TEMPERATURE-DEPENDENT ULTRAVIOLET-VISIBLE ABSORPTION CROSS-SECTIONS OF NO2 AND N2O4 - LOW-TEMPERATURE MEASUREMENTS OF THE EQUILIBRIUM-CONSTANT FOR 2NO(2)REVERSIBLE-ARROW-N2O4

A study of the temperature dependence of the ultraviolet-visible absorption cross sections for NO2 has been made in the temperature range 213-298 K and between 310 and 570 nm using a diode array spectrometer. Analysis of the experimental data allowed the simultaneous measurement of the NO2 and N2O4 cross sections and the equilibrium constant for the dimerization of NO2. The absorption cross sections were measured at a spectral resolution of 0.54 nm full width at half maximum (FWHM) and are presented for use in the analysis of differential absorption spectra of atmospheric NO2. Averaged cross section data for NO2 are presented for use in photolysis rate calculations. Values of cross sections for N2O4 and equilibrium constant for the association of NO2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO2 made by ultraviolet-visible absorption techniques.