Sentinel-5P OFFL AER LH: Offline UV Aerosol Layer Height

Dataset Availability
Dataset Provider
Earth Engine Snippet
aerosol air-quality alh copernicus esa eu knmi pollution s5p sentinel tropomi uvai



This dataset provides offline high-resolution imagery of the UV Aerosol Index (UVAI), also called the Absorbing Layer Height (ALH).

The ALH is very sensitive to cloud contamination. However, aerosols and clouds can be difficult to distinguish, and ALH is computed for all FRESCO effective cloud fractions smaller than 0.05. Cloud masks are available from FRESCO and VIIRS, and are strongly recommended to filter for residual clouds. A sunglint mask is also available to screen sunglint regions, which are not filtered beforehand.

It is known that high surface albedos negatively influence the ALH, biasing the ALH towards the surface. In general, the ALH over (dark) oceans is considered reliable to within the requirement of 1000 m or 100 hPa. Over land, especially bright surfaces, the accuracy may be lower, and the use of the ALH product over bright surfaces like deserts is not advisable.

For this L3 AER_LH product, the aerosol_mid_pressure is calculated with a pair of measurements at the 354 nm and 388 nm wavelengths. The COPERNICUS/S5P/OFFL/L3_SO2 product has the absorbing_aerosol_index calculated using the 340 nm and 380 nm wavelengths.

OFFL L3 Product

To make our OFFL L3 products, we find areas within the product's bounding box with data using a command like this:

harpconvert --format hdf5 --hdf5-compression 9
-a 'aerosol_height_validity>50;derive(datetime_stop {time})'

We then merge all the data into one large mosaic (area-averaging values for pixels that may have different values for different times). From the mosaic, we create a set of tiles containing orthorectified raster data.

Example harpconvert invocation for one tile: harpconvert --format hdf5 --hdf5-compression 9 -a 'aerosol_height_validity>50;derive(datetime_stop {time}); bin_spatial(2001, 50.000000, 0.01, 2001, -120.000000, 0.01); keep(aerosol_height,aerosol_pressure,aerosol_optical_depth, sensor_zenith_angle,sensor_azimuth_angle,solar_azimuth_angle,solar_zenith_angle)' output.h5

Sentinel-5 Precursor

Sentinel-5 Precursor is a satellite launched on 13 October 2017 by the European Space Agency to monitor air pollution. The onboard sensor is frequently referred to as Tropomi (TROPOspheric Monitoring Instrument).

All of the S5P datasets, except CH4, have two versions: Near Real-Time (NRTI) and Offline (OFFL). CH4 is available as OFFL only. The NRTI assets cover a smaller area than the OFFL assets, but appear more quickly after acquisition. The OFFL assets contain data from a single orbit (which, due to half the earth being dark, contains data only for a single hemisphere).

Because of noise on the data, negative vertical column values are often observed in particular over clean regions or for low SO2 emissions. It is recommended not to filter these values except for outliers, i.e. for vertical columns lower than -0.001 mol/m^2.

The original Sentinel 5P Level 2 (L2) data is binned by time, not by latitude/longitude. To make it possible to ingest the data into Earth Engine, each Sentinel 5P L2 product is converted to L3, keeping a single grid per orbit (that is, no aggregation across products is performed).

Source products spanning the antimeridian are ingested as two Earth Engine assets, with suffixes _1 and _2.

The conversion to L3 is done by the harpconvert tool using the bin_spatial operation. The source data is filtered to remove pixels with QA values less than:

  • 80% for AER_AI
  • 75% for the tropospheric_NO2_column_number_density band of NO2
  • 50% for all other datasets except for O3 and SO2

The O3_TCL product is ingested directly (without running harpconvert).


1113.2 meters


Name Units Min Max Description
aerosol_height m -78.91* 69370.9*

The aerosol layer pressure is converted into an aerosol layer altitude using an appropriate temperature profile, i.e. the temperature profile used in the retrieval. The value is given relative to the geoid.

aerosol_pressure Pa 2.06* 103896*

Pressure of an aerosol layer with an assumed pressure thickness of (currently) 50 hPa and a constant aerosol volume extinction coefficient and single scattering albedo.

aerosol_optical_depth Pa -0.6* 11.56*

Aerosol optical thickness τ of the assumed aerosol layer. The optical thickness holds for 760 nm.

sensor_azimuth_angle deg -180* 180*

Azimuth angle of the satellite at the ground pixel location (WGS84); angle measured East-of-North.

sensor_zenith_angle deg 0.098* 66.87*

Zenith angle of the satellite at the ground pixel location (WGS84); angle measured away from the vertical.

solar_azimuth_angle deg -180* 180*

Azimuth angle of the Sun at the ground pixel location (WGS84); angle measured East-of-North.

solar_zenith_angle deg 12.93* 74.7*

Zenith angle of the satellite at the ground pixel location (WGS84); angle measured away from the vertical.

* estimated min or max value

Image Properties

Image Properties

Name Type Description

The algorithm version used in L2 processing. It is separate from the processor (framework) version, to accommodate different release schedules for different products.


The date, expressed as milliseconds since 1 Jan 1970, when the software used to perform L2 processing was built.


The version of the HARP tool used to grid the L2 data into an L3 product.


The institution where data processing from L1 to L2 was performed.


The date, expressed as milliseconds since 1 Jan 1970, when Google processed the L2 data into L3 using harpconvert.


The maximum latitude of the asset (degrees).


The minimum latitude of the asset (degrees).


The maximum longitude of the asset (degrees).


The minimum longitude of the asset (degrees).


The orbit number of the satellite when the data was acquired.


Name of the platform which acquired the data.


The processing status of the product on a global level, mainly based on the availability of auxiliary input data. Possible values are "Nominal" and "Degraded".


The version of the software used for L2 processing, as a string of the form "major.minor.patch".


Id of the L2 product used to generate this asset.


Indicator that specifies whether the product quality is degraded or not. Allowed values are "Degraded" and "Nominal".


Name of the sensor which acquired the data.


Spatial resolution at nadir. For most products this is 3.5x7km2, except for L2__O3__PR, which uses 28x21km2, and L2__CO____ and L2__CH4___, which both use 7x7km2. This attribute originates from the CCI standard.


Days from 1 Jan 1950 to when the data was acquired.


The Julian day number when the data was acquired.


UUID for the L2 product file.

Terms of Use

Terms of Use

The use of Sentinel data is governed by the Copernicus Sentinel Data Terms and Conditions.

Explore with Earth Engine

Code Editor (JavaScript)

var collection = ee.ImageCollection('COPERNICUS/S5P/OFFL/L3_AER_LH')
                    .filterDate('2019-06-01', '2019-06-05');

var visualization = {
  min: 0,
  max: 6000,
  palette: ['blue', 'purple', 'cyan', 'green', 'yellow', 'red']

Map.setCenter(44.09, 24.27, 4);

Map.addLayer(collection.mean(), visualization, 'S5P Aerosol Height');
Open in Code Editor