SAOZ measurements of NO2 at Aberystwyth

Abstract

We present in this paper fifteen years’ measurements, from March 1991 to September 2005, of stratospheric NO₂ vertical columns measured by a SAOZ zenith-sky visible spectrometer. The instrument spent most of its time at Aberystwyth, Wales, with occasional excursions to other locations. The data have been analysed with the WinDOAS analysis program with low-temperature high-resolution NO₂ cross-sections and fitting a slit function to each spectrum. Because of a change in detector in May 1998 there is some uncertainty about the relative changes before and after this date, which are partially constrained by the results of an intercomparison exercise. However, the effect of the Mt Pinatubo aerosol cloud is very evident in the data from 1991–94, with a decrease of 10% in NO₂ in the summer of 1992 (the SAOZ was located in Lerwick, Scotland during the winter of 1991–92 and observed very low NO₂ values but these cannot be directly compared to the Aberystwyth data). To focus more on interannual and long-term variations in NO₂, a seasonal variation comprising an annual and semi-annual component was fitted to the morning and evening twilight separately from 1995 to the present. This fit yielded average NO₂ columns of 4.08 × 10¹⁵ cm⁻² and 2.68 × 10¹⁵ cm⁻² for the evening and morning twilight, respectively, with a corresponding annual amplitude of ±2.08 × 10¹⁵ cm⁻² and ±1.50 × 10¹⁵ cm⁻². Departures from the fitted curve show a trend of 6% per decade, consistent with that reported elsewhere, for the period 1998–2003, but in the past two years a distinct interannual variation of amplitude of ∼8% has emerged.