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Abstract
Abstract. The ozone radiometer GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) has been performing continuous observations of stratospheric ozone profiles since 1994 above Bern, Switzerland (46.95° N, 7.44° E, 577 m). GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). From November 1994 to October 2011, the ozone line spectra were measured by a filter bench (FB). In July 2009, a fast Fourier transform spectrometer (FFTS) was added as a back end to GROMOS. The new FFTS and the original FB measured in parallel for over 2 years. The ozone profiles retrieved separately from the ozone line spectra of FB and FFTS agree within 5 % at pressure levels from 30 to 0.5 hPa, from October 2009 to August 2011. A careful harmonisation of both time series has been carried out by taking the FFTS as the reference instrument for the FB. This enables us to assess the long-term trend derived from stratospheric ozone observations at Bern. The trend analysis was performed by using a robust multilinear parametric trend model which includes a linear term, the solar variability, the El Niño–Southern Oscillation (ENSO) index, the quasi-biennial oscillation (QBO), the annual and semi-annual oscillation and several harmonics with period lengths between 3 and 24 months. Over the last years, some experimental and modelling trend studies have shown that the stratospheric ozone trend is levelling off or even turning positive. With our observed ozone profiles, we are able to support this statement by reporting a statistically significant trend of +3.14 % decade−1 at 4.36 hPa (37.76 km), covering the period from January 1997 to January 2015, above Bern. Additionally, we have estimated a negative trend over this period of −3.94 % decade−1 at 0.2 hPa (59 km).
Highlights
For many decades it has been known that the stratospheric ozone layer shields the Earth’s surface from harmful solar ultraviolet radiation (UV), enabling life on Earth and protecting humans and the biosphere against adverse effects. Molina and Rowland (1974) were the first to propose that this protective layer could be depleted by anthropogenic emission of chlorofluorocarbons (CFCs) to the atmosphere
The selection of the time interval is based on the assumption that 1997 is the turnaround year of the equivalent effective stratospheric chlorine (EESC), since EESC concentration peaked in 1997 at midlatitudes (WMO, 2011)
From November 1994 to October 2011, the ozone line spectra were measured by a filter bench spectrometer
Summary
For many decades it has been known that the stratospheric ozone layer shields the Earth’s surface from harmful solar ultraviolet radiation (UV), enabling life on Earth and protecting humans and the biosphere against adverse effects. Molina and Rowland (1974) were the first to propose that this protective layer could be depleted by anthropogenic emission of chlorofluorocarbons (CFCs) to the atmosphere. For many decades it has been known that the stratospheric ozone layer shields the Earth’s surface from harmful solar ultraviolet radiation (UV), enabling life on Earth and protecting humans and the biosphere against adverse effects. The photodecomposition of CFCs and other long-lived organic molecules in the stratosphere releases chlorine (Cl) and bromine (Br) atoms that destroy ozone molecules in catalytic cycles. In the last Scientific Assessment of Ozone Depletion: 2014 of the World Meteorological Organization (WMO, 2014) it is stated that global ozone levels decreased through the 1980s and early 1990s while stratospheric abundances of ozone-depleting substances (ODSs) were increasing. In 1985, massive ozone losses in measured column abundances during the Antarctic spring were reported and heterogeneous chlorine chemistry on polar stratospheric clouds (PSCs) were implicated for the loss (WMO, 2011). Actions taken under the Montreal Protocol have led to decreases
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