The TROPOMI surface UV algorithm
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) is the only payload of the
Sentinel-5 Precursor (S5P), which is a polar-orbiting satellite mission of
the European Space Agency (ESA). TROPOMI is a nadir-viewing spectrometer
measuring in the ultraviolet, visible, near-infrared, and the shortwave
infrared that provides near-global daily coverage. Among other things,
TROPOMI measurements will be used for calculating the UV radiation reaching
the Earth's surface. Thus, the TROPOMI surface UV product will contribute to
the monitoring of UV radiation by providing daily information on the
prevailing UV conditions over the globe. The TROPOMI UV algorithm builds on
the heritage of the Ozone Monitoring Instrument (OMI) and the Satellite
Application Facility for Atmospheric Composition and UV Radiation (AC SAF)
algorithms. This paper provides a description of the algorithm that will be
used for estimating surface UV radiation from TROPOMI observations. The
TROPOMI surface UV product includes the following UV quantities: the UV
irradiance at 305, 310, 324, and 380  nm ; the erythemally weighted UV;
and the vitamin-D weighted UV. Each of these are available as (i) daily dose or
daily accumulated irradiance, (ii) overpass dose rate or irradiance, and
(iii) local noon dose rate or irradiance. In addition, all quantities are
available corresponding to actual cloud conditions and as clear-sky values,
which otherwise correspond to the same conditions but assume a cloud-free
atmosphere. This yields 36 UV parameters altogether. The TROPOMI UV algorithm
has been tested using input based on OMI and the Global Ozone Monitoring
Experiment-2 (GOME-2) satellite measurements. These preliminary results
indicate that the algorithm is functioning according to expectations.