Bernard Coquart

Measurements of the NO2 absorption cross-section from 42 000 cm−1 to 10 000 cm−1 (238–1000 nm) at 220 K and 294 K

Abstract The NO2 absorption cross-section has been measured from 42 000 to 10 000 cm−1 (238–1000 nm) with a Fourier transform spectrometer (at the resolution of 2 cm−1, 0.01 nm at 240 nm to 0.2 nm at 1000 nm) and a 5 m temperature controlled multiple reflection cell. The uncertainty on the cross-section is estimated to be less than 3% below 40 000 cm−1 (λ > 250 nm) at 294 K, 3% below 30 000 cm−1 (λ > 333 nm) at 220 K, but reaches 10% for higher wavenumbers. Temperature and pressure effects have been observed. Comparison with data from the literature generally shows a good agreement for wavenumbers between 37 500 and 20 000 cm−1 (267–500 nm). Outside these limits, the difference can reach several percent.

Absorption cross-sections of atmospheric constituents: NO2, O2, and H2O

Absorption spectroscopy, which is widely used for concentration measurements of tropospheric and stratospheric compounds, requires precise values of the absorption cross-sections of the measured species. NO2, O2 and its collision-induced absorption spectrum, and H2O absorption cross-sections have been measured at temperature and pressure conditions prevailing in the Earth’s atmosphere. Corrections to the generally accepted analysis procedures used to resolve the convolution problem are also proposed.

The NO2 absorption spectrum. I: Absorption cross-sections at ambient temperature in the 300–500 nm region

New laboratory measurements of NO2 absorption cross-sections have been performed between 300 and 500 nm at ambient temperature with improved experimental conditions: low gas pressures, long absorption paths, suitable absorbance values, narrow spectral bandwidths. The data, stored at 0.01 nm intervals, have been compared to those of the more recent studies and some reasons of disagreement are discussed.In the photolysis region below 400 nm, our absorption cross-sections are larger than those previously published, suggesting that the photodissociation coefficient calculated from the current data sets is underestimated. In the structured region of the spectrum above 400 nm, improvement of the resolution gives more precise values useful for optical measurements in atmosphere.

The NO2 absorption spectrum. II. Absorption cross-sections at low temperatures in the 400–500 nm region

With improved experimental conditions already used for measurements at ambient temperature (Mérienneet al., 1994), new values have been found for the absorption cross-sections of NO2 at 240 and 220 K in the 400–500 nm spectral region. Using a better resolution than in previous studies we show that the temperature effect is not negligible and should be taken into account for the optical measurements of atmospheric NO2 amounts by differential absorption methods.

Absorption Cross-section of the Collision-Induced Bands of Oxygen from the UV to the NIR

Collision-induced absorption (CIA) cross-sections of oxygen have been measured in the UV, Visible and near-IR regions from spectra recorded by Fourier Transform Spectroscopy at different pressures and room temperature. An extensive cross-sections dataset from 42000 to 7500 cm−1 (238–1330 nm) is presented. The separation procedure of the discrete and diffuse absorption features is described. Pressure and foreign gas effects are discussed, and a comparison with literature data is shown. A preliminary test on the influence of the choice of the dataset on atmospheric retrievals of O2, O3, BrO, and OClO is performed.

Fourier Transform Spectroscopy of the O2 Herzberg Bands

The absorption spectra of the O2 Herzberg band systems ( A S u –X S g , c S u –X S g , and A9 D u–X S g ) lying in the wavelength region 240–300 nm were reinvestigated. The coupling of a long absorption cell and a high-resolution Fourier transform spectrometer has allowed the observation of numerous weak lines which were not reported previously. From the rotational analysis of the line positions, determined with an accuracy of 0.005 cm, the molecular constants of the A S u , v 5 0–12, c S u , v 5 2–19, and A9 D u, v 5 2–12 levels are improved significantly. The interaction between the A and c states is described quantitatively. A new interpretation of the perturbations observed in the energy region close to the dissociation limit is given which involves a weakly bound P u state as the most probable perturbing state.

Fourier Transform Spectroscopy of the O2 Herzberg Bands: I. Rotational Analysis

The absorption spectra of the O2 Herzberg band systems ( A S u –X S g , c S u –X S g , and A9 D u–X S g ) lying in the wavelength region 240–300 nm were reinvestigated. The coupling of a long absorption cell and a high-resolution Fourier transform spectrometer has allowed the observation of numerous weak lines which were not reported previously. From the rotational analysis of the line positions, determined with an accuracy of 0.005 cm, the molecular constants of the A S u , v 5 0–12, c S u , v 5 2–19, and A9 D u, v 5 2–12 levels are improved significantly. The interaction between the A and c states is described quantitatively. A new interpretation of the perturbations observed in the energy region close to the dissociation limit is given which involves a weakly bound P u state as the most probable perturbing state.

Long pathlength measurements of oxygen absorption cross sections in the wavelength region 205–240 nm

New measurements of molecular oxygen photoabsorption cross sections and pressure coefficients have been made at room temperature in the wavelength region 205–240 nm of the Herzberg continuum. The experimental conditions [p(O2) < 100 Torr, pathlenths up to 1400 m] ensure accurate extrapolation to zero pressure of the pressure -dependent continuum cross sections. The results are in good agreement with recent laboratory data and are consistent with most of the stratospheric measurements. The O2 cross sections are found to be lower than values previously used in many photochemical stratospheric modelling calculations.

The NO2 Absorption Spectrum. IV: The 200–400 nm Region at 220 K

Previous experiments in the 400–500 nm region (Coquart et al., 1995) have been extended to the 200–400 nm region to determine the absorption cross-sections of NO2 at 220 K. The NO2 and N2O4 cross-sections are obtained simultaneously from a calculation applied to the data resulting from measurements at low pressures. A comparison between the NO2 cross-sections at 220 K and at ambient temperature shows that the low temperature cross-sections are generally lower, except in the region of the absorption peaks. Comparisons are also made with previous data at temperature close to 220 K.

The NO2 absorption spectrum. III : The 200-300 nm region at ambient temperature

Absorption cross-section measurements of NO2 performed in our laboratory have been extended to the 200–300 nm region at ambient temperature. Low pressures have been used, limiting the effects of the dimer N2O4 which has an absorption cross-section from one to two orders of magnitude larger than that of NO2 in this region. The results have been compared to those of previous authors and are now available for atmospheric purposes at 0.01 nm intervals.