Assessing Measurements of Pollution in the Troposphere (MOPITT) carbon monoxide retrievals over urban versus non-urban regions
Authors: Tang Wenfu , Worden Helen M. , Deeter Merritt N. , Edwards David P. , Emmons Louisa K. , Martínez-Alonso Sara , Gaubert Benjamin , Buchholz Rebecca R. , Diskin Glenn S. , Dickerson Russell R. , Ren Xinrong , He Hao , Kondo Yutaka 
Published: March 2020

Abstract. The Measurements of Pollution in the Troposphere (MOPITT) retrievals over
urban regions have not been validated systematically, even though MOPITT
observations are widely used to study CO over urban regions. Here we compare
MOPITT products over urban and non-urban regions with aircraft measurements
from the Deriving Information on Surface conditions from Column
and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ – 2011–2014), Studies of Emissions and Atmospheric Composition, Clouds, and Climate
Coupling by Regional Surveys (SEAC4RS – 2013), Air Chemistry Research In
Asia (ARIAs – 2016), A-FORCE
(2009, 2013), and Korea United States Air
Quality (KORUS-AQ – 2016) campaigns. In general, MOPITT agrees
reasonably well with the in situ profiles, over both urban and non-urban
regions. Version 8 multispectral product (V8J) biases vary from −0.7 % to
0.0 % and version 8 thermal-infrared product (TIR) biases vary from
2.0 % to 3.5 %. The evaluation statistics of MOPITT V8J and V8T over
non-urban regions are better than those over urban regions with smaller
biases and higher correlation coefficients. We find that the agreement of
MOPITT V8J and V8T with aircraft measurements at high CO concentrations is
not as good as that at low CO concentrations, although CO variability may
tend to exaggerate retrieval biases in heavily polluted scenes. We test the
sensitivities of the agreements between MOPITT and in situ profiles to
assumptions and data filters applied during the comparisons of MOPITT
retrievals and in situ profiles. The results at the surface layer are
insensitive to the model-based profile extension (required due to aircraft
altitude limitations), whereas the results at levels with limited aircraft
observations (e.g., the 600 hPa layer) are more sensitive to the model-based
profile extension. The results are insensitive to the maximum allowed time
difference criterion for co-location (12, 6, 3, and 1 h) and are generally insensitive to the radius for co-location, except
for the case where the radius is small (25 km), and hence few MOPITT
retrievals are included in the comparison. Daytime MOPITT products have
smaller overall biases than nighttime MOPITT products when comparing both
MOPITT daytime and nighttime retrievals to the daytime aircraft
observations. However, it would be premature to draw conclusions on the
performance of MOPITT nighttime retrievals without nighttime aircraft
observations. Applying signal-to-noise ratio (SNR) filters does not
necessarily improve the overall agreement between MOPITT retrievals and
in situ profiles, likely due to the reduced number of MOPITT retrievals for
comparison. Comparisons of MOPITT retrievals and in situ profiles over
complex urban or polluted regimes are inherently challenging due to spatial
and temporal variabilities of CO within MOPITT retrieval pixels (i.e.,
footprints). We demonstrate that some of the errors are due to CO
representativeness with these sensitivity tests, but further quantification
of representativeness errors due to CO variability within the MOPITT
footprint will require future work.