Attribution of stratospheric ozone trends

Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract The decrease of the concentration of ozone depleting substances (ODS) in the stratosphere over the past decade raises the question to what extent observed changes in stratospheric ozone over this period are consistent with known changes in chemical composition and possible changes in atmospheric transport. Here we present a series 5 of ozone sensitivity calculations with a stratospheric chemistry transport model (CTM) driven with meteorological reanalyses from the European Centre for Medium Range Weather Forecast, covering the period 1978–2009. In order to account for the reversal in ODS trends, ozone trends are analysed in two periods, 1979–1999 and 2000–2009. Effects of ODS changes on the ozone chemistry are either accounted for or left out, 10 allowing for a distinct attribution of ozone trends to the different factors of variability, namely ODS acting via gas phase chemistry, ODS acting via polar heterogeneous chemistry, and changes in transport and temperature. Modeled column ozone trends are in excellent agreement with observed trends from the Total Ozone Mapping Spectrometer (TOMS) and Solar Backscatter UV (SBUV/2) as well as the Global Ozone 15 Monitoring Experiment (GOME/GOME2) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments. For the 1979–1999 period we find that changes in ODS are the dominant source of the ozone trend, while changes in transport also contribute signifcantly to the overall trend. In contrast, for the period 2000–2009 the effect of ODS changes on total ozone is small. Observed ozone 20 changes can be reproduced well with the CTM driven with meteorological reanalyses, indicating that the observed evolution of ozone over the past decade is consistent with our current understanding of chemistry and transport.