Direct Sun measurements of NO₂ column abundances from Table Mountain, California: Intercomparison of low- and high-resolution spectrometers

Abstract

The NO₂ total column abundance, CNO₂ , was measured with a direct Sun viewing technique using three different instruments at NASA Jet Propulsion Laboratory’s (JPL)Table Mountain Facility in California during an instrument intercomparison campaign in July 2007. The instruments are a high‐resolution (∼0.001 nm) Fourier transform ultraviolet spectrometer (FTUVS) from JPL and two moderate‐resolution grating spectrometers, multifunction differential optical absorption spectroscopy (MF‐DOAS) (∼0.8 nm) from Washington State University and Pandora (∼0.4 nm) from NASA Goddard Space Flight Center. FTUVS uses high spectral resolution to determine the absolute NO₂ column abundance independently from the exoatmospheric solar irradiance using rovibrational NO₂ absorption lines. The NO2total column is retrieved after removing the solar background using Doppler‐shifted spectra from the east and west limbs of the Sun. The FTUVS measurements were used to validate the independently calibrated measurements of multifunction differential optical absorption spectroscopy (MF‐DOAS) and Pandora. The latter two instruments start with measured high‐Sun spectra as solar references to retrieve relative NO₂ columns and then apply modified Langley or “bootstrap” methods to determine the amounts of NO₂ in the references to obtain the absolute NO₂ columns. The calibration offset derived from the FTUVS measurements is consistent with the values derived from Langley and bootstrap calibration plots of the NO₂ slant column measured by the grating spectrometers. The calibrated total vertical column abundances of NO₂, CNO₂ from all three instruments are compared showing that MF‐DOAS and Pandora data agree well with each other, and both data sets agree with FTUVS data to within (1.5 ± 4.1)% and (6.0 ± 6.0)%, respectively.