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Abstract. We present a new algorithm for the near-real time retrieval -within 3 h of the actual satellite measurement -of tropospheric NO 2 columns from the Ozone Monitoring Instrument (OMI). The retrieval is based on the combined retrieval-assimilation-modelling approach developed at KNMI for off-line tropospheric NO 2 from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori informationprofile shapes and stratospheric background NO 2 -is now immediately available upon arrival (within 80 min of observation) of the OMI NO 2 slant columns and cloud data at KNMI. Slant columns for NO 2 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 O 2 -O 2 collision complex near 477 nm. Online availability of stratospheric slant columns and NO 2 profiles is achieved by running the TM4 chemistry transport model (CTM) forward in time based on forecast ECMWF meteo and assimilated NO 2 information from all previously observed orbits. OMI NO 2 slant columns, after correction for spurious across-track variability, show a random error for individual pixels of approximately 0.7×10 15

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We have re-analyzed the X-ray flare on Algol which was observed with EXOSAT (White et al. (1986)). The common practice of estimating loop volume and length from the decay time of the flare is discussed extensively. We show that during the decay phase of the flare both scaling laws for coronal loops are valid. This implies a unique determination of loop volume and length and allows a check whether additional heating occurs in the decay phase of a flare.

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