Accurate space-based NO<sub><i>x</i></sub> emission estimates with the flux divergence approach require fine-scale model information on local oxidation chemistry and profile shapes
<p>The flux divergence approach (FDA) is a popular technique for deriving <span class="inline-formula">NO<sub><i>x</i></sub></span> emission estimates from tropospheric <span class="inline-formula">NO<sub>2</sub></spa...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-02-01
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| Series: | Geoscientific Model Development |
| Online Access: | https://gmd.copernicus.org/articles/18/621/2025/gmd-18-621-2025.pdf |
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| Summary: | <p>The flux divergence approach (FDA) is a popular technique for deriving <span class="inline-formula">NO<sub><i>x</i></sub></span> emission estimates from tropospheric <span class="inline-formula">NO<sub>2</sub></span> columns measured by the TROPOspheric Monitoring Instrument (TROPOMI) satellite sensor. An attractive aspect of the FDA is that the method simplifies three-dimensional atmospheric chemistry and transport processes into a two-dimensional (longitude–latitude) steady-state continuity equation for columns that balances local <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions with the net outflow and chemical loss of <span class="inline-formula">NO<sub><i>x</i></sub></span>. Here we test the capability of the FDA to reproduce known <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions from synthetic <span class="inline-formula">NO<sub>2</sub></span> column retrievals generated with the LOTOS-EUROS chemistry transport model over the Netherlands at high spatial resolution of about <span class="inline-formula">2×2</span> <span class="inline-formula">km</span> during summer. Our results show that the FDA captures the magnitude and spatial distribution of the <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions to high accuracy (absolute bias <span class="inline-formula"><9</span> %), provided that the observations represent the <span class="inline-formula">NO<sub>2</sub></span> column in the boundary layer, that wind speed and direction are representative for the boundary layer (PBL) column, and that the high-resolution spatiotemporal variability of the <span class="inline-formula">NO<sub>2</sub></span> lifetimes and <span class="inline-formula">NO<sub><i>x</i></sub>:NO<sub>2</sub></span> ratio is accounted for in the inversion instead of using single fixed values. The FDA systematically overestimates <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions by 15 %–60 % when using tropospheric <span class="inline-formula">NO<sub>2</sub></span> columns as the driving observation, while using PBL <span class="inline-formula">NO<sub>2</sub></span> columns largely overcomes this systematic error. This merely reflects the fact that the local balance between emissions and sinks of <span class="inline-formula">NO<sub><i>x</i></sub></span> occurs in the boundary layer, which is decoupled from the <span class="inline-formula">NO<sub>2</sub></span> in the free troposphere. Based on the recommendations from this sensitivity test, we then applied the FDA using observations of <span class="inline-formula">NO<sub>2</sub></span> columns from TROPOMI, corrected for contributions from free-tropospheric <span class="inline-formula">NO<sub>2</sub></span>, between 1 June and 31 August 2018. The <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions derived from the default TROPOMI retrievals are biased low over cities and industrialized areas. However, when the coarse <span class="inline-formula">1×1</span>° TM5-MP <span class="inline-formula">NO<sub>2</sub></span> profile used in the retrieval is replaced by the high-resolution profile of LOTOS-EUROS, the TROPOMI <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions are enhanced by 22 % and are in better agreement with the inventory for the Netherlands. This emphasizes the importance of using realistic high-resolution a priori <span class="inline-formula">NO<sub>2</sub></span> profile shapes in the TROPOMI retrieval. We conclude that accurate quantitative <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions estimates are possible with the FDA, but they require sophisticated, fine-scale corrections for both the <span class="inline-formula">NO<sub>2</sub></span> observations driving the method and the estimates of the <span class="inline-formula">NO<sub>2</sub></span> chemical lifetime and <span class="inline-formula">NO<sub><i>x</i></sub>:NO<sub>2</sub></span> ratio. This information can be obtained from high-resolution chemistry transport model simulations at the expense of the simplicity and applicability of the FDA.</p> |
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| ISSN: | 1991-959X 1991-9603 |