Direct Sun measurements of NO_2 column abundances from Table Mountain, California: Intercomparison of low- and high-resolution spectrometers

摘要

The NO_2 total column abundance, C_(NO_2), 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 NO_2 column abundance independently from the exoatmospheric solar irradiance using rovibrational NO_2 absorption lines. The NO_2 total 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 NO_2 columns and then apply modified Langley or "bootstrap" methods to determine the amounts of NO_2 in the references to obtain the absolute NO_2 columns. The calibration offset derived from the FTUVS measurements is consistent with the values derived from Langley and bootstrap calibration plots of the NO_2 slant column measured by the grating spectrometers. The calibrated total vertical column abundances of NO_2,C_(NO_2), 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.