COMPARISONS OF OZONE MEASUREMENTS FROM THE MOZAIC AIRBORNE PROGRAM AND THE OZONE SOUNDING NETWORK AT 8 LOCATIONS

Automatic ozone measuring devices have been operating continuously on board the five long-range aircraft of the Measurement of Ozone and Wat er Vapor by Airbus In-Service Aircraft (MOZAIC) program since Septembe r 1994. This paper presents the main characteristics of the ozone syst em and the procedures followed to ensure its accurate calibration over long durations. Measurement accuracy was estimated at +/-[2 ppbv + 2% ], but much better in-flight levels were in fact observed: average dis crepancy (between different devices) ranging from 1 ppbv at tropospher ic concentrations to a few ppbv at stratospheric concentrations. This demonstrates the ability of the MOZAIC ozone data to produce accurate and reliable ozone climatologies. A 2-year ozone climatology (1994-199 6) generated from MOZAIC data collected at between 0 and 12 km altitud e was compared to longer and older measurements made at eight stations of the Ozone Sounding Network (OSN): Hohenpeissenberg, Wallops Island , Tateno, Palestine, Pretoria, Goose Bay, Biscarosse, and Poona. Despi te the different nature of the programs (techniques, platforms, sampli ng frequencies, spatial distribution, and operation periods), the OSN and MOZAIC climatologies were found to show a reasonably high level of agreement. Mean concentrations derived from ozone sondes are about 3 to 13% higher than those obtained by the MOZAIC program in the free tr oposphere, in a similar geographic location. These differences are wit hin the range of uncertainty of the two techniques. Larger discrepanci es observed in the boundary layer and in upper layers are explained by the influence of local pollution and the distance between measurement s, amongst other factors, limiting the reliability of comparisons. A c omparison of OSD and MOZAIC data at Hohenpeissenberg/Frankfurt and Wal lops Island/New York, over an overlapping period (1994-1995), shows go od agreement in the free troposphere (800-300 hPa), no detectable bias for Hohenpeissenberg/Frankfurt, when taking into consideration the va rious causes of discrepancies (Dobson normalization, ozone geographica l variations). Indeed, the results of this analysis support the hypoth esis that it is not advantageous to scale the ozone sonde data to the overhead ozone column; the scaling appears to cause overestimation of the tropospheric O-3 concentrations, by about 3-6% at Hohenpeissenberg , and to cause more scatter in the sonde-MOZAIC differences. The corre spondence between the OSN and MOZAIC climatologies obtained in very di fferent conditions demonstrates that they are representative of the at mosphere and that, being complementary while each retains its own adva ntages, they are therefore both useful for validation studies.