Articles | Volume 11, issue 12
Atmos. Meas. Tech., 11, 6651–6678, 2018
Atmos. Meas. Tech., 11, 6651–6678, 2018

Research article 17 Dec 2018

Research article | 17 Dec 2018

Improving algorithms and uncertainty estimates for satellite NO2 retrievals: results from the quality assurance for the essential climate variables (QA4ECV) project

K. Folkert Boersma1,2, Henk J. Eskes1, Andreas Richter3, Isabelle De Smedt4, Alba Lorente2, Steffen Beirle5, Jos H. G. M. van Geffen1, Marina Zara1, Enno Peters3, Michel Van Roozendael4, Thomas Wagner5, Joannes D. Maasakkers6, Ronald J. van der A1, Joanne Nightingale7, Anne De Rudder4, Hitoshi Irie8, Gaia Pinardi4, Jean-Christopher Lambert4, and Steven C. Compernolle4 K. Folkert Boersma et al.
  • 1Royal Netherlands Meteorological Institute, Satellite Observations, De Bilt, the Netherlands
  • 2Wageningen University, Meteorology and Air Quality Group, Wageningen, the Netherlands
  • 3Institute of Environmental Physics (IUP-UB), University of Bremen, Bremen, Germany
  • 4Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 5Max-Planck Institute for Chemistry (MPI-C), Mainz, Germany
  • 6Harvard University, Cambridge, Massachusetts, USA
  • 7National Physics Laboratory (NPL), Teddington, UK
  • 8Center for Environmental Remote Sensing (CEReS), Chiba University, Chiba, Japan

Abstract. Global observations of tropospheric nitrogen dioxide (NO2) columns have been shown to be feasible from space, but consistent multi-sensor records do not yet exist, nor are they covered by planned activities at the international level. Harmonised, multi-decadal records of NO2 columns and their associated uncertainties can provide crucial information on how the emissions and concentrations of nitrogen oxides evolve over time. Here we describe the development of a new, community best-practice NO2 retrieval algorithm based on a synthesis of existing approaches. Detailed comparisons of these approaches led us to implement an enhanced spectral fitting method for NO2, a 1°  ×  1° TM5-MP data assimilation scheme to estimate the stratospheric background and improve air mass factor calculations. Guided by the needs expressed by data users, producers, and WMO GCOS guidelines, we incorporated detailed per-pixel uncertainty information in the data product, along with easily traceable information on the relevant quality aspects of the retrieval. We applied the improved QA4ECV NO2 algorithm to the most current level-1 data sets to produce a complete 22-year data record that includes GOME (1995–2003), SCIAMACHY (2002–2012), GOME-2(A) (2007 onwards) and OMI (2004 onwards). The QA4ECV NO2 spectral fitting recommendations and TM5-MP stratospheric column and air mass factor approach are currently also applied to S5P-TROPOMI. The uncertainties in the QA4ECV tropospheric NO2 columns amount to typically 40 % over polluted scenes. The first validation results of the QA4ECV OMI NO2 columns and their uncertainties over Tai'an, China, in June 2006 suggest a small bias (−2 %) and better precision than suggested by uncertainty propagation. We conclude that our improved QA4ECV NO2 long-term data record is providing valuable information to quantitatively constrain emissions, deposition, and trends in nitrogen oxides on a global scale.

Short summary
This paper describes a new, improved data record of 22+ years of coherent nitrogen dioxide (NO2) pollution measurements from different satellite instruments. Our work helps to ensure that climate data are of sufficient quality to draw reliable conclusions and shape decisions. It shows how dedicated intercomparisons of retrieval sub-steps have led to improved NO2 measurements from the GOME, SCIAMACHY, GOME-2(A), and OMI sensors, and how quality assurance of the new data product is achieved.