Articles | Volume 8, issue 3
Research article
24 Mar 2015
Research article |  | 24 Mar 2015

An improved retrieval of tropospheric NO2 from space over polluted regions using an Earth radiance reference

J. S. Anand, P. S. Monks, and R. J. Leigh

Abstract. A novel tropospheric NO2 DOAS retrieval algorithm optimised for a nadir-viewing satellite instrument imaging polluted areas is proposed in this work. Current satellite DOAS retrievals have relied on using a solar reference spectrum to derive a total slant column, then using either model assimilation or spatial filtering to derive the tropospheric component. In the ERrs-DOAS (Earth radiance reference sector DOAS) algorithm, tropospheric NO2 slant columns are derived using spectra averaged from measurements over unpolluted regions, thus removing the need for post-hoc separation techniques, though some residual stratospheric biases may still remain. To validate the ERrs-DOAS algorithm, DOAS retrievals were performed on modelled spectra created by the radiative transfer model SCIATRAN, as well as L1B Earth radiance data measured by the NASA/KNMI Ozone Monitoring Instrument (OMI). It was found that retrievals using an Earth radiance reference produce spatial distributions of tropospheric NO2 over eastern China during June 2005 that highly correlate with those derived using existing retrieval algorithms. Comparisons with slant columns retrieved by the operational NO2 retrieval algorithm for OMI (OMNO2A) show that the ERrs-DOAS algorithm greatly reduces the presence of artificial across-track biases (stripes) caused by calibration errors, as well as the removal of path length enhancement in off-nadir pixels. Analysis of Pacific SCDs suggests that the algorithm also produces a 27% reduction in retrieval uncertainty, though this may be partially due to biases introduced by differences in the retrieval algorithm settings. The ERrs-DOAS technique also reveals absorption features over the Sahara and similar regions characteristic of sand and liquid H2O absorption, as first discovered in the analysis of GOME-2 NO2 retrievals.

Short summary
A new offline technique for retrieving tropospheric NO2 slant columns from space over urban regions is proposed in this work. An Earth radiance reference measurement from a remote unpolluted region is used instead of the standard solar irradiance measurement used by operational retrieval algorithms. Testing with spectra measured by the Ozone Monitoring Instrument (OMI) shows that the technique largely removes the presence of stratospheric NO2 and also greatly suppresses instrumental biases.