## 2 Materials and Methods
### Remotely Sensed Formaldehyde Columns from OMI  
We use formaldehyde observations retrieved from the Ozone Monitoring Instrument (OMI) spectrometer aboard the Aura mission launched in 2004 [77]. The OMI HCHO product used in this study was developed in the framework of the EU FP7 project QA4 ECV (Quality Assurance for Essential Climate Variables, [[http://www.qa4](http://www.qa4) ecv.eu]([http://www.qa4](http://www.qa4) ecv.eu), accessed on 1 June 2018) and is fully documented by [78; 79]. The retrievals of HCHO slant columns are based on improved Differential Optical Absorption Spectroscopy (DOAS) algorithms that reduce the effect of interferences between species using the 328.5-359 nm spectral window with the HCHO absorption cross-section from [80]. Further, the background normalization of the slant columns is performed using the equatorial Pacific Ocean in order to compensate for possible systematic latitude-dependent offsets in spectral fitting. The conversion into a vertical column is performed with the air mass factor (AMF) obtained by combining an altitude-resolved AMF look-up table derived at 340 nm from the VLIDORTv2.6 radiative transfer model [81] and the daily _a priori_ formaldehyde vertical profile shape calculated with the TM5-MP chemical transport model [82; 83]. The scattering due to clouds is corrected using an independent pixel approximation [84], whereas the effects of non-absorbing aerosols are implicitly accounted for through the cloud correction scheme. The effect of absorbing aerosols is currently not accounted for in the retrieval algorithm. Those effects can be significant over regions dominated by biomass burning or over heavily industrialized regions.  
The OMI QA4 ECV dataset provides separate estimates for the systematic and random components of the total uncertainty and a thorough description of the error estimation is provided in [85]. The total uncertainty on the monthly HCHO vertical column generally ranges between 20% and 40%, depending on the observation conditions. For clear sky conditions, in absence of aerosols, the total error on the AMFs is estimated at 18% and the error increases with cloud fraction, up to 50% for a cloud fraction of 0.5. The omission of the aerosol correction may lead to a significant underestimation of the derived HCHO column by up to 40% over fire scenes [80]. To ensure the best quality, we filter out cloudy pixels (fractions higher than 20%) in addition to the quality assurance criterion ([[http://www.tropomi.eu/sites/default/files/files/publicSentinel-5P-Formaldehyde-Readme.pdf](http://www.tropomi.eu/sites/default/files/files/publicSentinel-5P-Formaldehyde-Readme.pdf)]([http://www.tropomi.eu/sites/default/files/files/publicSentinel-5P-Formaldehyde-Readme.pdf](http://www.tropomi.eu/sites/default/files/files/publicSentinel-5P-Formaldehyde-Readme.pdf)), accessed on 25 September 2021). This already limits the potential impact of thick plumes on the retrievals. Furthermore, we conducted an additional sensitivity analysis aiming to minimize the potential impact of aerosols on the model comparisons with OMI HCHO data. This was done by filtering out OMI pixels with high aerosol optical depth (AOD), as described in Section 2.6 below and Section S6 of the Supplementary Materials.  
For the comparison with MOFLUX data, the OMI observations are averaged within a 20-km radius around the MOFLUX site. For the regional and global evaluation, observations at 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\) and 2\({}^{\circ}\)\(\times\) 2.5\({}^{\circ}\) are used, respectively.