An improved retrieval of tropospheric NO<sub>2</sub> from space over polluted regions using an Earth radiance reference

Abstract

Abstract.A novel tropospheric NO 2 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 NO 2 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 SCIA-TRAN, 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 NO 2 over eastern China during June 2005 that highly correlate with those derived using existing retrieval algorithms. Comparisons with slant columns retrieved by the operational NO 2 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 H 2 O absorption, as first discovered in the analysis of GOME-2 NO 2 retrievals.

High‐resolution NO2 observations from the Airborne Compact Atmospheric Mapper: Retrieval and validation

“…The reference location had to be changed daily to avoid any local NO 2 enhancements. The fitting algorithm QDOAS (http://uv-vis.aeronomie.be/software/QDOAS/) used here is a robust, flexible, and well‐validated open source software that has been used to perform DOAS retrievals of trace gases from satellite [e.g., Anand et al , ] and aircraft‐based [e.g., Merlaud et al , ] measurements. The spectral fitting is performed in the range of 425–460 nm.…”

Estimating daily surface NO<sub>2</sub> concentrations from satellite data – a case study over Hong Kong using land use regression models

“…A model combining both fixed and random effects is therefore known as a "mixed-effects" model, in which the concentration is expressed as the sum of fixed variables along with other variables whose effects vary with time or other properties classified by the dummy variables. In this work, these models are fitted from the observation dataset using the lme4 R software package (Bates et al, 2012).…”

“…Unlike in situ stations, satellite instruments can only be passively recalibrated over their lifetime, leading to a possible drift in retrieval precision that may progressively bias surface NO 2 models (e.g. Dikty, S. and Richter, A., 2011;Anand et al, 2015).…”

Discrete-wavelength DOAS NO<sub>2</sub> slant column retrievals from OMI and TROPOMI

“…The DW-DOAS method has only been tested using a solar spectrum as the reference (I 0 ) for the DOAS fit. However, Earthshine radiance spectra could be used instead, as demonstrated by Anand et al (2015).…”