Comparative assessment of TROPOMI and OMI formaldehyde observations and validation against MAX-DOAS network column measurements
Authors: De Smedt Isabelle , Pinardi Gaia , Vigouroux Corinne , Compernolle Steven , Bais Alkis , Benavent Nuria , Boersma Folkert , Chan Ka-Lok , Donner Sebastian , Eichmann Kai-Uwe , Hedelt Pascal , Hendrick François , Irie Hitoshi , Kumar Vinod , Lambert Jean-Christopher , Langerock Bavo , Lerot Christophe , Liu Cheng , Loyola Diego , Piters Ankie , Richter Andreas , Rivera Cárdenas Claudia , Romahn Fabian , Ryan Robert George , Sinha Vinayak , Theys Nicolas , Vlietinck Jonas , Wagner Thomas , Wang Ting , Yu Huan , Van Roozendael Michel 
Published: August 2021

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI), launched in
October 2017 on board the Sentinel-5 Precursor (S5P) satellite, monitors the
composition of the Earth's atmosphere at an unprecedented horizontal
resolution as fine as 3.5 × 5.5 km2. This paper assesses the performances
of the TROPOMI formaldehyde (HCHO) operational product compared to its
predecessor, the OMI (Ozone Monitoring Instrument) HCHO QA4ECV product, at different spatial and temporal
scales. The parallel development of the two algorithms favoured the
consistency of the products, which facilitates the production of long-term
combined time series. The main difference between the two satellite products
is related to the use of different cloud algorithms, leading to a positive
bias of OMI compared to TROPOMI of up to 30 % in tropical regions. We show
that after switching off the explicit correction for cloud effects, the two
datasets come into an excellent agreement. For medium to large HCHO vertical
columns (larger than 5 × 1015 molec. cm−2) the median bias between
OMI and TROPOMI HCHO columns is not larger than 10 % (< 0.4 × 1015 molec. cm−2). For lower columns, OMI observations present
a remaining positive bias of about 20 % (< 0.8 × 1015 molec. cm−2) compared to TROPOMI in midlatitude regions. Here, we also
use a global network of 18 MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments to validate both satellite
sensors for a large range of HCHO columns. This work complements the study
by Vigouroux et al. (2020), where a global FTIR (Fourier transform infrared) network is used to validate
the TROPOMI HCHO operational product. Consistent with the FTIR validation
study, we find that for elevated HCHO columns, TROPOMI data are
systematically low (−25 % for HCHO columns larger than 8 × 1015 molec. cm−2), while no significant bias is found for medium-range column
values. We further show that OMI and TROPOMI data present equivalent biases
for large HCHO levels. However, TROPOMI significantly improves the precision
of the HCHO observations at short temporal scales and for low HCHO columns.
We show that compared to OMI, the precision of the TROPOMI HCHO columns is
improved by 25 % for individual pixels and by up to a factor of 3 when
considering daily averages in 20 km radius circles. The validation precision
obtained with daily TROPOMI observations is comparable to the one obtained
with monthly OMI observations. To illustrate the improved performances of
TROPOMI in capturing weak HCHO signals, we present clear detection of HCHO
column enhancements related to shipping emissions in the Indian Ocean. This
is achieved by averaging data over a much shorter period (3 months) than
required with previous sensors (5 years) and opens new perspectives to
study shipping emissions of VOCs (volatile organic compounds) and related atmospheric chemical
interactions.