Knowledge Bank

Measurements of $NO2$ mixing ratios are very important in understanding the complex $NOx$ chemistry in both the lower and upper atmosphere. Many such measurements rely upon line strengths for transitions in the strongest band of $NO2(\nu 3)$. We report here new measurements for a number of transitions which are of interest to atmospheric chemists. In order to adequately measure the strengths of some of the transitions whose spin splittings are not resolved. It was necessary to make new determinations of the spin-spin interaction terms. Several higher order terms were found to be significant. The absorption measurements were carried out on known concentrations of $NO2$ employing permeation standards (50-100 ppm) which have been extensively characterized by gravimetric and chemical titration techniques. This approach is different from traditional measurements where the number density of the absorbing species is derived from pressure measurements alone. Because of the well characterized low concentration source of $NO2$ uncertainties in the number density due to decomposition and the presence of $N2O4$ are thus minimized. However, the low concentrations required the use of a long pass cell. A high resolution confocal interferometer and the technique of sweep integration were employed in these measurements. This both improved the signal to noise achieved and increased the accuracy of the line width measurements. Final results will be given and compared to unpublished results of Toth. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ R. Toth private communication.