitoring and character-
ters that measure the spectral sky radiance at known an-
ization network. We have assembled a reliable system
and offer it as a point of focus for further developmentgular distances from the Sun, have expanded the aerosol