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

Wenfu Tang,Merritt N Deeter,Xinrong Ren,Hao He,Sara Martínez-Alonso,Yutaka Kondo,Louisa K Emmons,Helen M Worden,Russell R Dickerson,Benjamin Gaubert,Glenn S Diskin,Rebecca R Buchholz,David P Edwards

doi:10.5194/amt-13-1337-2020

Abstract

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.

Highlights

Observations from the Measurements of Pollution in the Troposphere (MOPITT) instrument onboard the National Aeronautics and Space Administration (NASA) Terra satellite have been used for retrieving total column amounts and volume mixing ratio (VMR) profiles of carbon monoxide (CO) using both thermal-infrared (TIR) and near-infrared (NIR) measurements since March 2000
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
We find that applying the signal-to-noise ratio (SNR) filters does not significantly change the overall agreement between MOPITT retrievals and the in situ profiles used in this study

Summary

Introduction

Observations from the Measurements of Pollution in the Troposphere (MOPITT) instrument onboard the NASA Terra satellite have been used for retrieving total column amounts and volume mixing ratio (VMR) profiles of carbon monoxide (CO) using both thermal-infrared (TIR) and near-infrared (NIR) measurements since March 2000. Besides the TIRonly and NIR-only products, the MOPITT multispectral TIR–NIR product is provided, which has enhanced the sensitivity to near-surface CO (Deeter et al, 2011, 2013; Worden et al, 2010). In the Version 7 products, a new strategy for radiance bias correction and an improved method for calibrating MOPITT’s NIR radiances were included (Deeter et al, 2017). For the most recently released MOPITT Version 8 products, enhancements include a new radiance bias correction method (Deeter et al, 2019). The MOPITT products have been extensively evaluated and validated with in situ measurements, though this has been done primarily over nonurban areas (Deeter et al, 2010, 2012, 2013, 2014, 2016, 2017, 2019; Emmons et al, 2004, 2007, 2009). For the past 2 decades, MOPITT CO products have been widely used for various applications, including understanding atmospheric composition, evaluating atmospheric chemistry models, and constraining inverse analyses of CO emissions (e.g., Arellano et al, 2004, 2006, 2007; Chen et al, 2009; Edwards et al, 2006; Emmons et al, 2010; Fortems-Cheiney et al, 2011; Gaubert et al, 2016; Heald et al, 2004; Jiang et al, 2018; Kopacz et al, 2009, 2010; Kumar et al, 2012; Lamarque et al, 2012; Tang et al, 2018; Yurganov et al, 2005)

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