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

Abstract

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 ESrs-DOAS (EarthShine reference sector DOAS) algorithm, tropospheric NO2 slant columns are derived using spectra averaged from measurements over unpolluted regions, thus removing the need for a solar reference spectrum, though some residual stratospheric signal still has to be removed. To validate the ESrs-DOAS algorithm, DOAS retrievals were performed on modelled spectra created by the radiative transfer model- SCIATRAN, as well as L1B earthshine radiance data measured by the NASA/KNMI Ozone Monitoring Instrument (OMI). It was found that retrievals using an earthshine 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 ESrs-DOAS algorithm greatly reduces the presence of artificial across-track biases (stripes) caused by calibration errors, resulting in a 27% reduction in retrieval uncertainty. The ESrs-DOAS technique also reveals absorption features over the Sahara and similar regions characteristic of sand absorption, as first discovered in analysis of GOME-2 NO2 retrievals. Potentially, future satellite instruments optimised for using the ESrs-DOAS algorithm would not need a solar reference, which would simplify their optical and mission design.