Evaluating Sentinel-5P TROPOMI tropospheric NO2 column densities with airborne and Pandora spectrometers near New York City and Long Island Sound
Authors: Judd Laura M. , Al-Saadi Jassim A. , Szykman James J. , Valin Lukas C. , Janz Scott J. , Kowalewski Matthew G. , Eskes Henk J. , Veefkind J. Pepijn , Cede Alexander , Mueller Moritz , Gebetsberger Manuel , Swap Robert , Pierce R. Bradley , Nowlan Caroline R. , Abad Gonzalo González , Nehrir Amin , Williams David 
Published: November 2020

Abstract. Airborne and ground-based Pandora spectrometer NO2 column
measurements were collected during the 2018 Long Island Sound Tropospheric
Ozone Study (LISTOS) in the New York City/Long Island Sound region, which
coincided with early observations from the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) instrument.
Both airborne- and ground-based measurements are used to evaluate the
TROPOMI NO2 Tropospheric Vertical
Column (TrVC) product v1.2 in this region, which has high spatial and
temporal heterogeneity in NO2. First, airborne and Pandora TrVCs are
compared to evaluate the uncertainty of the airborne TrVC and establish the
spatial representativeness of the Pandora observations. The 171 coincidences
between Pandora and airborne TrVCs are found to be highly correlated
(r2= 0.92 and slope of 1.03), with the largest individual differences
being associated with high temporal and/or spatial variability. These
reference measurements (Pandora and airborne) are complementary with respect
to temporal coverage and spatial representativity. Pandora spectrometers can
provide continuous long-term measurements but may lack areal representativity
when operated in direct-sun mode. Airborne spectrometers are typically only
deployed for short periods of time, but their observations are more
spatially representative of the satellite measurements with the added
capability of retrieving at subpixel resolutions of 250 m × 250 m
over the entire TROPOMI pixels they overfly. Thus, airborne data are more
correlated with TROPOMI measurements (r2=0.96) than Pandora
measurements are with TROPOMI (r2=0.84). The largest outliers between TROPOMI and the reference measurements appear to stem from too spatially coarse a priori surface reflectivity (0.5∘) over bright urban scenes. In this work, this results during cloud-free scenes that, at times, are affected by errors in the TROPOMI cloud pressure retrieval impacting the calculation of tropospheric air mass factors. This
factor causes a high bias in TROPOMI TrVCs of 4 %–11 %. Excluding these
cloud-impacted points, TROPOMI has an overall low bias of 19 %–33 % during
the LISTOS timeframe of June–September 2018. Part of this low bias is caused
by coarse a priori profile input from the TM5-MP model; replacing these profiles
with those from a 12 km North American Model–Community Multiscale Air Quality (NAMCMAQ) analysis results in a 12 %–14 % increase in
the TrVCs. Even with this improvement, the TROPOMI-NAMCMAQ TrVCs have a
7 %–19 % low bias, indicating needed improvement in a priori assumptions in
the air mass factor calculation. Future work should explore additional
impacts of a priori inputs to further assess the remaining low biases in
TROPOMI using these datasets.