Temperature dependent ultraviolet‐visible absorption cross sections of N₂O and N₂O₄: Low‐temperature measurements of the equilibrium constant for 2NO₂ ⇌ N₂O₄

A study of the temperature dependence of the ultraviolet‐visible absorption cross sections for NO 2 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 NO 2 and N 2 O 4 cross sections and the equilibrium constant for the dimerization of NO 2 . 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 NO 2 . Averaged cross section data for NO 2 are presented for use in photolysis rate calculations. Values of cross sections for N 2 O 4 and equilibrium constant for the association of NO 2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO 2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO 2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO 2 made by ultraviolet‐visible absorption techniques.