Microtops II Hand-Held Sun Photometer Sun Pointing Error Correction for Sea Deployment

Hand held sun photometers, such as the Microtops II (manufactured by Solar Light, Inc.), provide simple and inexpensive means to measure in situ Aerosol Optical Thickness (AOT). Hand held sun photometers require that the user manually points the instrument at the sun. Unstable platforms, such as a ship at sea, can make this difficult, causing pointing errors. A poorly pointed instrument mistakenly records less than the full direct solar radiance, so the computed AOT is much higher than reality, and can be mistaken as cloud contamination or used incorrectly for validation with satellite derived AOT measurements. The relatively low sampling rate (3Hz) of the Microtops II leaves this instrument especially prone to this problem. Two steps were taken to reduce pointing errors. First, the measurement protocol was changed to keep the maximum (rather than average) value of a sequence of measurements. Several sets of these sequences are made for each intended data point. Once on shore, statistics are computed for each group of measurements. If the normalized variance of a group is above a threshold, the highest AOT measurement is discarded as a pointing error. The normalized variance is then recalculated. This is repeated until the normalized variance is reduced below the threshold or the number of points becomes too small to calculate variance. Several versions of this protocol were tested on a recent California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise, and a post processing algorithm was developed to remove pointing errors. These results were compared to concurrent measurements using the old protocol. Finally, a separate post processing algorithm was created for data already gathered with the old protocol, based upon statistics calculated by the instrument at the time of capture.