OMI Algorithm Theoretical Basis Document Volume II - NASA's Earth ...

90 ATBD-OMI-02km. So far, there is no evidence that the profile algorithm will be able to do significantly betterbelow 20 km than the total O 3 -dependent climatology described in Chapter 2. Though it cancertainly do better at higher altitudes, the profile above 20 km, does not become important fortotal O 3 retrieval until the slant column density (SCD) of O 3 becomes quite large. For TOMS V8this happens when SCD>3000, which typically occurs at solar zenith angles >82°, affecting onlyfew percent of the OMI data. The situation should be roughly the same for the DOAS algorithm.Therefore, the OMI-derived O 3 profiles would be useful only at very large solar zenith angles tocorrect TOMS and DOAS total O 3 .6.3. Combining TOMS and DOAS algorithmsA key difference between the TOMS and the DOAS algorithms is that TOMS takesadvantage of the strong continuum absorption of O 3 in the Huggins band (310-340 nm), whilethe DOAS algorithm uses the much weaker band absorption. As shown in Fig 6-1, the totalabsorption of the radiation is more than an order of magnitude larger than the differentialabsorption near 331 nm. Therefore, in principle, an algorithm that takes advantage of the totalabsorption of O 3 near 310 nm should be able to measure total O 3 with far greater precision thanthe DOAS algorithms. There are several reasons why such an algorithm has not yet beenFigure 6-1:The upper panel shows change in absolute radiance produced by 1 DU increase in stratospheric O 3 at45° SZA, nadir view. TOMS total O 3 wavelength is shown by the asterisk. The lower panel showsdifferential absorption obtained by subtracting a quadratic polynomial from the upper curve. OMIDOAS algorithm uses wavelengths longer than 331 nm.developed. First, as noted before, removal of continuum absorption from the radiance spectrumalso removes the interfering effects of aerosols, clouds, sea-glint etc.; indeed one removes anysmoothly varying errors in radiative transfer calculations or in the instrument calibration. Thus,apart from errors that might be caused by the Ring Effect, the DOAS algorithm is largelyinsensitive to many forward model and instrument errors that have plagued previous algorithms,such as TOMS, that do not use differential absorption. In order to develop a total-absorptionalgorithm, the forward model must explicitly account for aerosols and clouds (i.e., by treatingthem as Mie scatterers rather than Lambertian reflectors), and the parameters that are needed torun such a model, cloud/aerosol optical depth, single-scattering albedo etc. must be suppliedexternally. As discussed in another ATBD, there are plans to calculate these parameters using theVersion 2 – August 2002