A CORRECTION FOR TOTAL OZONE MAPPING SPECTROMETER PROFILE SHAPE ERRORS AT HIGH-LATITUDE

The total ozone mapping spectrometer (TOMS) ozone measurement is deriv ed by comparing measured backscatter ultraviolet radiances with theore tical radiances computed using standard climatological ozone profiles. Profile shape errors occur in this algorithm at high optical path len gths whenever the actual vertical ozone distribution differs significa ntly from the standard profile used. These errors are estimated using radiative transfer calculations and measurements of the actual ozone p rofile. These estimated errors include a shortterm component resulting from day-to-day variability in profile shape that gives rise to a sta ndard deviation of 10% in total column ozone amount, as well as a syst ematic error in the long-term trend at very high solar zenith angles. The trend error resulting from the long-term changes in the ozone prof ile shape is estimated using measurements from the solar backscattered ultraviolet instrument. At the maximum retrieval solar zenith angle o f 88 degrees, these calculations indicate that TOMS long-term ozone de pletions may be overestimated by 5% per decade. For trend studies that are restricted to latitudes lower than 60 degrees (a maximum of 83 de grees solar zenith angle), this error is reduced to no more than 1-2% per decade. Differential impact of the profile shape error at the vari ous TOMS wavelength pairs indicates that profile shape information is present in the TOMS measurements at high solar zenith angles. An inter polation method internal to TOMS is proposed to extract this informati on. It improves the retrieval at high solar zenith angle, reducing the shortterm variability to a standard deviation of 5%, and essentially eliminates the long-term error. The set of standard profiles used in t he algorithm are adjusted based on an analysis of empirical orthogonal functions derived from a composite climatology of Stratospheric Aeros ol and Gas Experiment II and balloonsonde profile measurements.