Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) aboard
the Sentinel-5 Precursor satellite (launched on 13 October 2017) is a
nadir-viewing spectrometer measuring reflected sunlight in the ultraviolet,
visible, near-infrared, and shortwave infrared spectral ranges. The measured
spectra are used to retrieve total columns of trace gases, including
nitrogen dioxide (NO2). For ground validation of these satellite
measurements, Pandora spectrometers, which retrieve high-quality NO2
total columns via direct-sun measurements, are widely used. In this study,
Pandora NO2 measurements made at three sites located in or north of the
Greater Toronto Area (GTA) are used to evaluate the TROPOMI NO2 data
products, including a standard Royal Netherlands Meteorological Institute
(KNMI) tropospheric and stratospheric NO2 data product and a TROPOMI
research data product developed by Environment and Climate Change Canada
(ECCC) using a high-resolution regional air quality forecast model (in the
air mass factor calculation). It is found that these current TROPOMI
tropospheric NO2 data products (standard and ECCC) met the TROPOMI
design bias requirement (< 10 %). Using the statistical
uncertainty estimation method, the estimated TROPOMI upper-limit precision
falls below the design requirement at a rural site but above in the other
two urban and suburban sites. The Pandora instruments are found to have
sufficient precision (< 0.02 DU) to perform TROPOMI validation work.
In addition to the traditional satellite validation method (i.e., pairing
ground-based measurements with satellite measurements closest in time and
space), we analyzed TROPOMI pixels located upwind and downwind from the
Pandora site. This makes it possible to improve the statistics and better
interpret the high-spatial-resolution measurements made by TROPOMI. By using
this wind-based validation technique, the number of coincident measurements
can be increased by about a factor of 5. With this larger number of
coincident measurements, this work shows that both TROPOMI and Pandora
instruments can reveal detailed spatial patterns (i.e., horizontal
distributions) of local and transported NO2 emissions, which can be
used to evaluate regional air quality changes. The TROPOMI ECCC NO2
research data product shows improved agreement with Pandora measurements
compared to the TROPOMI standard tropospheric NO2 data product (e.g.,
lower multiplicative bias at the suburban and urban sites by about 10 %),
demonstrating benefits from the high-resolution regional air quality
forecast model.