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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-430
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-430
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Dec 2019

Submitted as: research article | 18 Dec 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Impact of land-water sensitivity contrast on MOPITT retrievals and trends over a coastal city

Ian Ashpole1 and Aldona Wiacek1,2 Ian Ashpole and Aldona Wiacek
  • 1Department of Environmental Science, Saint Mary’s University, Halifax, Canada
  • 2Department of Astronomy and Physics, Saint Mary’s University, Halifax, Canada

Abstract. We compare MOPITT Version 7 (V7) Level 2 (L2) & Level 3 (L3) carbon monoxide (CO) products for the 1° x 1° L3 gridbox containing the coastal city of Halifax, Canada, for the seasons DJF and JJA, and highlight a limitation in the L3 products that has significant consequences for the temporal trends in near-surface CO identified using those data. Because this gridbox straddles the coastline, the MOPITT L3 products are created from the finer spatial resolution L2 products that are retrieved over both land and water, with a greater contribution from retrievals over water because more of the gridbox lies over water than land. We create alternative L3 products for this gridbox by separately averaging the bounded L2 retrievals over land (L3L) and water (L3W) and demonstrate that profile and total column CO (TCO) concentrations, retrieved at the same time, differ depending on whether the retrieval took place over land or water. These differences (ΔRET) are greatest, and most significant, in the lower troposphere (LT), with maximum mean differences of 11.4 % (14.9 ppbv, p = 0.116) at the 900 hPa level in DJF, and 10.8 % (12.4 ppbv, p = 0.005) at the surface profile level in JJA. Retrieved CO concentrations are more similar, on average, in the middle and upper troposphere (MT and UT), although large differences (in excess of 50 %) do infrequently occur. Significant (p < 0.1) TCO differences of ~ 5 % are also found in both seasons. By analyzing L3L and L3W retrieval averaging kernels and simulations of these retrievals, we demonstrate that, in JJA, ΔRET is strongly influenced by differences in retrieval sensitivity over land and water, especially close to the surface where L3L has significantly greater information content than L3W. In DJF, land-water differences in retrieval sensitivity are much less pronounced and appear to have less of an impact on ΔRET, which analysis of wind directions suggests is more likely to reflect differences in true profile concentrations (i.e. “real” differences). The original L3 timeseries for the 1° x 1° gridbox containing Halifax (L3O) corresponds much more closely to L3W than L3L, owing to the greater contribution from L2 retrievals over water than land. Thus, in JJA, variability in retrieved CO concentrations close to the surface in L3O is suppressed compared to L3L, and they decline at a significantly slower rate (surface profile level trends of −1.16 (± 0.32) ppbv y−1 vs −3.28 (± 0.68) ppbv y−1, from L3O and L3L respectively). This is because contributing L2 retrievals over water are closely tied to a priori CO concentrations used in the retrieval, owing to their lack of near-surface sensitivity in JJA, and these are based on monthly climatological CO profiles from a chemical transport model and therefore have no yearly change (surface profile level trend = −0.51 (± 0.28) ppbv y−1 in L3W). Although we focus only on the city of Halifax, our results imply potentially large differences in the results of near-surface CO analysis using the L2 and L3 datasets for other cities that are situated within a coastal L3 gridbox, among which are some of the most populous in the world.

Ian Ashpole and Aldona Wiacek
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Status: open (until 04 Mar 2020)
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Ian Ashpole and Aldona Wiacek
Ian Ashpole and Aldona Wiacek
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