Vertical profiles of global tropospheric nitrogen dioxide (NO<sub>2</sub>) obtained by cloud slicing the TROPOspheric Monitoring Instrument (TROPOMI)
<p>Routine observations of the vertical distribution of tropospheric nitrogen oxides (NO<span class="inline-formula"><sub><i>x</i></sub></span> <span class="inline-formula">≡</span> NO <span class="inline-formula"...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-11-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/24/13047/2024/acp-24-13047-2024.pdf |
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| Summary: | <p>Routine observations of the vertical distribution of tropospheric nitrogen oxides (NO<span class="inline-formula"><sub><i>x</i></sub></span> <span class="inline-formula">≡</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula"><sub>2</sub></span>) are severely lacking, despite the large influence of NO<span class="inline-formula"><sub><i>x</i></sub></span> on climate, air quality, and atmospheric oxidants. Here, we derive vertical profiles of global seasonal mean tropospheric NO<span class="inline-formula"><sub>2</sub></span> by applying the cloud-slicing method to TROPOspheric Monitoring Instrument (TROPOMI) columns of NO<span class="inline-formula"><sub>2</sub></span> retrieved above optically thick clouds. The resultant NO<span class="inline-formula"><sub>2</sub></span> is provided at a horizontal resolution of 1° <span class="inline-formula">×</span> 1° for multiple years (June 2018 to May 2022), covering five layers of the troposphere: two layers in the upper troposphere (180–320 hPa and 320–450 hPa), two layers in the middle troposphere (450–600 hPa and 600–800 hPa), and the marine boundary layer (800 hPa to the Earth's surface). NO<span class="inline-formula"><sub>2</sub></span> in the terrestrial boundary layer is obtained as the difference between TROPOMI tropospheric columns and the integrated column of cloud-sliced NO<span class="inline-formula"><sub>2</sub></span> in all layers above the boundary layer. Cloud-sliced NO<span class="inline-formula"><sub>2</sub></span> typically ranges from 20–60 pptv throughout the free troposphere, and spatial coverage ranges from <span class="inline-formula">></span> 60 % in the mid-troposphere to <span class="inline-formula"><</span> 20 % in the upper troposphere and boundary layer. When both datasets are abundant and sampling coverage is commensurate, our product is similar (within 10–15 pptv) to NO<span class="inline-formula"><sub>2</sub></span> data from NASA DC-8 aircraft campaigns. However, such instances are rare. We use cloud-sliced NO<span class="inline-formula"><sub>2</sub></span> to critique current knowledge of the vertical distribution of global NO<span class="inline-formula"><sub>2</sub></span>, as simulated by the GEOS-Chem chemical transport model, which has been updated to include peroxypropionyl nitrate (PPN) and aerosol nitrate photolysis, liberating NO<span class="inline-formula"><sub>2</sub></span> in the lower troposphere and mid-troposphere for aerosol nitrate photolysis and in the upper troposphere for PPN. Multiyear GEOS-Chem and cloud-sliced means are compared to mitigate the influence of interannual variability. We find that for cloud-sliced NO<span class="inline-formula"><sub>2</sub></span>, interannual variability is <span class="inline-formula">∼</span> 10 pptv over remote areas and <span class="inline-formula">∼</span> 25 pptv over areas influenced by lightning and surface sources. The model consistently underestimates NO<span class="inline-formula"><sub>2</sub></span> across the remote marine troposphere by <span class="inline-formula">∼</span> 15 pptv. At the northern midlatitudes, GEOS-Chem overestimates mid-tropospheric NO<span class="inline-formula"><sub>2</sub></span> by 20–50 pptv as NO<span class="inline-formula"><sub><i>x</i></sub></span> production per lightning flash is parameterised to be almost double that of the rest of the world. There is a critical need for in situ NO<span class="inline-formula"><sub>2</sub></span> measurements in the tropical terrestrial troposphere to evaluate cloud-sliced NO<span class="inline-formula"><sub>2</sub></span> there. The model and cloud-sliced NO<span class="inline-formula"><sub>2</sub></span> discrepancies identified here need to be investigated further to ensure confident use of models to understand and interpret factors affecting the global distribution of tropospheric NO<span class="inline-formula"><sub><i>x</i></sub></span>, ozone, and other oxidants.</p> |
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| ISSN: | 1680-7316 1680-7324 |