Sun-Pointing-Error Correction for Sea Deployment of the MICROTOPS II Handheld Sun Photometer

Abstract

Handheld sun photometers, such as the MICROTOPS II (manufactured by Solar Light, Inc.), provide a simple and inexpensive way to measure in situ aerosol optical thickness (AOT), ozone content, and water vapor. Handheld sun photometers require that the user manually point the instrument at the sun. Unstable platforms, such as a ship at sea, can make this difficult. A poorly pointed instrument mistakenly records less than the full direct solar radiance, so the computed AOT is much higher than reality. The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has been collecting maritime AOT data since 1997. As the dataset grew, a bias of the MICROTOPS II data with respect to other instrument data was noticed. This bias was attributed to the MICROTOPS II measurement protocol, which is intended for land-based measurements and does not remove pointing errors when used at sea. Based upon suggestions in previous literature, two steps were taken to reduce pointing errors. First, the measurement protocol was changed to keep the maximum (rather than average) voltage of a sequence of measurements. Once on shore, a second screening algorithm was utilized to iteratively reject outliers that represent sun-pointing errors. Several versions of this method were tested on a recent California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise, and were compared to concurrent measurements using the manufacturer-supplied protocol. Finally, a separate postprocessing algorithm was created for data previously gathered with the manufacturer-supplied protocol, based on statistics calculated by the instrument at the time of capture.