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A 5-channel handheld Microtops sunphotometer (Solar Light Co.) operating at 380, 440, 500, 675, and 870 nm was used. The full angular field of view is 2.5 deg. The instrument has built in pressure and temperature sensors and was operated with a GPS connection to obtain position and time of the measurements. A MATLAB routine, also used by the NASA SIMBIOS program and Brookhaven National Laboratory, was used to convert the raw signal voltages from the Microtops to aerosol optical depths. Included in the conversion is a correction for Rayleigh scattering [Penndorf, 1957], ozone optical depth, and an air mass that accounts for the Earth's curvature [Kasten and Young, 1989]. Ozone column amounts used to calculate the ozone optical depth were based on TOMS data. The instrument was calibrated using a Langley plot approach [Shaw, 1983] by the manufacturer prior to the cruise and again at Mauna Loa 5 months after the cruise. Calibration constants for the 5 wavelengths differed by less than 0.9% between the two calibrations, which corresponds to approximately 0.01 in optical depth.
Kasten, F. and A. T. Young, Revised optical air mass tables and approximation formula, Applied Optics, 28, 4735 - 4738, 1989.
Penndorf, R. , Tables of refractive index for standard air and the Rayleigh scattering coefficient for the spectral region between 0.2 and 20 um and their application to atmospheric optics, J. Opt. Soc. America, 47, 176 - 182, 1957.
Shaw, G. E., Sun Photometry, Bull. Am. Met. Soc., 64, 4-9, 1983.
Data can be downloaded in ACF format by following the ASCII link, or in binary netCDF file format by following the netCDF link