Visible‐ultraviolet absorption cross sections for NO2 as a function of temperature

A redetermination of the temperature dependence of the absorption cross section (σ) of NO2 in the visible-ultraviolet region (264–649 nm) has been made in order to provide a more reliable data base for the calculation of NO2 photolysis rates in the atmosphere. Experiments over a wide range of temperatures (−40.6° to 124°C) and NO2 concentrations (3.4×1013–7.3×1014 molecules cm−3) were made at relatively low and high spectral resolutions, employing diode array and Fourier transform spectrometers, respectively. There are three aspects of the temperature dependence which were noted: (1) the integral of a plot of σ versus 1/λ (264–649 nm) was essentially independent of temperature; (2) increasing temperature produced a shift of the spectrum toward longer wavelengths, resulting in a small negative temperature dependence of σ over the 264- to 400-nm range and a small positive dependence over the 450- to 649-nm range; (3) increasing temperature produced broadening of individual spectral features, resulting in a systematic lowering of peaks and filling of valleys. Recommended cross sections, averaged over 5-nm wavelength intervals, are presented for use in tropospheric NO2 photolysis rate calculations.