Intra-pixel variability in satellite tropospheric NO<sub>2</sub> column densities derived from simultaneous space-borne and airborne observations over the South African Highveld

Broccardo, Stephen; Heue, Klaus-Peter; Walter, David; Meyer, Christian; Kokhanovsky, Alexander; A, Ronald van der; Piketh, Stuart; Langerman, Kristy; Platt, Ulrich

doi:https://doi.org/10.5194/amt-11-2797-2018

Stephen Broccardo^1,a, Klaus-Peter Heue², David Walter³, Christian Meyer⁴, Alexander Kokhanovsky^5,6,b, Ronald van der A⁷, Stuart Piketh⁸, Kristy Langerman^9,c, and Ulrich Platt¹⁰ 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

Received: 03 Nov 2016 – Discussion started: 01 Feb 2017
Revised: 04 Jul 2017 – Accepted: 08 Aug 2017 – Published: 14 May 2018

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.
Citation: Broccardo, S., Heue, K.-P., Walter, D., Meyer, C., Kokhanovsky, A., van der A, R., Piketh, S., Langerman, K., and Platt, U.: Intra-pixel variability in satellite tropospheric NO₂ column densities derived from simultaneous space-borne and airborne observations over the South African Highveld, Atmos. Meas. Tech., 11, 2797-2819, https://doi.org/10.5194/amt-11-2797-2018, 2018.

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