Direct Sun Measurements of NO2 Column Abundances With FTUVS From Table Mountain, California: Retrieval Method, Instrument Intercomparison, and Results

NO2 total column abundances have been measured with a high resolution Fourier Transform Ultraviolet Spectrometer (FTUVS) instrument continuously during clear and lightly cloudy days at JPL's Table Mountain Facility (TMF) in California since 2005. The NO2 total column is retrieved after removing the solar background using Doppler shifted spectra from the east and west limbs of the rotating sun. The major advantage of this method over other direct Sun techniques is that it provides an absolute NO2 measurement technique and thus requires no calibration. An instrument intercomparison campaign was conducted at TMF during July, 2007, which involved the MF- DOAS and PANDORA from Washington State University and GSFC, respectively. Because the latter two instruments use a high-sun reference spectrum which contains an unknown NO2 column, the simultaneous measurements from FTUVS was shown to be useful as an independent calibration for the two grating spectrometers. The NO2 retrieval method and the intercomparison results are presented. Furthermore, the NO2 columns measured by FTUVS are compared to OMI measurements, after a terrain adjustment, for a period of near three years. While the linear correlation of the two datasets shows reasonable agreement, the comparison shows variability due to the distance between TMF and the center of OMI footprints. The interannual and seasonal behavior of NO2 abundances and their implications on atmospheric chemistry are discussed.