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 Platt10Stephen Broccardo et al.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
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.
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.
Measurements of nitrogen dioxide, known to originate from industrial and automotive combustion...