Temperature dependent NO2 cross sections at high spectral resolution

Abstract

The importance of nitrogen dioxide in both the troposphere and the stratosphere has been known for some years, and since the early 1970s, spectroscopic determinations have played an important role in understanding NOX chemistry. Spectroscopic measurements of the atmosphere have improved in quality in recent years to the point that an accurate determination of the NO2 absorption cross section is essential to accurate retrievals of not only NO2 but also less abundant species in the troposphere and stratosphere. NO2 is such a large absorber (approximately 1% at large air mass) in the stratosphere at twilight or in the troposphere under even mildly polluted conditions, that if it is not properly removed from observed spectra, the spectra of the more subtle species are masked and cannot be measured at all. We present cross sections of NO2 in the spectral region 350–585 nm at four temperatures between 217 and 298 K and total pressures between 100 and 600 torr at a mixing ratio of 84.1 ppmv and at a spectral resolution sufficient for accurate convolution with instruments typically used to measure atmospheric NO2. Data will be presented to demonstrate the presence of NO2 pressure dependence in high resolution. A detailed comparison with commonly used literature cross sections is made to show how such instrument parameters as wavelength accuracy, resolution, spectrograph scattered light, and data sampling affect the usefulness of the observed cross section.