1. Direct Sun measurements of NO 2 column abundances from Table Mountain, California: Retrieval method and intercomparison of low and high resolution spectrometers Shuhui Wang, Thomas J. Pongetti, and Stanley P. Sander Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California Elena Spinei and George Mount Washington State University, Pullman, Washington Alexander Cede University of Maryland, Earth System Science Interdisciplinary Center, College Park, Maryland Jay Herman NASA/Goddard Space Flight Center, Greenbelt, Maryland JP L

2. An improved NO 2 total column retrieval method. Intercomparison of NO 2 column measurements using high and low resolution spectrometers. ObjectiveApproach NO 2 column abundance has been measured with a high resolution Fourier Transform Ultraviolet Spectrometer (FTUVS) instrument at NASA/JPL’s Table Mountain Facility (TMF). With a high resolution, FTUVS measures the absolute NO 2 columns. The solar lines are removed using Doppler shifted spectra from the east and west limbs of the rotating Sun. Instrument intercomparison campaign at TMF during July, 2007 MF-DOAS and PANDORA use a high Sun reference spectrum which contains an unknown NO 2 column (SC ref ). The high resolution FTUVS measurements can be used as an absolute calibration for the two low resolution spectrometers.  Calibration FTUVS – JPLHigh resolution (0.001 nm) MF-DOAS – WSULow resolution (0.8 nm) PANDORA– GSFCLow resolution (0.4 nm)

3. East and west limb solar spectra are taken alternatively. The solar lines are Doppler-shifted, while the NO 2 absorption features remain at the same wavenumber. Solar lines are removed by matching and ratioing east and west spectra. Solar signal Atmospheric NO 2 signal FTUVS Absolute NO 2 Column Retrieval  Removal of solar lines FFT high pass filter is used to further remove the broad- band solar baseline, leaving only high frequency NO 2 features.

4. FTUVS Absolute NO 2 Column Retrieval  Reference The reference cross sections were measured using FTS at various temperatures at the Kitt Peak National Observatory [Nizkorodov et al., 2004]. Low pressure cross sections followed by convolution (normalized Lorentz function) to various pressures are used to create references for each atmospheric layer. Based on A priori profiles, average NO 2 references in the Stratosphere and the Troposphere are calculated. Total reference is obtained assuming a Strat/Trop partition (typically ~3 at TMF but could go to 1~5 in extreme cases). The total reference is convolved to 2 atm to reduce the sensitivity to the assumed partition.

5. A weighted fit is applied to block the spectral regions with solar residuals, leaving numerous micro-windows with NO 2 features but few solar features. FTUVS Absolute NO 2 Column Retrieval  Weighted fit

6. Correlation of NO 2 Slant Columns from PANDORA / MF-DOAS vs. FTUVS LangleyFTUVS PANDORA0.12 DU.0.14 DU. MF-DOAS0.12 DU.0.14 DU. The NO 2 slant column in the high- Sun reference (SC ref ) determined with FTUVS agrees with that from the Langley method. SC ref

7. Comparison of NO 2 Vertical Columns from PANDORA, MF-DOAS and FTUVS

8. Summary Direct Sun measurements of NO 2 total columns have been conducted using three instruments at TMF in July, 2007. The high-resolution FTUVS measures the absolute NO 2 columns, while the low- resolution instruments, PANDORA and MF-DOAS, use a high Sun spectrum as the reference to retrieve NO 2 columns. The FTUVS absolute NO 2 measurements can thus provide a calibration for the two low resolution spectrometers. During this campaign, the NO 2 slant columns from PANDORA and MF-DOAS are compared to those from FTUVS to determine the amount of NO 2 in the high Sun reference. The (calibrated) total NO 2 vertical columns from all three instruments show good agreement to ~10%.