Minimizing aerosol effects on the OMI tropospheric NO2 retrieval – An improved use of the 477 nm O2 − O2 band and an estimation of the aerosol correction uncertainty
Authors: Chimot Julien , Veefkind J. Pepijn , de Haan Johan F. , Stammes Piet , Levelt Pieternel F. 
Published: January 2019

Abstract. Global mapping of satellite tropospheric NO2 vertical column
density (VCD), a key gas in air quality monitoring, requires accurate
retrievals over complex urban and industrialized areas and under any
atmospheric conditions. The high abundance of aerosol particles in regions
dominated by anthropogenic fossil fuel combustion, e.g. megacities, and/or
biomass-burning episodes, affects the space-borne spectral measurement.
Minimizing the tropospheric NO2 VCD biases caused by aerosol
scattering and absorption effects is one of the main retrieval challenges
from air quality satellite instruments. In this study, the reference Ozone
Monitoring Instrument (OMI) DOMINO-v2 product was reprocessed over cloud-free
scenes, by applying new aerosol correction parameters retrieved from the
477 nm O2−O2 band, over eastern China and South America for 2 years
(2006–2007). These new parameters are based on two different and separate
algorithms developed during the last 2 years in view of an improved use of
the OMI 477 nm O2−O2 band:
the updated OMCLDO2 algorithm, which derives improved effective cloud
parameters, the aerosol neural network (NN), which retrieves explicit aerosol
parameters by assuming a more physical aerosol model.
The OMI aerosol NN is a step ahead of OMCLDO2 because it primarily estimates an
explicit aerosol layer height (ALH), and secondly an aerosol optical
thickness τ for cloud-free observations. Overall, it was found that all
the considered aerosol correction parameters reduce the biases identified in
DOMINO-v2 over scenes in China with high aerosol abundance dominated by fine
scattering and weakly absorbing particles, e.g. from
[-20%:-40%] to
[0 %:20 %] in summertime. The use of the retrieved OMI aerosol
parameters leads in general to a more explicit aerosol correction and higher
tropospheric NO2 VCD values, in the range of [0 %:40 %], than
from the implicit correction with the updated OMCLDO2. This number overall
represents an estimation of the aerosol correction strategy uncertainty
nowadays for tropospheric NO2 VCD retrieval from space-borne visible
measurements. The explicit aerosol correction theoretically includes a more
realistic consideration of aerosol multiple scattering and absorption
effects, especially over scenes dominated by strongly absorbing particles,
where the correction based on OMCLDO2 seems to remain insufficient. However,
the use of ALH and τ from the OMI NN aerosol algorithm is not a
straightforward operation and future studies are required to identify the
optimal methodology. For that purpose, several elements are recommended in
this paper. Overall, we demonstrate the possibility of applying a more explicit
aerosol correction by considering aerosol parameters directly derived from
the 477 nm O2−O2 spectral band, measured by the same satellite
instrument. Such an approach can, in theory, easily be transposed to the
new-generation of space-borne instruments (e.g. TROPOMI on board Sentinel-5
Precursor), enabling a fast reprocessing of tropospheric NO2 data
over cloud-free scenes (cloudy pixels need to be filtered out), as well as
for other trace gas retrievals (e.g. SO2, HCHO).