Ground-based observations of the slowdown in ozone decline and onset of ozone increase

This paper presents the evidence for a slowdown in ozone decline and subsequent ozone increase using ground-based data, i.e., Dobson, Brewer, and Russian filter ozonometer total-ozone data and Umkehr and ozonesonde-derived layer-ozone data. The impacts of quasi-biennial oscillation (QBO) and 11-year solar cycle were minimized by determining 5-year trends based on 11-year running means. On the basis of 50-60 total-ozone stations, the global trends vary from a maximum negative value of -2.1 +/- 0.6%/decade in 1988 to a value of 0.7 +/- 0.5%/decade at the end of the record in 2000, where the confidence intervals are 2 standard deviations of the mean of the individual station trends. Because of the use of 5-year trends and 11-year running means, the actual year of slowdown in total-ozone decline may be up to 7 years later than the year of maximum negative trend in Our analysis, or close to the 1993-1995 peak, in ozone-depleting substances (ODS) in the atmosphere. Umkehr and ozonesonde-derived layer-ozone trends were determined for 32- to 53-, 24- to 32-, 19- to 24-, 10- to 19-, and 0- to 10-km layers of the north temperate zone. On the basis of four Umkehr stations and about five ozonesonde stations, the integrated layer-weighted Umkehr trends vary from a maximum negative value of -3.8 +/- 0.3%/decade in 1989 to a value of 1.2 +/- 2.1%/decade in 2000, while the integrated sonde trends vary from a maximum negative value of -4.8 +/- 1.6%/decade in 1989 to a value of 1.8 +/- 2.1/decade in 2000. Both Umkehr and sonde data show that nearly half of the increase in north temperate total-ozone trend between 1989 and 2000 is due to trend increase in the low-stratospheric 10- to 19-km layer, with the troposphere contributing only about 5%.