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. The Measurements of Pollution in the Troposphere (MOPITT) retrievals overurban regions have not been validated systematically, even though MOPITTobservations are widely used to study CO over urban regions. Here we compareMOPITT products over urban and non-urban regions with aircraft measurementsfrom the Deriving Information on Surface conditions from Columnand Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ – 2011–2014), Studies of Emissions and Atmospheric Composition, Clouds, and ClimateCoupling by Regional Surveys (SEAC 4 RS – 2013), Air Chemistry Research InAsia (ARIAs – 2016), A-FORCE(2009, 2013), and Korea United States AirQuality (KORUS-AQ – 2016) campaigns. In general, MOPITT agreesreasonably well with the in situ profiles, over both urban and non-urbanregions. Version 8 multispectral product (V8J) biases vary from  −0.7  % to0.0 % and version 8 thermal-infrared product (TIR) biases vary from2.0 % to 3.5 %. The evaluation statistics of MOPITT V8J and V8T overnon-urban regions are better than those over urban regions with smallerbiases and higher correlation coefficients. We find that the agreement ofMOPITT V8J and V8T with aircraft measurements at high CO concentrations isnot as good as that at low CO concentrations, although CO variability maytend to exaggerate retrieval biases in heavily polluted scenes. We test thesensitivities of the agreements between MOPITT and in situ profiles toassumptions and data filters applied during the comparisons of MOPITTretrievals and in situ profiles. The results at the surface layer areinsensitive to the model-based profile extension (required due to aircraftaltitude limitations), whereas the results at levels with limited aircraftobservations (e.g., the 600 hPa layer) are more sensitive to the model-basedprofile extension. The results are insensitive to the maximum allowed timedifference criterion for co-location (12, 6, 3, and 1 h) and are generally insensitive to the radius for co-location, exceptfor the case where the radius is small (25 km), and hence few MOPITTretrievals are included in the comparison. Daytime MOPITT products havesmaller overall biases than nighttime MOPITT products when comparing bothMOPITT daytime and nighttime retrievals to the daytime aircraftobservations. However, it would be premature to draw conclusions on theperformance of MOPITT nighttime retrievals without nighttime aircraftobservations. Applying signal-to-noise ratio (SNR) filters does notnecessarily improve the overall agreement between MOPITT retrievals andin situ profiles, likely due to the reduced number of MOPITT retrievals forcomparison. Comparisons of MOPITT retrievals and in situ profiles overcomplex urban or polluted regimes are inherently  challenging due to spatialand temporal variabilities of CO within MOPITT retrieval pixels (i.e.,footprints). We demonstrate that some of the errors are due to COrepresentativeness with these sensitivity tests, but further quantificationof representativeness errors due to CO variability within the MOPITTfootprint will require future work.

Assessing Measurements of Pollution in the Troposphere (MOPITT) carbon monoxide retrievals over urban versus non-urban regions