uv_aerosol_index

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Responsible:Niccolò Calandri

UV Aerosol Index — Sentinel-5P TROPOMI

Overview

The UV Aerosol Index (UVAI) is derived from the TROPOMI instrument aboard Sentinel-5P and provides a qualitative measure of UV-absorbing aerosols in the atmosphere. Unlike traditional aerosol optical depth (AOD), the UVAI is sensitive specifically to absorbing aerosol types:

Aerosol Type	Typical UVAI Range	Sources
Mineral dust	1–4	Saharan dust, desert storms
Wildfire smoke	2–5	Biomass burning, forest fires
Volcanic ash	2–4	Volcanic eruptions
Black carbon	1–3	Fossil fuel combustion, industrial pollution

The UVAI is a dimensionless index:

Positive values → UV-absorbing aerosols (dust, smoke, ash, black carbon)
Near-zero values → Molecular atmosphere or non-absorbing aerosols
Negative values → Non-absorbing aerosols (sulfate, sea salt) or sub-pixel clouds

The native resolution is approximately 1113 m, with output at 1000 m.

Calculation Methodology

Retrieval algorithm:

The UVAI is computed from the difference between measured and calculated (Rayleigh-only) top-of-atmosphere reflectances at 354 nm and 388 nm:

Measure spectral reflectances at 354 nm and 388 nm
Calculate expected reflectances for a pure Rayleigh atmosphere with the same surface albedo
Compute residual — the difference between measured and expected reflectances
UVAI derivation — the residual is converted to the aerosol index using a parametric formula

Key properties:

UVAI > 1.0 indicates significant aerosol absorption
UVAI > 2.0 indicates strong absorption events (dust storms, fire plumes)
UVAI > 4.0 indicates extreme events (major volcanic eruptions, large wildfires)
Negative UVAI values are associated with non-absorbing aerosols or cloud artifacts

Data Sources

Code	Name	Provider	Type	Coverage	Resolution	Availability
`COPERNICUS/S5P/OFFL/L3_AER_AI`	Sentinel-5P TROPOMI UV Aerosol Index L3	ESA/Copernicus	Aerosol index	Global	~1113 m	2018-07 — present

Documentation: GEE S5P AER AI

Technical Notes

The UVAI is qualitative, not quantitative: it indicates the presence and type of aerosols but not their exact concentration
Cloud contamination produces artifacts (typically negative UVAI values)
Over bright surfaces (desert, snow), the UVAI baseline is elevated, requiring regional calibration
The UVAI is most useful for detecting episodic events (dust storms, fire plumes) rather than persistent background aerosol
Sun glint over water surfaces can produce false positive signals

References

Veefkind, J.P. et al. (2012). "TROPOMI on the ESA Sentinel-5 Precursor: A GMES mission for global observations". Remote Sensing of Environment, 120, pp. 70–83.
Torres, O., Bhartia, P.K., Jethva, H., Ahn, C. (2018). "Impact of the ozone monitoring instrument row anomaly on the long-term record of aerosol products". Atmospheric Measurement Techniques, 11, pp. 2701–2715.
Herman, J.R., Bhartia, P.K., Torres, O. et al. (1997). "Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data". Journal of Geophysical Research, 102(D14), pp. 16911–16922.

Data Sources & References

Sources

WRD_S5AER_99

Methodology

L'UV Aerosol Index (UVAI) viene mappato utilizzando i dati Sentinel-5P TROPOMI, prodotto OFFL L3_AER_AI disponibile su Google Earth Engine, con risoluzione nativa di circa 1113 metri. L'UVAI è un indice sensibile agli aerosol che assorbono radiazione ultravioletta, con valori adimensionali tipicamente nel range da -2 a +5. L'interpretazione dei valori è: - Valori positivi indicano aerosol UV-assorbenti: polveri minerali come Saharan dust (UVAI 1-4), fumo da incendi boschivi (UVAI 2-5), ceneri vulcaniche (UVAI 2-4), black carbon (UVAI 1-3); - Valori negativi indicano aerosol non assorbenti (nuvole) o superfici altamente riflettenti (neve). La banda utilizzata è 'absorbing_aerosol_index'. Nell'elaborazione vengono selezionati solo i valori > 0, che indicano la presenza di aerosol UV-assorbenti. Viene applicato un ricampionamento bilineare a 1000m (risoluzione più bassa degli altri S5P data la natura dell'indice), quindi calcolata la media pixel-wise sul periodo.