Knowledge Bank

Up to now, infrared atmospheric $NO2$ measurements have mainly been performed in the 6.2$\mu$m region from aircraft or balloon. Indeed this spectral region covers the strongest infrared band of $NO2$ which is $\nu 3$. However, it is not possible for measuring $NO2$ from the ground to work in this spectral region because the $NO2$ absorption is hindered by the strong absorbing $\nu 2$ band of water vapor. The second strongest absorbing band of $NO2$ which is $\nu 1+\nu 3$ falls in the 3.4$\mu$m region which is a relatively clear atmospheric window. However, since $\nu 1+\nu 3$ is about 20 times weaker than $\nu 3$ , it is necessary to have the best knowledge of the line parameters of the $\nu 1+\nu 3$ and $\nu 1+\nu 2+\nu 3-\nu 2$ bands of $NO2$ to be able to recognize unambiguously the weak absorption peaks of this molecule in atmospheric spectra. Using an hamiltonian which takes explicitly into account the Coriolis interaction between the $(v1v2v3)$ and the $(v1v2+2v3-1)$ viorational states of $NO2$, it has been possible to reproduce very satisfactorily the rotational levels of the interacting states (1 2 0),(1 0 1) and (1 3 0),(1 1 1). Also a least squares fit of the available experimental intensities of $\nu 1+\nu 3$ has provided us with the expansion of the transformed transition moment of this band which has then been used together with the wavefunctions resulting from the diagonalization of the hamiltonian matrix, to generate an improved set of line parameters for the $\nu 1+\nu 3$ and $\nu 1+\nu 2+\nu 3-\nu 2$ bands absorbing in the 3,4 $\mu$m region. This compilation has then been used to detect for the first time $NO2$ absorption peaks in atmospheric spectra recorded from the ground. Thirteen well isolated features appearing clearly in high resolution Fourier transform atmospheric spectra recorded at Kitt Peak have allowed a precise determination of $NO2$ vertical column densities. Indeed from to analysis of 4 spectra recorded at air masses of 10.7, 5.4, 3.1 in the morning and of 10.8 in the afternoon, vertical column densities of $1.8\pm 0.3$ and $3.25\pm 0.46$ (in $1015$ molecule $cm-2$) have been derived for the morning and the evening.