Differences in MOPITT surface level CO retrievals and trends from Level 2 and Level 3 products in coastal grid boxes
<p><span id="page1924"/>Users of MOPITT (Measurement of Pollution in the Troposphere) data are advised to discard retrievals performed over water from analyses. This is because MOPITT retrievals are more sensitive to near-surface CO when performed over land than water, meaning...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-04-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://amt.copernicus.org/articles/16/1923/2023/amt-16-1923-2023.pdf |
| Summary: | <p><span id="page1924"/>Users of MOPITT (Measurement of Pollution in the Troposphere) data are advised to discard retrievals performed over water from analyses. This is because MOPITT retrievals are more sensitive to near-surface CO when performed over land than water, meaning that they have a greater measurement component and are less tied to the a priori CO concentrations (which are taken from a model climatology) that are necessarily used in their retrieval. MOPITT Level 3 (L3) products are a
1<span class="inline-formula"><sup>∘</sup></span> <span class="inline-formula">×</span> 1<span class="inline-formula"><sup>∘</sup></span> gridded average of finer-resolution (<span class="inline-formula">∼</span> 22 <span class="inline-formula">×</span> 22 km) Level 2 (L2) retrievals. In the case of coastal L3 grid boxes, L2
retrievals performed over both land and water may be averaged together to
create the L3 product, with L2 retrievals over land not contributing to the
average at all in certain situations. This conflicts with data usage
recommendations. The aim of this paper is to highlight the consequences that this has on surface level retrievals and their temporal trends in
“as-downloaded” L3 data (L3O), by comparing them to those obtained if
only the L2 retrievals performed over land are averaged to create the L3
product (L3L), for all identified coastal L3 MOPITT grid boxes. First,
the difference between surface level retrievals in L3L and the corresponding
L2 retrievals performed over water (L3W) is established for days when
they are averaged together to create the L3O product for coastal grid boxes
(yielding an L3O surface index of “mixed”, L3O<span class="inline-formula"><sub>M</sub></span>). Mean retrieved volume mixing ratios (VMRs) in L3L differ by over 10 ppbv from those in L3W, and temporal trends detected in L3L are between 0.28 and 0.43 ppbv yr<span class="inline-formula"><sup>−1</sup></span>
stronger than in L3W, on average. These L3L <span class="inline-formula">−</span> L3W differences are clearly
linked to retrieval sensitivity differences, with L3W being more heavily
tied to the a priori CO profiles used in the retrieval, which are a
model-derived monthly mean climatology that, by definition, has no trend
year to year. VMRs in the resulting L3O<span class="inline-formula"><sub>M</sub></span> are significantly different to L3L for 45 % of all coastal grid boxes, corresponding to 75 % of grid boxes where the L3L <span class="inline-formula">−</span> L3W difference is also significant. Just under half of the grid boxes that featured a significant L3L <span class="inline-formula">−</span> L3W trend difference also see trends differing significantly between L3L and L3O<span class="inline-formula"><sub>M</sub></span>. Factors that determine whether L3O<span class="inline-formula"><sub>M</sub></span> and L3L differ significantly include the proportion of the surface covered by land/water and the magnitude of land–water contrast in retrieval sensitivity. Comparing the full L3O dataset to L3L, it is shown that if L3O is filtered so that only retrievals over land (L3O<span class="inline-formula"><sub>L</sub></span>) are analysed – as recommended – there is a huge loss of
days with data for coastal grid boxes. This is because L2 retrievals over
land are routinely discarded during the L3O creation process for these grid
boxes. There is less data loss if L3O<span class="inline-formula"><sub>M</sub></span> retrievals are also retained, but the resulting L3O “land or mixed” (L3O<span class="inline-formula"><sub>LM</sub></span>) subset still has fewer data days than L3L for 61 % of coastal grid boxes. As shown, these additional days with data feature some influence from retrievals made over water, demonstrably affecting mean VMRs and their trends. Coastal L3 grid boxes contain 33 of the 100 largest coastal cities in the world, by population. Focusing on the L3 grid boxes containing these cities, it is shown that mean VMRs in L3O<span class="inline-formula"><sub>L</sub></span> and L3L differ significantly for 11 of the 27 grid boxes that can be compared (there are no L3O<span class="inline-formula"><sub>L</sub></span> data for 6 of the
grid boxes studied), with 9 of the 18 grid boxes where temporal trend
analysis can be performed in L3O<span class="inline-formula"><sub>L</sub></span> featuring a trend that is
significantly different to that in L3L. These differences are a direct
result of the data loss in L3O<span class="inline-formula"><sub>L</sub></span> – data that are available in L2 data (and are incorporated into the L3L product created for this study). The L3L <span class="inline-formula">−</span> L3O<span class="inline-formula"><sub>LM</sub></span> mean VMR difference exceeds 10 (22) ppbv for 11 (3) of these 33 grid boxes, significant in 13 cases, with significant temporal trend differences in 5 cases. It is concluded that an L3 product based only on L2 retrievals over land – the L3L product analysed in this paper, available for public download – could be of benefit to MOPITT data users.</p> |
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| ISSN: | 1867-1381 1867-8548 |