Articles | Volume 10, issue 3
Atmos. Meas. Tech., 10, 759–782, 2017
Atmos. Meas. Tech., 10, 759–782, 2017

Research article 07 Mar 2017

Research article | 07 Mar 2017

Structural uncertainty in air mass factor calculation for NO2 and HCHO satellite retrievals

Alba Lorente1, K. Folkert Boersma1,2, Huan Yu3, Steffen Dörner4, Andreas Hilboll5,6, Andreas Richter5, Mengyao Liu7, Lok N. Lamsal8, Michael Barkley9, Isabelle De Smedt3, Michel Van Roozendael3, Yang Wang4, Thomas Wagner4, Steffen Beirle4, Jin-Tai Lin7, Nickolay Krotkov8, Piet Stammes2, Ping Wang2, Henk J. Eskes2, and Maarten Krol1,10,11 Alba Lorente et al.
  • 1Wageningen University, Meteorology and Air Quality Group, Wageningen, the Netherlands
  • 2Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 3Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 4Max-Planck Institute for Chemistry (MPI-C), Mainz, Germany
  • 5Institute of Environmental Physics (IUP-UB), University of Bremen, Bremen, Germany
  • 6MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
  • 7Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China
  • 8Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • 9EOS Group, Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 10Netherlands Institute for Space Research (SRON), Utrecht, the Netherlands
  • 11Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands

Abstract. Air mass factor (AMF) calculation is the largest source of uncertainty in NO2 and HCHO satellite retrievals in situations with enhanced trace gas concentrations in the lower troposphere. Structural uncertainty arises when different retrieval methodologies are applied within the scientific community to the same satellite observations. Here, we address the issue of AMF structural uncertainty via a detailed comparison of AMF calculation methods that are structurally different between seven retrieval groups for measurements from the Ozone Monitoring Instrument (OMI). We estimate the escalation of structural uncertainty in every sub-step of the AMF calculation process. This goes beyond the algorithm uncertainty estimates provided in state-of-the-art retrievals, which address the theoretical propagation of uncertainties for one particular retrieval algorithm only. We find that top-of-atmosphere reflectances simulated by four radiative transfer models (RTMs) (DAK, McArtim, SCIATRAN and VLIDORT) agree within 1.5 %. We find that different retrieval groups agree well in the calculations of altitude resolved AMFs from different RTMs (to within 3 %), and in the tropospheric AMFs (to within 6 %) as long as identical ancillary data (surface albedo, terrain height, cloud parameters and trace gas profile) and cloud and aerosol correction procedures are being used. Structural uncertainty increases sharply when retrieval groups use their preference for ancillary data, cloud and aerosol correction. On average, we estimate the AMF structural uncertainty to be 42 % over polluted regions and 31 % over unpolluted regions, mostly driven by substantial differences in the a priori trace gas profiles, surface albedo and cloud parameters. Sensitivity studies for one particular algorithm indicate that different cloud correction approaches result in substantial AMF differences in polluted conditions (5 to 40 % depending on cloud fraction and cloud pressure, and 11 % on average) even for low cloud fractions (<  0.2) and the choice of aerosol correction introduces an average uncertainty of 50 % for situations with high pollution and high aerosol loading. Our work shows that structural uncertainty in AMF calculations is significant and that it is mainly caused by the assumptions and choices made to represent the state of the atmosphere. In order to decide which approach and which ancillary data are best for AMF calculations, we call for well-designed validation exercises focusing on polluted conditions in which AMF structural uncertainty has the highest impact on NO2 and HCHO retrievals.

Short summary
Choices and assumptions made to represent the state of the atmosphere introduce an uncertainty of 42 % in the air mass factor calculation in trace gas satellite retrievals in polluted regions. The AMF strongly depends on the choice of a priori trace gas profile, surface albedo data set and the correction method to account for clouds and aerosols. We call for well-designed validation exercises focusing on situations when AMF structural uncertainty has the highest impact on satellite retrievals.