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Comparison of OMI NO2 with Ground-based Direct Sun Measurements at NASA GSFC and JPL Table Mountainduring Summer 2007George H. Mount & Elena Spinei Washington State UniversityStanley P. Sander, Thomas Pongetti**, and Shuhui Wang**NASA JPLJay Herman and Alexander CedeNASA GSFC** = next talks
Participating Instruments WSU MultiFunction DOAS (MFDOAS) - basic measurement: slant column • observes direct sun or scattered skylight (DS + MAXDOAS) • spectral coverage: 281 - 498 nm • CCD - 13.5 µm pixel size • spectral resolution 0.83 nm • spectral sampling on FWHM 7.8 pixels • DS uses an integrating sphere • integration time: sub-second GSFC PANDORA - basic measurement: slant column • observes direct sun • spectral coverage: 265 - 500 nm • CCD - 25 µm pixels • spectral resolution 0.4 nm • spectral sampling on FWHM 2 pixels • integration time: sub-second time averaged to 20-sec JPL UVFTS - basic measurement absolute slant column • observes direct sun • spectral regions used for analysis: 488 - 498 nm • spectral resolution: 0.001 nm • integration time: 17 min - observes opposite limbs of sun
data analysis MFDOAS and PANDORA • reduce raw data to slant column using a common reference spectrum from a particular date/time • nonlinear least squares using common cross section (Harder et al., 1997) --> difference between absolute slant column at time of measurement and the absolute slant column in the reference spectrum • determine absolute slant column in the reference spectrum using Langley method, bootstrap method, UVFTS - all are consistent at TMF where clean skies were observed part of the time • correction: 0.13 + 0.01 DU at TMF, 0.25 + 0.01 DU at GSFC for the reference spectra used at each location • apply AMF from direct sun (simple geometry), remove data with SZA > 80° to avoid problems with layer height in AMF calculation UVFTS • Tom Pongetti and Shuhui Wang will talk about the FTS data analysis technique in the next talks
Campaigns Goddard Space Flight Center • data every day 14 - 25 May 2007 • PANDORA - direct sun on ~ 20 sec centers • MFDOAS • direct sun on ~ 1 sec centers • direct sun + scattered sky at 4 azimuths and 4 elev angles ~ 20 min • about half the days were devoted to scattered sky + DS measurements JPL Table Mountain • data every day from 2 July - 12 July 2007 • PANDORA and MFDOAS as above • UVFTS - 17 min cycles on each of the two solar limbs Only direct sun observations are presented here OMI data is collection 3
DS-DOAS vs UVFTS measurements at TMF ground = 0.93 * UVFTS
OMI vs. Ground Based Total NO2 Vertical Column at GSFC fitted slope: ~ 1.2 OMI = 1.2 x ground
Temporal Variability of Total NO2 Vertical Columns (GSFC: 14-May-07)
__ 10a 10 km 2z 6 11z 4 12b 3b 9 10b 7 12a 8 3a JPL TMF region - numbers are dates in July of the OMI observations and are placed at the pixel center; “z” = OMI zoom mode, “a” and “b” indicate overpasses on the same day
OMI vs. Ground Based Total NO2 Vertical Column at JPL-TMF (July 2007)
Conclusions • successful campaigns were carried out in summer 2007 at GSFC and JPL TMF with 3 instrument groups measuring NO2 • excellent agreement between instruments was found, even on time scales of minutes • UVFTS was extremely valuable --> absolute column & confirmation of calibration for the reference column of NO2 for PANDORA and MFDOAS • need a large number of days to get a solid comparison with OMI • comparisons with OMI total NO2 were difficult due to: • spatial inhomogeneities • especially true at TMF - the high elevation and proximity to LA (Tom’s talk next) • times when agreement with OMI should have been good and was not • wave structure at GSFC showed changes on order 10% in minutes above the site • vertical inhomogeneities • movement of polluted air “under” the observing site and inside an OMI pixel • OMI NO2 vertical columns are ~ 15% low at GSFC & poor correlation at TMF • use of scattered sky data is valuable (e.g. MFDOAS at PNNL in Washington), but requires a much more difficult interpretation due to calculation of AMF, vertical structure of NO2, aerosol loading, and spatial inhomogeneities - we are working with PK on use of a combination of near simultaneous DS and scattered sky measurements to improve the analysis
Future • MFDOAS • detailed reduction of campaign scattered sky data • determinations of ozone and formaldehyde • campaigns which will be used for OMI comparison (DS+scatt sky): • WSU fall/winter 2007/2008 • PNNL Richland, WA - spring 2008 • wildfire measurements late summer 2008 • Boise, Idaho December 2008 • Houston, Texas April-May 2009 • US SE for formaldehyde validation • Spain(?) international campaign - summer 2009 or 2010 • PANDORA • deployment of PAN-3 for sun, sky, moon • improved tracking and s/n • build network of 15 instruments to map OMI sub-pixel variation and regional variation • UVFTS - Tom and Shuhui in next talks
