Shuhui Wang, Thomas J. Pongetti, and Stanley P. Sander

1. JPL JPL Direct Sun measurements of NO2 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

2. Objective Approach  • An improved NO2 total column retrieval method. • Intercomparison of NO2 column measurements using high and low resolution spectrometers. • NO2 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 NO2 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 NO2 column (SCref). • The high resolution FTUVS measurements can be used as an absolute calibration for the two low resolution spectrometers. Calibration

3. FTUVS Absolute NO2 Column Retrieval  Removal of solar lines Atmospheric NO2 signal Solar signal • East and west limb solar spectra are taken alternatively. Thesolar lines are Doppler-shifted, while the NO2 absorption features remain at the same wavenumber. • Solar lines are removed by matching and ratioing east and west spectra. • FFT high pass filter is used to further remove the broad-band solar baseline, leaving only high frequency NO2 features.

4. FTUVS Absolute NO2 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 NO2 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. FTUVS Absolute NO2 Column Retrieval  Weighted fit • A weighted fit is applied to block the spectral regions with solar residuals, leaving numerous micro-windows with NO2 features but few solar features.

6. Correlation of NO2 Slant Columns from PANDORA / MF-DOAS vs. FTUVS • The NO2 slant column in the high-Sun reference (SCref) determined with FTUVS agrees with that from the Langley method. SCref

7. Comparison of NO2 Vertical Columns from PANDORA, MF-DOAS and FTUVS

8. Summary • Direct Sun measurements of NO2 total columns have been conducted using three instruments at TMF in July, 2007. • The high-resolution FTUVS measures the absolute NO2 columns, while the low-resolution instruments, PANDORA and MF-DOAS, use a high Sun spectrum as the reference to retrieve NO2 columns. The FTUVS absolute NO2 measurements can thus provide a calibration for the two low resolution spectrometers. • During this campaign, the NO2 slant columns from PANDORA and MF-DOAS are compared to those from FTUVS to determine the amount of NO2 in the high Sun reference. • The (calibrated) total NO2 vertical columns from all three instruments show good agreement to ~10%.