EVIDENCE OF A LONG-TERM INCREASE IN TROPOSPHERIC OZONE FROM PIC DU MIDI DATA SERIES - CONSEQUENCES - POSITIVE RADIATIVE FORCING

The rate at which ozone is increasing in the troposphere is uncertain due to the lack of accurate long-term measurements. Old ozone measurem ents obtained at the Pic du Midi Observatory (3000 m high, southwester n France) were recently rediscovered. Four sets of data available at t his station are presented herein: (1) 1874-1881 and (2) 1881-1909 by t he Schonbein method and (3) 1982-1984 and (4) 1990-1993 by UV absorpti on analyzers. The results show an increase in ozone by a factor of 5 s ince the beginning of the twentieth century, corresponding to an expon ential increase of 1.6% per year, although this trend is probably high er (2.4% per year) for the last few decades. A stable 10 ppb ozone mix ing ratio is observed during the first 20 years of the series, which i s representative of the preindustrial era ozone level. The increase is seen to start around 1895. Other data, obtained at various European h igh-altitude stations between 1920 and 1980, tie in closely with the P ic du Midi observations. A tentative evaluation of the impact of tropo spheric ozone on radiative forcing confirms that ozone is currently th e second most significant greenhouse gas, responsible for 22% and 13% of radiative forcing changes since 1800 in the northern and southern h emispheres, respectively. If these rates were to be maintained in the future, ozone would continue to evolve differently in the two hemisphe res (maximum level in the northern hemisphere) and could make an even more significant contribution to the radiative forcing of the northern hemisphere.