Sneep M

Publications (2)0 Total impact

• Article: Accounting for surface reflectance anisotropy in satellite retrievals of tropospheric NO₂

  Zhou Y, Brunner D, R. J. D. Spurr, K. F. Boersma, Sneep M, Popp C, Buchmann B
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  ABSTRACT: Surface reflectance is a key parameter in satellite trace gas retrievals in the UV/visible range and in particular for the retrieval of nitrogen dioxide (NO₂) vertical tropospheric columns (VTCs). Current operational retrievals rely on coarse-resolution reflectance data and do not account for the generally anisotropic properties of surface reflectance. Here we present a NO₂ VTC retrieval that uses MODIS bi-directional reflectance distribution function (BRDF) data at high temporal (8 days) and spatial (1 km×1 km) resolution in combination with the LIDORT radiative transfer model to account for the dependence of surface reflectance on viewing and illumination geometry. The method was applied to two years of NO₂ observations from the Ozone Monitoring Instrument (OMI) over Europe. Due to its wide swath, OMI is particularly sensitive to BRDF effects. Using representative BRDF parameters for various land surfaces, we found that in July (low solar zenith angles) and November (high solar zenith angles) and for typical viewing geometries of OMI, differences between MODIS black-sky albedos and surface bi-directional reflectances are of the order of 0–10% and 0–40%, respectively, depending on the position of the OMI pixel within the swath. In the retrieval, black-sky albedo was treated as a Lambertian (isotropic) reflectance, while for BRDF effects we used the kernel-based approach in the MODIS BRDF product. Air Mass Factors were computed using the LIDORT radiative transfer model based on these surface reflectance conditions. Differences in NO₂ VTCs based on the Lambertian and BRDF approaches were found to be of the order of 0–3% in July and 0–20% in November with the extreme values found at large viewing angles. The much larger differences in November are partly due to higher solar zenith angles and partly to the choice of a priori NO₂ profiles – the latter typically have more pronounced maxima in the boundary layer during the cold season. However, BRDF impacts vary considerably across Europe due to changes in land surface type and increasing solar zenith angles at higher latitude. Finally, we compare BRDF-based NO₂ VTCs with those retrieved using the GOME/TOMS Lambertian equivalent reflectance (LER) data set. Our results indicate that the specific choice of albedo data set is even more important than accounting for surface BRDF effects, and this again demonstrates the strong requirement for more accurate surface reflectance data sets.
  Atmospheric Measurement Techniques Discussions. 01/2010;
• Source

  Available from: atmos-chem-phys-discuss.net

  Article: 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, Sneep M, G.H.J. van den Oord, P. F. Levelt, Stammes P, J. F. Gleason, E. J. Bucsela
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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 NO₂ 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 NO₂ from the GOME and SCIAMACHY satellite instruments. We have adapted the off-line system such that the required a priori information – profile shapes and stratospheric background NO₂ – is now immediately available upon arrival (within 80 min of observation) of the OMI NO₂ slant columns and cloud data at KNMI. 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 0.7×10¹⁵ molec cm⁻². Cloud parameters from OMI's O₂-O₂ algorithm have similar frequency distributions as retrieved from SCIAMACHY's Fast Retrieval Scheme for Cloud Observables (FRESCO) for August 2006. On average, OMI cloud fractions are higher by 0.011, and OMI cloud pressures exceed FRESCO cloud pressures by 60 hPa. A sequence of OMI observations over Europe in October 2005 shows OMI's capability to track changeable NO_x air pollution from day to day in cloud-free situations.
  Atmospheric Chemistry and Physics. 01/2007;