Update to Ancillary Ozone

The first step in atmospheric correction is to remove the effects of gaseous absoption due to passage through the ozone layer. The losses due to ozone absorption are modulated by ozone concentration. The OBPG uses data from Earth Probe Total Ozone Mapping Spectrometer (EP-TOMS) as the primary source for ozone concentration knowledge. Recently, the TOMS project updated the EP-TOMS ozone processing with improved algorithms and instrument calibration, calling the update version 8 (V8). The OBPG began using V8 TOMS in mid-2004, when the new products were first made available. Subsequently, the TOMS project has reprocessed all past data to V8, and the OBPG is taking this opportunity to update the existing V7 TOMS archive to V8. A synopsis of the differences between V7 and V8 is available here, and additional information about the use of ozone ancillary data by the OBPG is available here.

The effect of ozone absorption is greatest in the SeaWiFS 555nm band and the MODIS 551nm band, so any change in ozone inputs will impact the water-leaving radiance retrievals in those bands the most. A mission test was performed to determine the impact of the ozone change on SeaWiFS. The complete results are available here. Over most of the mission, the ozone change resulted in a mean reduction in nLw(555) radiances of a few percent, with much smaller changes in other bands. This can be seen in the ratio plot of global deep-water radiances, before/after the ozone update. The effect is more significant at higher latitudes, where the change in TOMS ozone was largest and the solar and sensor path length through the ozone layer is longer. The effect at 40S-50S in the Pacific is shown below.

The standard chlorophyll algorithm for SeaWiFS is based on a ratio to 555nm water-leaving reflectance, so the ozone change is directly reflected in the chlorophyll product, as seen in the following chlorophyll ratio trends.

The effect to MODIS/Aqua should be similar.