formaldehyde

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Responsible:Niccolò Calandri

Formaldehyde — Sentinel-5P TROPOMI HCHO

Overview

The Formaldehyde (HCHO) layer measures the tropospheric column density of formaldehyde using the TROPOMI instrument aboard Sentinel-5P. Formaldehyde is a short-lived volatile organic compound (VOC) that serves as a key tracer for:

Biogenic emissions (~65 %): oxidation of isoprene emitted by vegetation, particularly broadleaf forests in warm climates
Biomass burning (~20 %): incomplete combustion of organic matter
Anthropogenic sources (~15 %): industrial solvents, vehicle exhaust, petrochemical processing

HCHO has an atmospheric lifetime of only a few hours, making it a powerful indicator of local and regional emission sources. Elevated HCHO columns over vegetated areas indicate active biogenic VOC emissions, while enhanced columns over urban or industrial areas signal anthropogenic pollution.

The native spatial resolution is approximately 1113 m, with output maps at 100 m through spatial interpolation.

Calculation Methodology

Retrieval algorithm:

The HCHO tropospheric column is retrieved using a Differential Optical Absorption Spectroscopy (DOAS) approach in the UV spectral range (328.5–359 nm):

Slant column fitting — HCHO absorption features are fitted in the measured spectrum
Stratospheric correction — Removal of the stratospheric background contribution
Air mass factor calculation — Conversion from slant to vertical column using radiative transfer modeling
Quality filtering — Based on cloud fraction (< 0.4), solar zenith angle, and retrieval uncertainty

Post-processing:

Spatial resampling from native footprint to 100 m output
Temporal averaging over the requested period
Outlier removal based on physical plausibility checks

Data Sources

Code	Name	Provider	Type	Coverage	Resolution	Availability
`COPERNICUS/S5P/OFFL/L3_HCHO`	Sentinel-5P TROPOMI HCHO L3	ESA/Copernicus	Atmospheric column	Global	~1113 m	2018-06 — present

Documentation: GEE S5P HCHO

Technical Notes

HCHO retrievals are sensitive to cloud contamination; data availability is reduced in persistently cloudy regions
The short atmospheric lifetime means that HCHO columns are strongly influenced by local emission conditions
Over forested regions, seasonal HCHO patterns closely follow temperature-driven isoprene emission cycles
Urban HCHO signals may be confounded by co-located NO₂ emissions which affect the DOAS retrieval

References

Veefkind, J.P. et al. (2012). "TROPOMI on the ESA Sentinel-5 Precursor". Remote Sensing of Environment, 120, pp. 70–83.
De Smedt, I., Theys, N., Yu, H. et al. (2018). "Algorithm theoretical baseline for formaldehyde retrievals from S5P TROPOMI and from the QA4ECV project". Atmospheric Measurement Techniques, 11, pp. 2395–2426.
Zhu, L., Jacob, D.J., Kim, P.S. et al. (2016). "Observing atmospheric formaldehyde (HCHO) from space: validation and intercomparison of six retrievals from four satellites". Atmos. Chem. Phys., 16, pp. 13477–13490.

Data Sources & References

Sources

WRD_S5HCO_992d

Methodology

La formaldeide (HCHO) viene mappata utilizzando i dati Sentinel-5P TROPOMI, prodotto OFFL L3_HCHO disponibile su Google Earth Engine, con risoluzione nativa di circa 1113 metri. La formaldeide è un composto organico volatile (VOC) che svolge un ruolo chiave nella chimica atmosferica come prodotto intermedio nella degradazione ossidativa del metano e di altri idrocarburi. Le fonti principali sono: 1) Emissioni biogeniche: ossidazione dell'isoprene emesso dalla vegetazione (principale fonte naturale); 2) Incendi boschivi (biomass burning): emissione diretta durante la combustione; 3) Fonti antropiche: combustione di combustibili, processi industriali; 4) Fotochimica: ossidazione atmosferica di metano e altri VOC. Per questi motivi, l'HCHO è un tracciante utile per l'attività fotosintetica e le emissioni biogeniche, il rilevamento di incendi forestali, e l'inquinamento atmosferico urbano. La banda utilizzata è 'tropospheric_HCHO_column_number_density', che misura la densità colonnare troposferica di formaldeide in mol/m². L'elaborazione è analoga agli altri prodotti S5P: ricampionamento bilineare a 100m e composizione media.