Sentinel-5P OFFL CH4: Offline Methane

OFFL/L3_CH4

This dataset provides offline high-resolution imagery of methane concentrations.

Methane (CH₄) is, after carbon dioxide (CO₂), the most important contributor to the anthropogenically enhanced greenhouse effect. Roughly three-quarters of methane emissions are anthropogenic and as such it is important to continue the record of satellite based measurements. TROPOMI aims at providing CH₄ column concentrations with high sensitivity to the Earth's surface, good spatiotemporal coverage, and sufficient accuracy to facilitate inverse modeling of sources and sinks. TROPOMI uses absorption information from the Oxygen-A Band (760nm) and the SWIR spectral range to monitor CH₄ abundances in the Earth's atmosphere. More information.

Currently, the following data quality issues are known, are not covered by the quality flags, and should be kept in mind when looking at the methane product and also at preliminary validation results. For more details, see the MPC VDAF website.

Filtering on qa_value < 0.5 does not remove all pixels considered bad. Some pixels with too low methane concentrations are still present:

• Single TROPOMI overpasses show stripes of erroneous CH₄ values in the flight direction.

• Not all pixels above inland water are filtered.

• Uncertainties for the XCH₄ is only based on the single sounding precision due to measurement noise. For applications requiring an overall uncertainty estimate, we propose to multiply the provided error by a factor 2, which reflects the scatter of single sounding errors in the TCCON validation.

• Data prior to November 2021 only provides XCH₄ over land, after which glint ocean observations were added.

• No data are present between 2022-07-26 and 2022-08-31 due to a provider outage.

OFFL L3 Product

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

harpconvert --format hdf5 --hdf5-compression 9
-a 'CH4_column_volume_mixing_ratio_dry_air_validity>50;derive(datetime_stop {time})'
S5P_OFFL_L2__CH4____20190223T202409_20190223T220540_07072_01_010202_20190301T221106.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.

Example harpconvert invocation for one tile: harpconvert --format hdf5 --hdf5-compression 9 -a 'CH4_column_volume_mixing_ratio_dry_air_validity>50; derive(datetime_stop {time}); bin_spatial(2001, 50.000000, 0.01, 2001, -120.000000, 0.01); keep(CH4_column_volume_mixing_ratio_dry_air, aerosol_height, aerosol_optical_depth, sensor_azimuth_angle, sensor_zenith_angle, solar_azimuth_angle, solar_zenith_angle)' S5P_OFFL_L2__CH4____20190223T202409_20190223T220540_07072_01_010202_20190301T221106.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).