Articles | Volume 11, issue 5
Atmos. Meas. Tech., 11, 2797–2819, 2018
Atmos. Meas. Tech., 11, 2797–2819, 2018
Research article
14 May 2018
Research article | 14 May 2018

Intra-pixel variability in satellite tropospheric NO2 column densities derived from simultaneous space-borne and airborne observations over the South African Highveld

Stephen Broccardo1,a, Klaus-Peter Heue2, David Walter3, Christian Meyer4, Alexander Kokhanovsky5,6,b, Ronald van der A7, Stuart Piketh8, Kristy Langerman9,c, and Ulrich Platt10 Stephen Broccardo et al.
  • 1School of Geography, Archaeology and Environmental Science, University of the Witwatersrand, Johannesburg, 2030, South Africa
  • 2Deutsches Zentrum für Luft- und Raumfahrt, Münchener Str. 20, 82234 Oberpfaffenhofen, Germany
  • 3Max Planck Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
  • 4IDT Europe GmbH, Grentzstr 28, 01109 Dresden, Germany
  • 5EUMETSAT, Eumetsat Allee 1, 64295 Darmstadt, Germany
  • 6Moscow Engineering Physics Institute, National Research Nuclear University, Kashirskoe Ave. 31, 115409, Moscow, Russia
  • 7R&D Satellite Observations, KNMI, Utrechtseweg 297, 3731GA, De Bilt, the Netherlands
  • 8Climatology Research Group, Unit for Environmental Science and Management, North-West University, Potchefstroom, 2531, South Africa
  • 9Eskom Holdings SOC Ltd, Megawatt Park, Maxwell Drive, Sandton, 2157, South Africa
  • 10Institut für Umweltphysik, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
  • anow at: Climatology Research Group, Unit for Environmental Science and Management, North-West University, Potchefstroom, 2531, South Africa
  • bnow at: Vitrociset Belgium SPRL, Bratustrasse 7, 64293 Darmstadt, Germany
  • cnow at: Department of Geography, Environmental Management and Energy Studies, University of Johannesburg, Johannesburg, South Africa

Abstract. Aircraft measurements of NO2 using an imaging differential optical absorption spectrometer (iDOAS) instrument over the South African Highveld region in August 2007 are presented and compared to satellite measurements from OMI and SCIAMACHY. In situ aerosol and trace-gas vertical profile measurements, along with aerosol optical thickness and single-scattering albedo measurements from the Aerosol Robotic Network (AERONET), are used to devise scenarios for a radiative transfer modelling sensitivity study. Uncertainty in the air-mass factor due to variations in the aerosol and NO2 profile shape is constrained and used to calculate vertical column densities (VCDs), which are compared to co-located satellite measurements. The lower spatial resolution of the satellites cannot resolve the detailed plume structures revealed in the aircraft measurements. The airborne DOAS in general measured steeper horizontal gradients and higher peak NO2 vertical column density. Aircraft measurements close to major sources, spatially averaged to the satellite resolution, indicate NO2 column densities more than twice those measured by the satellite. The agreement between the high-resolution aircraft instrument and the satellite instrument improves with distance from the source, this is attributed to horizontal and vertical dispersion of NO2 in the boundary layer. Despite the low spatial resolution, satellite images reveal point sources and plumes that retain their structure for several hundred kilometres downwind.

Short summary
Measurements of nitrogen dioxide, known to originate from industrial and automotive combustion sources, have been made from space for two decades. Successive generations of instrument bring improvements in ground-pixel resolution; however features in the atmosphere are known to be smaller than what the satellites can resolve. Measurements of urban and industrial areas using a high-resolution airborne instrument allow the impact of the satellite's relatively low resolution to be evaluated.