Abstract

Abstract. We have studied data from two satellite occultation instruments in order to generate a high vertical resolution homogeneous ozone time series of 26 yr. The Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation instrument and the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument measured ozone profiles in the stratosphere and mesosphere from 1984–2005 and 2002–2012, respectively. Global coverage, good vertical resolution, and the self-calibrating measurement method make data from these instruments valuable for the detection of changes in vertical distribution of ozone over time. As both instruments share a common measurement period from 2002–2005, it is possible to inter-calibrate the data sets. We investigate how well these measurements agree with each other and combine all the data to produce a new stratospheric ozone profile data set. Above 55 km, SAGE II measurements show much less ozone than the GOMOS nighttime measurements as a consequence of the well-known diurnal variation of ozone in the mesosphere. Between 35–55 km, SAGE II sunrise and sunset measurements differ from GOMOS' measurements to different extents. Sunrise measurements show 2% less ozone than GOMOS, whereas sunset measurements show 4% more ozone than GOMOS. Differences can be explained qualitatively by the diurnal variation of ozone in the stratosphere recently observed by SMILES and modeled by chemical transport models. Between 25–35 km, SAGE II sunrise and sunset measurements and GOMOS measurements agree within 1%. The observed ozone bias between collocated measurements of SAGE II sunrise/sunset and GOMOS night measurements is used to align the two data sets. The combined data set covers the time period 1984–2011, latitudes 60° S–60° N, and the altitude range of 20–60 km. Profile data are given on a 1 km vertical grid, and with a resolution of 1 month in time and 10° in latitude. The combined ozone data set is analyzed by fitting a time series model to the data. We assume a linear trend with an inflection point (so-called "hockey stick" form). The best estimate for the point of inflection was found to be the year 1997 for ozone between altitudes 35 and 45 km. At all latitudes and altitudes from 35 to 50 km we find a clear change in ozone trend before and after the inflection time. From 38 to 45 km, a negative trend of 4% per decade (statistically significant at 95% level) at the equator has changed to a small positive trend of 0–2% per decade. At mid-latitudes, the negative trend of 4–8% per decade has changed to to a small positive trend of 0–2% per decade. At mid-latitudes near 20 km, the ozone loss has still increased whereas in the tropics a recovery is ongoing.

Highlights

  • The stratospheric ozone decline, especially the drastic decrease of ozone over the Antarctic, has been a focus of middle atmosphere research during the past 25 yr

  • Between 25–35 km, Stratospheric Aerosol and Gas Experiment (SAGE) II sunrise and sunset measurements and Global Ozone Monitoring by Occultation of Stars (GOMOS) measurements agree within 1 %

  • We have created a homogenized ozone profile data set from SAGE II and GOMOS measurements for the period 1984– 2011 (Data are available from http://igaco-o3.fmi.fi/VDO/ index.html)

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Summary

Introduction

The stratospheric ozone decline, especially the drastic decrease of ozone over the Antarctic, has been a focus of middle atmosphere research during the past 25 yr (for reviews, see Solomon, 1999; Staehelin et al, 2001; WMO, 2011). The differences between the SAGE II and GOMOS ozone profiles are studied in Sect. Due to the short measurement series, no conclusion can be made at this time These studies have used the previous data versions of SAGE II and GOMOS. The final number of profiles used in this study is about 215 000 (in 60◦ S–60◦ N) Both SAGE II and GOMOS took measurements during the period 2002 to 2005. The large number of measurements allows for a direct comparison between SAGE II and GOMOS ozone profiles. That the atmospheric transmission is different in nighttime, sunset, and sunrise measurements because NO2 has a strong diurnal variation This was clearly seen when we compared GOMOS and SAGE II transmissions at collocation points. The vertical grid is 20– 60 km with a 1 km step

Common time–latitude grid
Removal of bias
The combined data set
Findings
Conclusions
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