Knowledge Bank

Nitrous acid (HONO) has been proposed to be a significant source of HO-radicals in sunlight-irradiated, $NOX$-rich exhaust gases $environments.1$ To facilitate the identification and detection of this molecule in the atmosphere, moderately high resolution ($0.125cm-1$) spectra were obtained in the mid-IR region by Fourier transform spectroscopy using a path length of 43 m and a nitrous acid concentration of atm. The $\nu 4$ bands of both the trans- and the cis-isomers (at 790 and $852cm-1$, respectively) and the $\nu 3$ band of the trans-isomer (at $1263cm-1$) are A-type transitions and are mildly degraded to higher energies. The positions and relative intensities of these bands are in agreement with the assignments by McGraw $etal.2$ A partial rotational analysis of each of these bands has been carried out using Watson’s Hamilton and the lower state constants given by Cox $etal.3$ A much weaker band of HONO has also been observed near $1068cm-1$. This band exhibits B-type structure and it is rather strongly degraded to lower energies. A preliminary analysis of the and heads suggests that it is an overtone band due to $2\nu 6$ of the trans-isomer.