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widely used for high volume data collection programs (Harrison et al., 1994). Sky scanning radiometers 
have also been used to derive the aerosol optical thickness and more importantly the aerosol properties such 
as size distribution and phase function.(Nakajima et al. (1983), Tanre et al. (1988), Shiobara et al. (1991) 
and Kaufman et al., (1993)). This technique requires precise aureole measurements near the solar disc and 
extremely good collimation. Unfortunately most systems have been rather cumbersome and beyond the 
scope of many investigators resources however the following description of a new sun and sky scanning 
spectral radiometer overcomes most such limitations providing the first of a three component monitoring 
system. 
The second and often neglected component of sun photometry systems is that of data handling 
including delivery of the raw data to a central archive and its subsequent processing and availability to the 
user. We have utilized the simple and inexpensive Data Collection System used by GOES, METEOSAT 
and GMS which provides global coverage in near real time at very little expense (NOAA/NESDIS, 1990). 
Finally there are the very contentious issues of processing the data archive. Although Beers law is 
very straight forward, its implementation has as many variations as there are investigators who use it. The 
central problem being agreement on the accuracy which the aerosol optical thickness is derived. The 
uncertainties in computation of the airmass (m), the calculations for the Raleigh, ozone and water vapor 
optical depths (Tr, To and Tw) as well as strategies for calibration of the instruments and monitoring the 
long term change in calibration all combine to preclude any globally accepted processing scheme. Our near 
term solutions make the original data available to everyone and provide a basic processing package available 
to all users with sufficient friendliness and flexibility that all data may be accessed globally through 
common forms of electronic communication. 
Following is our version of a ground based aerosol monitoring system that addresses, 
compromises and offers a solution to the three components of a sun photometer network system. We 
suggest that future networks must integrate all three components if they are to be successful and which is 
dependent on further development of the individual parts. We have assembled a reliable system and offer it 
as a point of focus for further development of each component. As an example of the system's performance 
we present data collected in the Brazilian Amazon during the dry season of 1993. Owing to the 
fundamental importance of these and similar data for basic and applied research, our philosophy is for an 
open, honor system whereby all contributed data may be accessed by anyone but publication of results 
requires permission of the contributing investigators. 
2 - AUTOMATIC SUN AND SKY SCANNING SPECTRAL RADIOMETER 
Most if not all sun photometer networks have had limited success when people are required to make routine 
observations. Therefore an automatic instrument is a fundamental component to any network where routine 
observations are required. Additionally the instrument protocol must be reasonably robust such that the data 
that are not wanted can be screened from that which are useful. Lastly the instrument must collect data for 
monitoring its calibration. Following is our assessment of an instrument that meets these criteria and our 
requirements of a field hardy sun and sky scanning spectral radiometer. The reason for a sky scanning 
radiometer rather than a traditional sun photometer is to provide duplicity in aerosol optical thickness 
retrievals and aerosol properties from inverting sky radiance data. 
2.1. General Description 
The Cimel Electronique 318A radiometer manufactured in Paris, France is independent of public electrical 
and communication systems. This instrument has a 1.2 degree full angle field of view, dual detector for 
measurement of direct sun, aureole and sky radiance with 33 cm collimators for 10"^ straylight rejection. 
The sensor head is mounted on the robot such that the optics are protected from rain and entrance of foreign 
particles into the system in the non active position. The sun aureole collimator is protected by a quartz 
window allowing observation with a UV enhanced silicon detector with sufficient signal to noise for 
spectral observations from approximately 300 nm to 1020 nm. The sky collimator has the same field of 
view but an aperture approximately 10 times larger for better dynamic range for the sky radiances. The 
components of the sensor head are o-ring sealed from moisture and desiccated to prevent damage to the 
electrical components and interference filters. Eight interference filters are located in a filter wheel which is 
rotated by a direct drive stepping motor. A thermister measures the temperature of the detector allowing 
compensation for any temperature dependence in the silicon detector.