the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2024
Natural emissions of VOC and NOx over Africa constrained by TROPOMI HCHO and NO2 data using the MAGRITTEv1.1 model
Abstract. Natural emissions (vegetation, soil, lightning) are the dominant sources of non-methane biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NOx ≡ NO + NO2) released into the atmosphere over Africa. BVOCs and NOx interact with each other and strongly impact their own chemical lifetimes and degradation pathways, in particular through their influence on hydroxyl radical levels. To account for this intricate interplay between NOx and VOCs, we design and apply a novel inversion setup aiming at the simultaneous optimisation of monthly VOC and NOx emissions in 2019 in a regional chemistry-transport model, based on TROPOMI HCHO and NO2 satellite observations. The TROPOMI-based inversions suggest substantial underestimations of natural NOx and VOC emissions used as a priori in the model. The annual flux over Africa is increased from 125 to 165 Tg yr−1 for isoprene, and from 1.9 to 2.4 TgN yr−1 and from 0.5 to 2.0 TgN yr−1 for the soil and lightning NO emissions, respectively. Despite the NOx emission increase, evaluation against in situ NO2 measurements at seven rural sites in Western Africa displays significant model underestimations after optimisation. The large increases in lightning emissions are supported by comparisons with TROPOMI cloud-sliced upper-tropospheric NO2 volume mixing ratios, which even remain underestimated by the model after optimisation. Our study strongly supports the application of a bias correction to the TROPOMI HCHO data and the use of a double-species constraint (vs single-species inversion), based on comparisons with isoprene columns retrieved from the Cross-track Infrared Sensor (CrIS).
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CC1: 'Citations for CAMS-GLOB-SOIL', David Simpson, 10 Oct 2024
I just came across this interesting-looking paper, and noticed a few things that should be corrected concerning the CAMS soil emissions:
1. I see that the citation given for the CAMS-GLOB_SOIL v2.4 inventory is Simpson & Darras, 2021. As that paper was blocked (inexplicably in my view!) by the editor without the chance to resubmit or respond to referee comments, it is better to refer to two more recent documents:
- Simpson, D., Benedictow, A., and Darras, S.: The CAMS soil emissions: CAMS-GLOB-SOIL, in: CAMS2_61 – Global and European emission inventories. Documentation of CAMS emission inventory products, Ch. 9, pp59–70, https://doi.org/10.24380/q2si-ti6i, 2023.
- Simpson, D. and Segers, A.: Soil NO emissions, in: Transboundary particulate matter, photo-oxidants, acidifying and eutrophying components. EMEP Status Report 1/2024, The Norwegian Meteorological Institute, Oslo, Norway, 125–136, 2024, https://www.emep.int/publ/emep2024_publications.html.
2. Table 2 - cites "Darras et Simpson, 2021", which doesn't exist. The above refs are better.
3. On L146 the text says that the CAMs inventory follows Hudman, but this is not the case. The CAMS emissions related far more closely to methods developed by Yienger and Levy (1995) and Steinkamp and Lawrence, 2011, though adapted to cope with the availability of some key data.
Best Regards,
David Simpson
Citation: https://doi.org/10.5194/egusphere-2024-2912-CC1 -
AC1: 'Reply on CC1', Beata Opacka, 11 Oct 2024
Dear Mr. Simpson,
Thank you for your valuable comment. I will make sure that the correct references and information are accounted for in the revised version.
Best regards,
Beata Opacka
Citation: https://doi.org/10.5194/egusphere-2024-2912-AC1
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RC1: 'Comment on egusphere-2024-2912', Anonymous Referee #1, 28 Oct 2024
The paper provides a comprehensive overview of the NOx and VOCs sources in sub-Saharan Africa. It is well written and has a clear outline.
There is a large increase in isoprene for region 1 and region 3 in the northern hemisphere during the dry season, peaking in March for SVOC, STD and ALBE inversions. At this time and location, the HCHO columns are quite high (figure 10), and this coincide with the peak in fire emissions, which must explain part of increase in isoprene. At the same, these inversions do not capture the increase in isoprene during fall in region 1.
While the precise source attribution will not be solved in this study, you could still provide some context at whether why the isoprene columns are not well represented in both the prior simulations and inversions and provide some hypothesis(es) to guide future studies. A figure of the seasonal cycle of NOx and VOCs surface emissions by source (anthropogenic, biogenic and pyrogenic) for both the prior and the STD inversions (having the other inversions might make the plot too busy) could illustrate this issue.
Even though the increments on anthropogenic emissions are not large, it would be appreciated to add a key point about main takeaways on this subject in the conclusion section.
Minor comments:
L47: “Global bottom-up inventories for soil and lightning fluxes range from 1.3 to 6.6 TgN yr-1 (Murray, 2016; Vinken et al., 2014; Weng et al., 2020), and from 4 to 34 TgN yr-1 (Steinkamp and Lawrence, 2011; Yan et al., 2005), respectively”
You switched up soil and lightning.
L60: remove the parenthesis after “Lebel et al., 2011);”
L63: “By virtue of their global coverage and continuous monitoring, spaceborne data are great alternatives to studying air composition in Africa and are often used to constrain model emission estimates obtained with chemistry-transport models (CTMs).” At this stage of the discussion, you could say: can be used to constrain instead of are often.
L98: first definition of TROPOMI and S5P acronyms, so no need to redefine them at line 177.
L101: first instance of MAGRITTE, please define the acronym here.
L105: make it 3 sentences for section 2, 3 and 4. Please be consistent and choose either Sect. or Section.
2.1 MAGRITTEv1.1 chemistry-transport model
If you are using a limited area version of the model, you should specify how you prescribed the boundary conditions.
Citation: https://doi.org/10.5194/egusphere-2024-2912-RC1 -
RC2: 'Comment on egusphere-2024-2912', Anonymous Referee #2, 27 Nov 2024
The authors conducted adjoint-based inverse modelling to update emissions of NOx and VOC over Africa for 2019 using a chemistry-transport model MAGRITTEv1.1 and TROPOMI HCHO and NO2 retrievals. This is a robust study. Even though there are uncertainties associated with assumptions and methods in the approach, the authors conduct additional analyses (an ensemble of inversions) to evaluate and address these uncertainties. The updated emissions have also been evaluated using available observations. This study is important and informative given limited knowledge and observations of Africa NOx and VOC emissions. The manuscript is well written. I have no major concerns or comments on this manuscript. Below are only a few minor suggestions.
Please consider adding latitude and longitude to your map plots. For example, Line 393 says “decreases in soil emissions in the Northern Hemisphere”. Without latitude and longitude, it’s hard to visualize in Figure 11.
Caption of Figure 17: Please change “Histogram of average soil NO flux measurements” to “Average soil NO flux”.
I wonder if the author could add more discussion on the implication on MEGAN.
Citation: https://doi.org/10.5194/egusphere-2024-2912-RC2
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