Abstract. Ozone in the troposphere affects humans and ecosystems as a pollutant and as a
greenhouse gas. Observing, understanding and modelling this dual role, as
well as monitoring effects of international regulations on air quality and
climate change, however, challenge measurement systems to operate at opposite
ends of the spatio-temporal scale ladder. Aboard the ESA/EU Copernicus
Sentinel-5 Precursor (S5P) satellite launched in October 2017, the TROPOspheric
Monitoring Instrument (TROPOMI) aspires to take the next leap forward by
measuring ozone and its precursors at unprecedented horizontal resolution
until at least the mid-2020s. In this work, we assess the quality of
TROPOMI's first release (V01.01.05–08) of tropical tropospheric ozone column (TrOC) data. Derived with the convective cloud differential (CCD) method,
TROPOMI daily TrOC data represent the 3 d moving mean ozone column
between the surface and 270 hPa under clear-sky conditions gridded at
0.5∘ latitude by 1∘ longitude resolution. Comparisons to
almost 2 years of co-located SHADOZ ozonesonde and satellite data (Aura OMI
and MetOp-B GOME-2) conclude to TROPOMI biases between −0.1 and
+2.3 DU (<+13 %) when averaged over the tropical belt.
The field of the bias is essentially uniform in space (deviations <1 DU) and stable in time at the 1.5–2.5 DU level. However,
the record is still fairly short, and continued monitoring will be key to
clarify whether observed patterns and stability persist, alter behaviour or
disappear. Biases are partially due to TROPOMI and the reference data records
themselves, but they can also be linked to systematic effects of the non-perfect co-locations. Random uncertainty due to co-location mismatch
contributes considerably to the 2.6–4.6 DU (∼14 %–23 %)
statistical dispersion observed in the difference time series. We circumvent
part of this problem by employing the triple co-location analysis technique
and infer that TROPOMI single-measurement precision is better than
1.5–2.5 DU (∼8 %–13 %), in line with uncertainty
estimates reported in the data files. Hence, the TROPOMI precision is judged
to be 20 %–25 % better than for its predecessors OMI and
GOME-2B, while sampling at 4 times better spatial resolution and almost
2 times better temporal resolution. Using TROPOMI tropospheric ozone columns at
maximal resolution nevertheless requires consideration of correlated errors at
small scales of up to 5 DU due to the inevitable interplay of
satellite orbit and cloud coverage. Two particular types of sampling error
are investigated, and we suggest how these can be identified or remedied. Our
study confirms that major known geophysical patterns and signals of the
tropical tropospheric ozone field are imprinted in TROPOMI's 2-year data
record. These include the permanent zonal wave-one pattern, the pervasive
annual and semiannual cycles, the high levels of ozone due to biomass burning
around the Atlantic basin, and enhanced convective activity cycles associated
with the Madden–Julian Oscillation over the Indo-Pacific warm pool.
TROPOMI's combination of higher precision and higher resolution reveals details
of these patterns and the processes involved, at considerably smaller spatial
and temporal scales and with more complete coverage than contemporary
satellite sounders. If the accuracy of future TROPOMI data proves to remain
stable with time, these hold great potential to be included in Climate Data
Records, as well as serve as a travelling standard to interconnect the
upcoming constellation of air quality satellites in geostationary and low
Earth orbits.
TROPOMI tropospheric ozone column data: Geophysical assessment and comparison to ozonesondes, GOME-2B and OMI
