OClO slant column densities derived from GOMOS averaged transmittance measurements

[摘要] The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard the European platform ENVISAT (ENVironment SATellite) was dedicated to the study of theof Earth's atmosphere using the stellar occultation technique. The spectralrange of the GOMOS spectrometer extends from the UV (ultra violet) to the near infrared,allowing for the retrieval of species such as O₃, NO₂, NO₃, H₂O,O₂, air density, aerosol extinction and OClO. Nevertheless, OClO cannotbe retrieved using a single GOMOS measurement because of the weaksignal-to-noise ratio and the small optical thickness associated with thismolecule. We present here the method used to detect this molecule by usingseveral GOMOS measurements. It is based on a two-step approach. First,several co-located measurements are combined in a statistical way to build anaveraged measurement with a higher signal-to-noise ratio; then, adifferential optical absorption spectroscopy (DOAS) method is applied toretrieve OClO slant column densities (SCD). The statistics of the sets ofGOMOS measurements used to build the averaged measurement and the spectralwindow selection are analyzed. The obtained retrievals are compared toresults from two balloon-borne instruments. It appears that theinter-comparisons of OClO are generally satisfying (relative differences areabout 15–60%). Two nighttime climatologies of OClO based onGOMOS averaged measurements are presented. The first depicts annual globalpictures of OClO from 2003 to 2011. From this climatology, the presence of anOClO SCD peak in the equatorial region at about 35 km is confirmed andstrong OClO SCD in both polar regions are observed (more than10¹⁶ cm⁻² in the Antarctic region and slightly less in theArctic region), a sign of chlorine activation. The second climatology is amonthly time series. It clearly shows the chlorine activation of the lowerstratosphere during winter. Moreover the equatorial OClO SCD peak is observedduring all years without any significant variations. This very promisingmethod, applied on GOMOS measurements, allowed us to build the firstnighttime climatology of OClO using limb-viewing instruments.

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