The temperature and pressure dependence of the absorption cross-sections of NO 2 in the 250–800 nm region measured by Fourier-transform spectroscopy

Abstract

The absorption cross-sections of NO 2 at atmospheric temperatures (223–293 K) and pressures (100 and 1000 mbar) were measured in the 250–800 nm (12500–40000 cm −1) region using Fourier-transform spectroscopy, at spectral resolutions of 0.5 cm −1 above 435 nm and 1.0 cm −1 below 435 nm (corresponding to about 8 and 16 pm at this wavelength). The wavenumber accuracy of the new cross-sections is better than 0.1 cm −1 (about 0.5 pm at 250 nm and about 6.4 pm at 800 nm), validated by recording of I 2 absorption spectra in the visible using the same experimental set-up (light source, beam splitter, interferometer optics). The NO 2 absorption spectra were recorded at five different sample temperatures between 223 and 293 K, and at each temperature at two total pressures (100 and 1000 mbar) using pure N 2 as buffer gas. Despite the weakness of this effect compared to the density of the NO 2 absorption structures, pressure-broadening was clearly observed at all temperatures. The pressure-broadening was partially modeled using a convolution of the low-pressure NO 2 absorption spectra with a Lorentzian lineshape. The pressure-broadening coefficient increases significantly with decreasing temperature, as already observed in the mid- and near-infrared vibration–rotation spectra of NO 2. This effect is of importance for high-resolution spectroscopy of the earth’s atmosphere in the UV–visible region.