Continuous Light Absorption Photometer
Earth System Research Laboratory
Absorption of sunlight by atmospheric aerosols is important to Earth's energy budget, and the climate forcing caused by the dominant light absorbing aerosol, black carbon, is second in magnitude only to carbon dioxide. In recognition of this important role in Earth's climate, aerosol light absorption is continuously monitored at the nearly two dozen stations that collaborate closely with the Global Monitoring Division (GMD) of NOAA's Earth System Research Laboratory, and at many more stations around the globe that contribute to the World Meteorological Organization's Global Atmosphere Watch. There are several different types of instruments in use for this measurement, but all the long-term records are based on continuous measurements of light transmitted through a filter while the aerosols are being deposited on the filter. However, none of the commercial instruments for measuring spectral aerosol light absorption were optimized for making long-term, research-quality measurements.
Based on experience gained from years of operating commercial instruments continuously at field sites and intermittently in laboratory studies, staff at GMD designed a new instrument, the Continuous Light Absorption Photometer (CLAP), to be functionally comparable to a widely-used commercial instrument, the Particle/Soot Absorption Photometer (PSAP), but that did not suffer from the same operating limitations as the PSAP. The choice of a functionally comparable design to the PSAP allowed measurements from the CLAP to use the well-established PSAP correction schemes that account for know imperfections in all filter-based absorption measurements.
The primary advantages of the CLAP, compared to the PSAP, are that it is one-tenth the size, can sample for roughly ten times as long before the operator needs to change the filter, and is temperature-stabilized to reduce sensitivity to changes in room temperature. In addition, the computer software running on the internal microprocessor is completely open on the CLAP. These features make the CLAP much better suited for long-term monitoring applications, and currently NOAA-built CLAPs are deployed at 23 stations around the globe. Five of these stations are operated by NOAA, two by the U.S. Department of Energy, and three by U.S. universities; the remaining 13 stations are outside the U.S. and are operated by foreign universities and research institutions.
During the initial field deployment of the NOAA-built CLAPs, they were run in parallel with PSAPs for over a year at multiple stations to confirm that the CLAPs and PSAPs give comparable results. Subsequently, NOAA has retired the PSAPs from service at its monitoring stations.
During the initial deployment period, quite a few researchers asked whether they could purchase a CLAP for their own projects. It became clear that there was sufficient demand to support further production, but it was also apparent that providing support for additional instruments would not always advance the goals of GMD's long-term aerosol research program. That initiated a search for a commercial manufacturer of aerosol instrumentation to take over production, marketing, and support of the CLAP.
Four companies were contacted and one of them, Brechtel Manufacturing, Inc. (BMI, Fremont, CA), accepted the challenge to make a commercial version of the CLAP. GMD provided all the information needed to build a CLAP, including schematic diagrams, construction drawings, parts lists, computer code, and fabrication instructions, and consulted regularly with BMI to transfer both the techniques and "art" needed to build a properly-functioning instrument. BMI made an initial production run of six instruments to become familiar with the instrument, and then modified the initial design to make it more suitable for a broader market. The second production run consisting of eight instruments of the modified design, renamed "Tricolor Absorption Photometer" (TAP, http://www.brechtel.com/products-item/tricolor-absorption-photometer/) was recently completed.
The on-going operation of CLAPs at 23 long-term aerosol monitoring stations, operated by NOAA, other U.S. federal organizations, U.S. universities, and foreign organizations satisfies the criterion for exemplary activities that lead to the use of NOAA R&D to increase productivity in American industry or business, universities, State or local Governments, or other Federal agencies. GMD provides software for controlling and acquiring data from the CLAP, training and written documentation for operating the CLAP and interpreting the data, and assistance through personal visits to collaborating institutions. This involvement in operation of the CLAPs leads to high-quality observations of aerosol light absorption and equivalent black carbon that is contributing to increased understanding to the role of light absorbing aerosols in forcing climate change.