Sentinel-5P NRTI HCHO: Near Real-Time Formaldehyde Data (COPERNICUS/S5P/NRTI/L3_HCHO)

###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 have two versions: Near Real-Time (NRTI) and Offline
(OFFL). 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.

###NRTI/L3_HCHO

This dataset provides near real-time high-resolution imagery of atmospheric
formaldehyde (HCHO) concentrations.

To make our NRTI L3 products, we use harpconvert
to grid the data.

Example harpconvert invocation for one tile:

harpconvert --format hdf5 --hdf5-compression 9
-a 'tropospheric_HCHO_column_number_density_validity>50;derive(datetime_stop {time});
bin_spatial(2001, 50.000000, 0.01, 2001, -120.000000, 0.01);
keep(tropospheric_HCHO_column_number_density,
     tropospheric_HCHO_column_number_density_amf,
     HCHO_slant_column_number_density,cloud_fraction,sensor_altitude,
     sensor_azimuth_angle, sensor_zenith_angle,solar_azimuth_angle,
     solar_zenith_angle)'
S5P_NRTI_L2__HCHO___20181017T181013_20181017T181513_05241_01_010102_20181017T185718.nc
output.h5