An extended intercomparison of simultaneous ground-based Fourier transform infrared spectrometer measurements at the Toronto Atmospheric Observatory

Simultaneous measurements of O3, HCl, N2O, and CH4 were recorded by two infrared Fourier transform spectrometers of differing resolution (0.004 and 0.02cm-1) over a period of four months in the summer of 2005. These coincident observations were made at the Toronto Atmospheric Observatory, a complementary site of the Network for the Detection of Atmospheric Composition Change, and provide one of the longest records of simultaneously recorded ground-based infrared spectra to date. Retrievals performed on the spectra utilized the SFIT2 optimal estimation algorithm with HITRAN 2004 spectroscopic parameters. The influence of instrument resolution was considered in relation to the respective averaging kernels, with the predicted influence of multiplicative bias agreeing well with the observed influence for the stratospheric species. The retrieved column amounts correlated well for the stratospheric gases (R2>0.6) but poorer correlations were observed for the well-mixed tropospheric species that were investigated. The median column differences observed by the instruments are -1.7% and 2.7% in two different micro-windows of O3, 2.2% for HCl, -0.36% for N2O, and 3.7% for CH4.