Ship Based Sun Photometer Measurements

As part of previous Simbios ocean color efforts we employed a hand held sun photometer to measure aerosol optical depths on ship cruises north of Oahu, Hawaii (the Hawaii Ocean Time Series,  HOTS). Due to the difficulty of manually pointing a narrow field of view sensor at the sun on a moving platform we developed a simple approach to reduce (but not really eliminate) the number of bad misaligned measurements (Porter et al., 2001). This involved a somewhat subjective method of removing the larger values which can occur if the photometer is not correctly pointed at the sun. This approach was later adopted by the NASA SIMBIOS program as a way to process hand held sun photometer measurements carried out on ships.

The problem is illustrated in the following figure. On the first day the ocean was calm and the majority of the hand held sun photometer measurements form a reasonably tight cluster. On subsequent days the winds picked up and the ocean grew rougher. As this occured it became more difficult to point the photometer at the sun and the number of misaligned values (larger points) increased.