Evaluation of the first year of Pandora NO2 measurements over Beijing and application to satellite validation

Nitrogen dioxide (NO2) is a highly photo-
chemically reactive gas, has a lifetime of only a few
hours, and at high concentrations is harmful to human
beings. Therefore, it is important to monitor NO2 with
high-precision, time-resolved instruments. To this end, a
Pandora spectrometer has been installed on the roof of
the laboratory building of the Aerospace Information Re-
search Institute of the Chinese Academy of Sciences in
the Olympic Park, Beijing, China. The concentrations
of trace gases (including NO2, HCHO, O3) measured
with Pandora are made available through the open-access
Pandora database (https://data.pandonia-global-network.org/
Beijing-RADI/Pandora171s1/, last access: 11 July 2023). In
this paper, an overview is presented of the Pandora total and
tropospheric NO2 vertical column densities (VCDs) and sur-
face concentrations collected during the first year of oper-
ation, i.e., from August 2021 to July 2022. The data show
that NO2 concentrations were high in the winter and low in
the summer, with a diurnal cycle where the concentrations
reached a minimum during the daytime. The concentrations
were significantly lower during the 2022 Winter Olympics
in Beijing, showing the effectiveness of the emission con-
trol measures during that period. The Pandora observations
show that during northerly winds, clean air is transported
to Beijing with low NO2 concentrations, whereas during
southerly winds, pollution from surrounding areas is trans-
ported to Beijing and NO2 concentrations are high. The con-
tribution of tropospheric NO2 to the total NO2 VCD varies
significantly on daily to seasonal timescales; i.e., monthly
averages vary between 50 % and 60 % in the winter and be-
tween 60 % and 70 % in the spring and autumn. A compari-
son of Pandora-measured surface concentrations with collo-
cated in situ measurements using a Thermo Scientific 42i-
TL analyzer shows that the Pandora data are low and that
the relationship between Pandora-derived surface concen-
trations and in situ measurements is different for low and
high NO2 concentrations. Explanations for these differences
are offered in terms of measurement techniques and phys-
ical (transport) phenomena. The use of Pandora total and
tropospheric NO2 VCDs for validation of collocated TRO-
POspheric Monitoring Instrument (TROPOMI) data, resam-
pled to 100 m × 100 m, shows that although on average the
TROPOMI VCDs are slightly lower, they are well within
the expected error for TROPOMI of 0.5 Pmolec. cm−2 + (0.2
to 0.5) · VCDtrop (1 Pmolec. cm−2 = 1 × 1015 molec cm−2).
The location of the Pandora instrument within a sub-orbital TROPOMI pixel of 3.5 km × 5.5 km may result in an error
in the TROPOMI-derived tropospheric NO2 VCD between
0.223 and 0.282 Pmolec. cm−2, i.e., between 1.7 % and 2 %.
In addition, the data also show that the Pandora observations
at the Beijing-RADI site are representative of an area with a
radius of 10 km.