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

Abstract

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%.