Investigating the long-term evolution of subtropical ozone profiles applying ground-based FTIR spectrometry

Abstract

Abstract. We investigate the long-term evolution of subtropical ozone profile time series (1999–2010) obtained from different ground-based FTIR (Fourier Transform InfraRed) retrieval setups. We examine the influence of an additional temperature retrieval and different constraints. The study is performed at the Izaña Observatory ozone super-site (Tenerife Island, Spain). The FTIR system is able to resolve four independent ozone layers with a theoretical precision of better than 7.5% in the troposphere, lower, middle and upper stratosphere. This total error includes the smoothing error, which is dominating the random error budget. Furthermore, our theoretical calculations indicate that a very precise knowledge of the instrumental line shape is mandatory for a precise ground-based FTIR remote sensing of stratospheric ozone. Likewise, we show that a simultaneous temperature retrieval is highly recommended. We empirically confirm our precision estimates by daily intercomparisons with Electro Chemical Cell (ECC) sonde profiles. Both FTIR and ECC sonde profile time series show similar seasonality: in winter stratospheric ozone profiles are typical middle latitude profiles (low tropopause, low ozone maximum concentrations) and in summer/autumn they are typical tropical profiles (high tropopause, high maximum concentrations). Good agreement is also observed for the linear trends estimated from the FTIR and the ECC datasets: a negative trend in the upper troposphere/lower stratosphere region of about −0.3% yr−1 and a positive trend in the middle/upper stratosphere of about +0.3% yr−1. Admittedly, a 12-yr time series is too short for reliable trend studies, however, it is worthwhile mentioning that such subtropical ozone profile trends are predicted by climate models due to an increased stratospheric circulation in response to climate change.