Measurements of stratospheric ozone and nitrogen dioxide at Èvora, Portugal

Abstract

The Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) has been developed as a result of collaboration between the Geophysics Centre of Évora University (CGE-UE), the Institute for Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR) in Italy and the Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multipurpose ultraviolet (UV)–visible scanning spectrometer (250–950 nm). It has been installed at the Observatory of the CGE, in Évora, since April 2004 and is currently used to carry out measurements of the zenith scattered radiation, the so-called ‘Passive mode’, to retrieve the vertical content and distribution of some atmospheric tracers such as ozone (O₃) and nitrogen dioxide (NO₂) using Differential Optical Absorption Spectroscopy (DOAS) methodology. The lack of such measurements taken automatically on a routine basis in southwestern European regions, specifically in Portugal, motivated the effort for its installation and constitutes a major driving force for the present work. For continuous NO₂ monitoring the 425–455 nm spectral range is investigated. For O₃ retrieval the spectral interval 320–340 nm is chosen. The measurements are in good agreement with the photochemical theory of NO₂ (O₃), showing maximum values during the summer (spring) and minimum values during the winter (autumn) seasons. Moreover, the application of sophisticated inversion schemes to the output of the DOAS program, using the Air Mass Factor (AMF) matrix as the kernel of the inversion algorithm, allowed for the determination of the vertical distribution of NO₂ and O₃ atmospheric compounds. In addition, the influence of desert dust aerosol absorption on ozone retrieval is assessed, revealing values of about 3.5% for an aerosol optical depth (AOD) of 1.0, in the case simulated. A correction factor is derived and applied whenever desert dust is detected. The ground-based results obtained for the ozone column content are compared with data from the satellite-borne Ozone Monitoring Instrument (OMI), and the two data sets are found to be in good agreement, with a correlation coefficient of 0.96.

Acknowledgements

We thank the reviewer for helpful comments and suggestions that have improved the paper considerably. This work was funded by the Portuguese Science Foundation (FCT) through projects PDCTE/CTA/49828/2003 and POCI/AMB/59774/2004. D.B. was supported by the FCT through grant SFRH/BPD/22160/2005. A.F.D. was in receipt of a grant from the FCT project POCI/AMB/59774/2004.