Sentinel-5P OFFL AER AI: Offline UV Aerosol Index

COPERNICUS/S5P/OFFL/L3_AER_AI
Dataset Availability
2018-07-04T13:34:21 - Present
Dataset Provider
Earth Engine Snippet
ee.ImageCollection("COPERNICUS/S5P/OFFL/L3_AER_AI")
Tags
tropomi aerosol aai uvai pollution air-quality eu esa copernicus sentinel s5p knmi

Description

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).

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 any values with QA values of less than 50.

OFFL/L3_AER_AI

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

The AAI is based on wavelength-dependent changes in Rayleigh scattering in the UV spectral range for a pair of wavelengths. The difference between observed and modelled reflectance results in the AAI. When the AAI is positive, it indicates the presence of UV-absorbing aerosols like dust and smoke. It is useful for tracking the evolution of episodic aerosol plumes from dust outbreaks, volcanic ash, and biomass burning.

The wavelengths used have very low ozone absorption, so unlike aerosol optical thickness measurements, AAI can be calculated in the presence of clouds. Daily global coverage is therefore possible.

For this L3 AER_AI product, the absorbing_aerosol_index 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 'absorbing_aerosol_index_validity>50;derive(datetime_stop {time})'
S5P_OFFL_L2__AER_AI_20181030T213916_20181030T232046_05427_01_010200_20181105T210529.nc
grid_info.h5

We then merge all the data into one large mosaic (taking only one value 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 'absorbing_aerosol_index_validity>50;derive(datetime_stop {time}); bin_spatial(2001, 50.000000, 0.01, 2001, -120.000000, 0.01); keep(absorbing_aerosol_index,sensor_altitude,sensor_azimuth_angle, sensor_zenith_angle,solar_azimuth_angle,solar_zenith_angle)' S5P_OFFL_L2__AER_AI_20181030T213916_20181030T232046_05427_01_010200_20181105T210529.nc output.h5

Bands

Resolution
0.01 arc degrees

Bands

Name Units Min Max Description
absorbing_aerosol_index -21* 39*

A measure of the prevalence of aerosols in the atmosphere, calculated by this equation using the 354/388 wavelength pair.

sensor_altitude m 828543* 856078*

Altitude of the satellite with respect to the geodetic sub-satellite point (WGS84).

sensor_azimuth_angle degrees -180* 180*

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

sensor_zenith_angle degrees 0.098* 66.87*

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

solar_azimuth_angle degrees -180* 180*

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

solar_zenith_angle degrees 8* 88*

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
ALGORITHM_VERSION STRING

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

BUILD_DATE STRING

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

HARP_VERSION INT

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

INSTITUTION STRING

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

L3_PROCESSING_TIME INT

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

LAT_MAX DOUBLE

The maximum latitude of the asset (degrees).

LAT_MIN DOUBLE

The minimum latitude of the asset (degrees).

LON_MAX DOUBLE

The maximum longitude of the asset (degrees).

LON_MIN DOUBLE

The minimum longitude of the asset (degrees).

ORBIT INT

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

PLATFORM STRING

Name of the platform which acquired the data.

PROCESSING_STATUS STRING

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".

PROCESSOR_VERSION STRING

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

PRODUCT_ID STRING

Id of the L2 product used to generate this asset.

PRODUCT_QUALITY STRING

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

SENSOR STRING

Name of the sensor which acquired the data.

SPATIAL_RESOLUTION STRING

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.

TIME_REFERENCE_DAYS_SINCE_1950 INT

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

TIME_REFERENCE_JULIAN_DAY DOUBLE

The Julian day number when the data was acquired.

TRACKING_ID STRING

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 in Earth Engine

var collection = ee.ImageCollection('COPERNICUS/S5P/OFFL/L3_AER_AI')
  .select('absorbing_aerosol_index')
  .filterDate('2018-10-15', '2018-10-20');

var val_max = 2.0;
var val_min = -1;

var band_viz = {
  min: val_min,
  max: val_max,
  opacity: 1.0,
  palette: ["black", "blue", "purple", "cyan", "green", "yellow", "red"]
};

Map.addLayer(collection.mean(), band_viz, 's5p');
Map.setCenter(17.06, 23.1, 4);