The long‐term evolution of upper stratospheric ozone has been recorded by lidars and microwave radiometers within the ground‐based Network for the Detection of Stratospheric Change (NDSC), and by the space‐borne Solar Backscatter Ultra‐Violet instruments (SBUV), Stratospheric Aerosol and Gas Experiment (SAGE), and Halogen Occultation Experiment (HALOE). Climatological mean differences between these instruments are typically smaller than 5% between 25 and 50 km. Ozone anomaly time series from all instruments, averaged from 35 to 45 km altitude, track each other very well and typically agree within 3 to 5%. SBUV seems to have a slight positive drift against the other instruments. The corresponding 1979 to 1999 period from a transient simulation by the fully coupled MAECHAM4‐CHEM chemistry climate model reproduces many features of the observed anomalies. However, in the upper stratosphere the model shows too low ozone values and too negative ozone trends, probably due to an underestimation of methane and a consequent overestimation of ClO. The combination of all observational data sets provides a very consistent picture, with a long‐term stability of 2% or better. Upper stratospheric ozone shows three main features: (1) a decline by 10 to 15% since 1980, due to chemical destruction by chlorine; (2) two to three year fluctuations by 5 to 10%, due to the Quasi‐Biennial Oscillation (QBO); (3) an 11‐year oscillation by about 5%, due to the 11‐year solar cycle. The 1979 to 1997 ozone trends are larger at the southern mid‐latitude station Lauder (45°S), reaching −8%/decade, compared to only about −6%/decade at Table Mountain (35°N), Haute Provence/Bordeaux (≈45°N), and Hohenpeissenberg/Bern(≈47°N). At Lauder, Hawaii (20°N), Table Mountain, and Haute Provence, ozone residuals after subtraction of QBO‐ and solar cycle effects have levelled off in recent years, or are even increasing. Assuming a turning point in January 1997, the change of trend is largest at southern mid‐latitude Lauder, +11%/decade, compared to +7%/decade at northern mid‐latitudes. This points to a beginning recovery of upper stratospheric ozone. However, chlorine levels are still very high and ozone will remain vulnerable. At this point the most northerly mid‐latitude station, Hohenpeissenberg/Bern differs from the other stations, and shows much less clear evidence for a beginning recovery, with a change of trend in 1997 by only +3%/decade. In fact, record low upper stratospheric ozone values were observed at Hohenpeissenberg/Bern, and to a lesser degree at Table Mountain and Haute Provence, in the winters 2003/2004 and 2004/2005.

In March 2002 the European Space Agency (ESA) launched the polar‐orbiting environmental satellite Envisat. One of its nine instruments is the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument, which is a medium‐resolution stellar occultation spectrometer measuring vertical profiles of ozone. In the first year after launch a large group of scientists performed additional measurements and validation activities to assess the quality of Envisat observations. In this paper, we present validation results of GOMOS ozone profiles from comparisons to microwave radiometer, balloon ozonesonde, and lidar measurements worldwide. Thirty‐one instruments/launch sites at twenty‐five stations ranging from the Arctic to the Antarctic joined in this activity. We identified 6747 collocated observations that were performed within an 800‐km radius and a maximum 20‐hour time difference of a satellite observation, for the period between 1 July 2002 and 1 April 2003. The GOMOS data analyzed here have been generated with a prototype processor that corresponds to version 4.02 of the operational GOMOS processor. The GOMOS data initially contained many obviously unrealistic values, most of which were successfully removed by imposing data quality criteria. Analyzing the effect of these criteria indicated, among other things, that for some specific stars, only less than 10% of their occultations yield an acceptable profile. The total number of useful collocated observations was reduced to 2502 because of GOMOS data unavailability, the imposed data quality criteria, and lack of altitude overlap. These collocated profiles were compared, and the results were analyzed for possible dependencies on several geophysical (e.g., latitude) and GOMOS observational (e.g., star characteristics) parameters. We find that GOMOS data quality is strongly dependent on the illumination of the limb through which the star is observed. Data measured under bright limb conditions, and to a certain extent also in twilight limb, should be used with caution, as their usability is doubtful. In dark limb the GOMOS data agree very well with the correlative data, and between 14‐ and 64‐km altitude their differences only show a small (2.5–7.5%) insignificant negative bias with a standard deviation of 11–16% (19–63 km). This conclusion was demonstrated to be independent of the star temperature and magnitude and the latitudinal region of the GOMOS observation, with the exception of a slightly larger bias in the polar regions at altitudes between 35 and 45 km.

[1] One of the difficulties arising when intercomparing independent measurements of atmospheric constituent profiles consists in homogenizing their respective profile coordinates in order to allow quantitative comparison results. Special care should be devoted in particular to the homogenization of remote sensor measurements, those being indeed intricately bound to their respective numerical grids through discretization rules implied for the evaluation of the retrieval algorithms. Recently, a method of intercomparing remote sounders while allowing for different observational characteristics was proposed by Rodgers and Connor (2003). However, at the time of publication, application of this method was restricted to comparisons of identical state vectors. We propose to relax this condition by the use of linear transformation functions to homogenize the products of independent retrievals. We combine this technique with the Rodgers and Connor procedure to compare independent ozone profile measurements by the Global Ozone Monitoring Experiment (GOME) and a ground-based microwave radiometer (MW). Verification of the achieved results is obtained by considering a second series of MW retrievals, evaluated directly on the GOME numerical grid.

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