Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-5183-2005 http://www.atmos-chem-phys-discuss.net/5/5183/2005/ http://www.atmos-chem-phys-discuss.net/5/5183/2005/acpd-5-5183-2005.html http://www.atmos-chem-phys-discuss.net/5/5183/2005/acpd-5-5183-2005.pdf 5183 5221 2005-07-22 Effects of column density on I₂ spectroscopy and a determination of I₂ absorption cross section at 500 nm P. Spietz J. C. Gómez Martín J. P. Burrows Institute of Environmental Physics (IUP), University of Bremen, PO Box 330440, 28334 Bremen, Germany The use of ro-vibronic spectra of I₂ in the region of 543 nm to 578 nm as reference spectra for atmospheric Differential Optical Absorption Spectroscopy is studied. In this study it is shown that the retrieval of atmospheric column densities with Differential Optical Absorption Spectroscopy set-ups at FWHM at and above 1nm depends critically on the column density, under which the used reference spectrum was recorded. Systematic overestimation of the comparatively low atmospheric column density of I₂ of the order of 13% is possible. Under low pressure conditions relevant in laboratory studies, the systematic deviations may grow up to 45%. To avoid such effects with respect to field measurements, new reference spectra of I₂ were determined under column density of the order of 10¹⁶ molec/cm² close to that expected for the atmospheric measurement. Thereby the described systematic deviations are avoided. Two typical configurations of Differential Optical Absorption Spectroscopy, which use grating spectrometers, were chosen for the spectroscopic set-up. One spectrum was recorded at similar resolution (0.25 nm FWHM) but finer binning (0.035 nm/pixel) than previously published data. For the other (0.59 nm FWHM, 0.154 nm/pixel) no previously published spectra exist. Wavelength calibration is accurate to ±0.04 nm and ±0.11 nm respectively. The absorption cross section for the recordings was determined under low column density with an accuracy of ±4% and ±3% respectively. The absolute absorption cross section of I₂ at 500 nm (in standard air) in the continuum absorption region was determined using a method independent of iodine vapour pressure. Obtained was σ_I₂(500 nm)=(2.186±0.021)·10^−18 cm²·molec^−1 in very good agreement with previously published results, but at 50% smaller uncertainty. From this and previously published results a weighted average of σ_I₂(500 nm)=(2.191±0.02) ·10^−18 cm²·molec^−1 is determined.