NO2 Corrections for Reprocessing 2009

Corrections were added to account for absorption by nitrogen dioxide (NO2), using the approach described in Ahmad et al. 2007 and a time-series of tropospheric and stratopheric NO2 data from ancillary sources.

NO2 Data Source

The OBPG has been working to produce a continuouse NO2 data set spanning the temporal range of satellite ocean color measurements (at least back to the OCTS mission). To that end, NO2 data from GOME, SCIAMACHY, and OMI were combined and adjusted, as described in Robinson et al. to create a consistent ancillary NO2 dataset of the required temporal extent at the highest temporal resolution possible. In addition, a monthly climatology was also constructed based on the multi-sensor record. Given the difficulties in constructing and maintaining the daily NO2 dataset, it was decided that the monthly climatology will be utilized for this reprocessing. However, the daily NO2 will be produced and made available for community evaluation and regional processing.

Method and Results

The NO2 correction approach is full described in Ahmad et al. 2007. Results generally show a reduction in the occurrance of very low or negative water-leaving radiances in blue wavelengths, in regions of persistently elevated tropospheric NO2 concentrations, with mean regionalized increases at 412nm as large as 10%. Significant effects can also be seen at higher latitudes, due to the large slant-path of the radiance through the stratospheric NO2. The impact on the global deep-water mean water-leaving radiances, however, is negligible. See Figures 1-3.



Figure 1: Ratio of MODIS/Aqua water-leaving radiance retrievals, before and after NO2 corrections, for a global subset consisting of all waters deeper than 1000m.



Figure 2: Ratio of MODIS/Aqua water-leaving radiance retrievals, before and after NO2 corrections, for a global subset of coastal (typically eutrophic) waters.



Figure 3: Ratio of MODIS/Aqua water-leaving radiance retrievals, before and after NO2 corrections, for a region of the North Pacific between 40 and 50-deg latitude.