Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 6 2006 10.5194/acpd-6-12301-2006 http://www.atmos-chem-phys-discuss.net/6/12301/2006/ http://www.atmos-chem-phys-discuss.net/6/12301/2006/acpd-6-12301-2006.html http://www.atmos-chem-phys-discuss.net/6/12301/2006/acpd-6-12301-2006.pdf 12301 12345 2006-11-29 Near-real time retrieval of tropospheric NO₂ from OMI K. F. Boersma H. J. Eskes J. P. Veefkind E. J. Brinksma R. J. van der A M. Sneep G. H. J. van der Oord P. F. Levelt P. Stammes J. F. Gleason E. J. Bucsela KNMI, De Bilt, The Netherlands NASA GSFC, Greenbelt, Maryland, USA now at: Harvard University, Cambridge, USA We present a new algorithm for the near-real time retrieval – within 3 h of the actual satellite measurement – of tropospheric NO₂ columns from the Ozone Monitoring Instrument (OMI). The retrieval system is based on the combined retrieval-assimilation-modelling approach developed at KNMI for off-line tropospheric NO₂ from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori information ndash; profile shapes and stratospheric background NO₂ ndash; is now immediately available upon arrival of the OMI NO₂ slant columns and cloud data at KNMI. Slant column NO₂ and cloud information arrives at KNMI typically within 80 min of actual OMI observations. Slant columns for NO₂ are retrieved using differential optical absorption spectroscopy (DOAS) in the 405–465 nm range. Cloud fraction and cloud pressure are provided by a new cloud retrieval algorithm that uses the absorption of the O₂–O₂ collision complex near 477 nm. On-line availability of stratospheric slant columns and NO₂ profiles is achieved by running the TM4 chemistry transport model (CTM) forward in time based on forecast ECMWF meteo and assimilated NO₂ information from all previously observed orbits. OMI NO₂ slant columns, after correction for spurious across-track variability, show a random error for individual pixels of approximately 0.7×10¹⁵molec.cm^–2. As NO₂ retrievals are very sensitive to clouds, we evaluated the consistency of cloud fraction and cloud pressure from the new O₂–O₂ (OMI) algorithm and from the Fast Retrieval Scheme for Cloud Observables (FRESCO). Cloud parameters from the O₂–O₂ (OMI) algorithm have similar frequency distributions as cloud parameters retrieved from FRESCO (SCIAMACHY) for August 2006. On average, OMI cloud fractions are higher by 0.011, and OMI cloud pressures exceed FRESCO cloud pressures by 60 hPa. As a consistency check, we intercompared OMI near-real time NO₂ columns measured at 13:45 h local time to SCIAMACHY off-line NO₂ columns measured at 10:00 h local time. In August 2006, both instruments observe very similar spatial patterns of tropospheric NO₂ columns, and small differences for most locations on Earth where tropospheric NO₂ columns are small. For regions that are strongly polluted, SCIAMACHY observes higher tropospheric NO₂ columns than OMI.