Several existing techniques for inferring column SO2 amounts from satellite UV measurements were developed for specific instruments and derived in rather different ways [Krueger 1983; Krueger et al 1995; Eisinger and Burows 1998; Krueger 2000]. In the current presentation we try to clarify connections between different inversion methods and unify the key ideas of these methods in a single flexible inversion procedure based on the principles of the maximum likelihood parameter estimation [Dubovik and King 2000]. The inversion procedure lays the foundation of the operational SO2 algorithm proposed for the Ozone Monitoring Instrument (OMI) on board NASA EOS-Aura (CHEM) satellite to be launched in 2003. The algorithm is based on the heritage of TOMS sulfur dioxide and ozone discrimination methods [Krueger et al 1995; Krueger et al. 2000], and is enhanced to take advantage of 1) the additional spectral information of OMI (additional wavelengths as well as spectral S/N characteristics), 2) available climatological data and, 3) other standard OMI products (ozone, aerosol, clouds, ring). Special emphasis is placed on the retrieval of small amounts of SO2 in the lower troposphere.
The OMI SO2 algorithm is based on detailed radiative transfer calculations used to build lookup tables of backscattered radiances in the 305 to 330nm spectral region at the OMI spectral resolution (0.45nm) as a function of O3 and SO2 vertical profiles and the conditions of the measurement: geometry, surface/cloud pressure, reflectivity and latitude. The inversion algorithm works with an arbitrary set of OMI wavelengths within 305-330nm fitting window, which can range from just five "TOMS-like" wavelengths to the full set of wavelengths at the highest OMI spectral resolution. Like in the TOMS algorithm (and OMI standard ozone algorithm), the clouds, aerosols and surface albedo variations within OMI FOV are treated implicitly in the forward model through the concept of Lambertian Equivalent Reflectivity (LER). The radiances and weighting functions to tropospheric SO2 are pre-calculated for two constant mixing ratio SO2 vertical profiles: a) from the ground to 900mbar and b) from the ground to 700mbar. The SO2 inversion strategy is based on the statistical approach given by Dubovik and King [2000]. The strategy is to consider OMI measurements together with a climatological data as a single set of multisource data. The inversion technique is designed as a search for the best overall fit of all data considered by our forward model (in a least-square sense) taking into account the different accuracy of the fitted (measured and a priori) data. The errors in OMI radiances measured by two different detectors (OMI "UV1" and "UV2") are considered as independent even for overlapping spectral regions. The choice of the numerical technique for minimization of the least square quadratic form is also discussed.
-
Dubovik, O. and M. King, A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements, J. Geophys. Res.,105, 20673-20696, 2000
-
Eisinger, M. and J.P. Burrows, Tropospheric sulfur dioxide observed by the ERS-2 GOME instrument, Geophys. Res. Lett., 25, 4177-4180, 1998.
-
Krueger. A.J., Sighting of El Chichon sulfur dioxide clouds with the Nimbus 7 total Ozone Mapping Spectrometer, Science, 220, 1377-1379, 1983
-
Krueger, A.J., L.S. Walter, P.K. Bhartia, C.C. Schnetzler, N. A. Krotkov, I. Sprod, and G.J.S. Bluth, Volcanic sulfur dioxide measurements from the Total Ozone Mapping Spectrometer instruments, J. Geophys. Res., 100,14057-14076, 1995
-
Krueger, A.J., S.J. Schaefer, N. Krotkov, G. Bluth, and S. Barker, Ultraviolet Remote Sensing of Volcanic Emissions, in Remote Sensing of Active Volcanism, ed by P. Mouginis Mark, J.A. Crisp, and J. H. Fink, Geophysical Monograph 116, American Geophysical Union, Washington, DC, 2000.
