Abstract

Last year we reported some results from field-testing the GLOBE sun photometer. During the past year, the GLOBE office has incorporated all the new atmosphere protocols into its training and we have presented the aerosol protocols to European country coordinators in September 2000 and to teachers, country coordinators, and others from Africa in April 2001. This year we can report that the GLOBE sun photometer is an operational instrument.

We have further refined the design of the sun photometer and we have produced a new printed circuit board with room for expansion from two to as many as four detectors. We currently have available parts kits (for students and teachers to assemble their own sun photometers) and some assembled instruments. We are working toward commercial manufacturing, but instrument calibration cannot be done as a manufacturing step and this remains the most significant challenge for this kind of work. We have established a three-tier approach:

1. "reference calibrations" for instruments calibrated using the Langley method at Mauna Loa Observatory;
2. "transfer calibrations" for instruments calibrated in sunlight side-by-side with a reference instrument;
3. "standard lamp calibrations" for detector assemblies calibrated in the lab against a detector assembly calibrated at Mauna Loa Observatory.

The first method is the most desirable but it is impractical for large numbers of instruments. The second method works well but is very weather dependent. The third method is the quickest, but also the least accurate.

We have published a paper on calibrating and interpreting data from LED-based sun photometers in the Journal of Geophysical Research – Atmospheres, the leading peer-reviewed journal for atmospheric science.

During the past year we have reached an agreement with NASA's EOS-Terra project to include GLOBE aerosol optical thickness (AOT) data as part of the ground validation database for the MODIS instrument. We are working with other projects, too, such as EOS-Aura, PICASSO/CENA, and GIFTS. These links allow students and teachers to participate directly in major space-based earth science projects for the 21st century.

This year we have made significant progress in several important areas.

Last year we reported some results from field-testing the GLOBE sun photometer. During the past year, the GLOBE office has incorporated all the new atmospheric science protocols into its train-the-trainer workshops. We have presented the aerosol protocol to more than 20 European country coordinators in September 2000. In April 2001, we have presented both the aerosol and surface ozone protocols to 30 teachers from Bénin and to government officials and country coordinators from Cameroon, Ghana, and Mali at the First GLOBE International Atmosphere Symposium held in Cotonou, Bénin. As a result of these activities, we can now report that the aerosol protocol and the GLOBE sun photometer have both achieved operational status.

We continue to refine the design of the sun photometer and we have produced a new printed circuit board with room for expansion from two to as many as four detectors. There has been a gratifyingly high demand for sun photometers in kit form, which can be obtained through the Science Team.

Assembled and calibrated sun photometers are also available through the Science Team. However, it is important to note that, at least in the near future, the GLOBE sun photometer will not be an "off the shelf" instrument. Indeed, sun photometers of all kinds are essentially "research" instruments that will not be generically available in the same way as, for example, a max/min thermometer. We are currently working toward commercial assembly, but instrument calibration cannot be done as a manufacturing step and this remains the most significant challenge for this kind of work. We have established a three-tier approach to calibration:

1. "reference calibrations" for instruments calibrated using the Langley method at Mauna Loa Observatory;
2. "transfer calibrations" for instruments calibrated in sunlight side-by-side with a reference instrument;
3. "standard lamp calibrations" for detector assemblies calibrated in the lab against a detector assembly calibrated at Mauna Loa Observatory.

The first method is the most desirable but it is impractical for large numbers of instruments. The second method works well but is very weather dependent. The third method is the quickest, but also the least accurate.

Science Initiatives

We are committed to broadening scientific acceptance for GLOBE's aerosol monitoring project. Toward this end, we have published a paper on calibrating and interpreting data from LED-based sun photometers in the Journal of Geophysical Research – Atmospheres, the leading peer-reviewed journal for atmospheric science (Brooks and Mims, 2001).

The agreement with EOS-Terra represents a major opportunity for GLOBE schools to participate as real partners in one of the major earth science undertakings of the 21st century. We are involved with other projects, too, such as NASA Goddard Space Flight Center's EOS-Aura, and NASA Langley Research Center's PICASSO/CENA and GIFTS projects.

For EOS-Aura, we are developing handheld LED-based instruments to measure UV-A radiation at a wavelength of about 375 nm. These instruments are similar to the GLOBE sun photometers and will be used in a program of GLOBE "special measurements." The measurement protocol for EOS-Aura includes both aerosol optical thickness, with a collimated beam of direct sunlight and full-sky radiation, using the same detector without a collimator. In the latter configuration, the measurement can be made under any sky condition, as it does not depend on having an unobstructed view of the sun. Taken together at the time of an EOS-Aura overflight of an observation site, the GLOBE sun photometer and UV-A measurements will provide data of great scientific interest. We hope that schools will take both sets of measurements simultaneously, weather permitting, and we hope that an "all weather" protocol will encourage GLOBE schools to incorporate these measurements into their every-day atmospheric measurements.

The PICASSSO/CENA and GIFTS projects also use GLOBE's sun photometer technology. (PICASSO/CENA is an acronym for: Pathfinder Instruments for Cloud and Aerosol Spaceborne Observations/Climatologie Étendue des Nuages et des Aerosols. (The project is run jointly by NASA and CNES, the French aerospace research agency.) As a result of objections by the Picasso family to the use of this name, NASA is in the process of renaming this project. At the time this paper was submitted, the new name had not been selected. Hence, the reference to PICASSO/CENA is retained even tough it will be changed by the time this paper is presented.)

The PICASSO project can use instruments identical to the GLOBE sun photometer. One of the GIFTS measurements is atmospheric water vapor. This can be measured with two near-IR detectors, one in the water vapor absorption band and one just outside. For this measurement, GIFTS will use the newly developed four-channel pc boards with the red and green GLOBE detectors and two near-IR detectors. Thus, students can again extend the GLOBE aerosol protocol to include another scientifically significant quantity.

Each of these new projects relies on the basic instrument developed by GLOBE, but each brings its own special features. One important difference is that for each of these projects, the cost of equipment is part of the project cost and those instruments will be provided directly to teachers at no charge. Project personnel will then provide extensive teacher training and support for a limited set of measurements in the context of a specific space-based mission.

Progress With EOS-Terra/MODIS

As noted above, the agreement to include student data as part of the MODIS ground validation database is an exciting opportunity for GLOBE schools. Unfortunately, the processing of EOS-Terra data is lagging behind data collection by more than five months at the time this paper was submitted at the end of May, 2001. Although this is a common and not entirely unanticipated problem for space-based missions, we view it as a significant impediment to recruiting teachers and students to participate in this project.

Brooks, D. R., and Mims, F. M. III, Development of an inexpensive handheld LED-based Sun photometer for the GLOBE program. Journal Geophys. Res. 106(D5):4733-4740, 2001.

Mims, F. M., III, An international haze-monitoring network for students. Bull. Am. Meteorol. Soc., 80, 1421-1431, 1999.