Sentinel-5P OFFL SO2: Offline Sulfur Dioxide

OFFL/L3_SO2

This dataset provides offline high-resolution imagery of atmospheric sulfur dioxide (SO₂) concentrations.

Sulfur dioxide (SO₂) enters the Earth's atmosphere through both natural and anthropogenic processes. It plays a role in chemistry on a local and global scale and its impact ranges from short-term pollution to effects on climate. Only about 30% of the emitted SO₂ comes from natural sources; the majority is of anthropogenic origin. SO₂ emissions adversely affect human health and air quality. SO₂ has an effect on climate through radiative forcing, via the formation of sulfate aerosols. Volcanic SO₂ emissions can also pose a threat to aviation, along with volcanic ash. S5P/TROPOMI samples the Earth's surface with a revisit time of one day with unprecedented spatial resolution of 3.5 x 7 km which allows the resolution of fine details including the detection of much smaller SO₂ plumes. More information.

For this L3 SO₂ product, the absorbing_aerosol_index is calculated with a pair of measurements at the 340 nm and 380 wavelengths. The COPERNICUS/S5P/OFFL/L3_AER_AI product has the absorbing_aerosol_index calculated using the 354 nm and 388 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 'SO2_column_number_density_validity>50;derive(datetime_stop {time})'
S5P_OFFL_L2__SO2____20181228T231131_20181229T005301_06265_01_010105_20190104T083244.nc
grid_info.h5

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.

The qa value is adjusted before running harpconvert to satisfy all of the following criteria:

• snow_ice < 0.5
• sulfurdioxide_total_air_mass_factor_polluted > 0.1
• sulfurdioxide_total_vertical_column > -0.001
• qa_value > 0.5
• cloud_fraction_crb < 0.3
• solar_zenith_angle < 60

The 15km SO2 band is ingested only when solar_zenith_angle < 70.

Example harpconvert invocation for one tile: harpconvert --format hdf5 --hdf5-compression 9 -a 'SO2_column_number_density_validity>50;derive(datetime_stop {time}); bin_spatial(2001, 50.000000, 0.01, 2001, -120.000000, 0.01); keep(SO2_column_number_density,SO2_column_number_density_amf, SO2_slant_column_number_density,absorbing_aerosol_index,cloud_fraction, sensor_altitude, sensor_azimuth_angle, sensor_zenith_angle,solar_azimuth_angle, solar_zenith_angle)' S5P_OFFL_L2__SO2____20181228T231131_20181229T005301_06265_01_010105_20190104T083244.nc 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).