the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Dec 2024
Natural and anthropogenic influence on tropospheric ozone variability over the Tropical Atlantic unveiled by satellite and in situ observations
Abstract. Tropospheric ozone over the South and Tropical Atlantic plays an important role in the photochemistry and energy budget of the atmosphere. In this remote region, tropospheric ozone estimates from reanalysis datasets show the largest discrepancies. The present study characterises the vertical and horizontal distribution of tropospheric ozone over the South and Tropical Atlantic during February 2017 using a multispectral satellite approach called IASI+GOME2 and in situ airborne measurements from the Atmospheric Tomography Mission (ATom). These observations are compared with three global chemistry reanalysis products: the Copernicus Atmosphere Monitoring Service reanalysis (CAMS reanalysis), the Tropospheric Chemistry Reanalysis version 2 (TCR-2), and the second Modern-Era Retrospective Analysis for Research and Applications (MERRA-2). The CO-enriched air masses from Western and Central Africa are lifted into the middle and upper troposphere over the ocean by strong upward motions. In the descending branches of the Hadley cells over the Southern Atlantic, stratospheric intrusions are observed. Air masses in the Southern Hemisphere are influenced by biomass burning sources from Central and Eastern Africa and lightning, as well as downdrafts from the stratosphere. According to in situ measurements of chemical tracers, tropospheric ozone attributed to biomass burning emissions of ozone precursors is approximately 13 ppb (~17 %) over 7 km (25° S–5° N) and approximately 38 ppb (~50 %) over 3 km (25° S–15° S). The intercomparison suggests a significant overestimation of three chemistry reanalysis products of lowermost troposphere ozone over the Atlantic in the Northern Hemisphere because of the overestimations of ozone precursors from anthropogenic sources from North America.
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RC1: 'Comment on egusphere-2024-3758', Anonymous Referee #1, 15 Jan 2025
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In “Natural and anthropogenic influence on tropospheric ozone variability over the Tropical Atlantic unveiled by satellite and in situ observations” Okamoto et al. discuss ozone levels over the Atlantic Ocean based on satellite observations, reanalysis products and in situ observations from the ATom 2 campaign in February 2017. The authors show that ozone can be attributed to various different sources in the studied region including biomass burning, urban influences and stratospheric intrusion. They report an overprediction of ozone in lower altitudes in all investigated reanalysis products.
This is an interesting study. However, I have several questions and comments, which need to be addressed before I can recommend this paper for publication. The manuscript is very long and contains too many figures. I recommend creating a Supplement and moving some figures and explanations there. Overall, the manuscript would benefit from being more concise. I further suggest including the other three ATom campaigns in this study. The authors investigate seasonality and as the ATom deployments took place in all seasons and all cover the studied region, the in situ data will be a valuable addition. While the authors present the differences in the reanalysis products for only two days in February 2017, the comparison with further in situ data could help to draw general conclusions on the performance of these products. Please find my detailed comments in the following.
Lines 35 f.: Stratospheric intrusion only contributes a small fraction to the overall tropospheric ozone (Lelieveld & Dentener, 2000, doi: 10.1029/1999JD901011)
Line 41 f.: biogenic sources? – There seems to be a word missing. I further recommend briefly discussing the formation mechanism of ozone from these precursors here (or in the previous paragraph).
Lines 45: MOZAIC?
Line 48: Several studies have shown that lightning is the dominant source of ozone in the upper troposphere, e.g. Schumann & Huntrieser, 2007 (doi: 10.5194/acp-7-3823-2007) Nussbaumer et al., 2023 (doi:10.5194/acp-23-12651-2023). Please consider citing this literature.
Line 54: Over the tropics, 9-13km is still tropospheric. The UTLS region only applies to Southern or Northern Extratropical Latitudes.
Line 57: What do the authors mean by “the observational gap of air pollution” and how does it relate to the tropical Atlantic?
Line 69: What “major outbreaks” are the authors referring to?
Lines 181 f.: Please briefly describe the measurements / instruments including uncertainties and detection limits of the trace gases used in this analysis.
Lines 187 f.: “(…) they use a pair of HCN biomass of a burning tracer and C2Cl4 of an urban tracer.” This sentence is difficult to read, I recommend rephrasing.
Line 190: The conditions for polluted and aged air are the same. I assume the first “<” should be “>” instead? Is well-mixed and aged air equivalent to clean / unpolluted air? What’s the lifetime of HCN and C2Cl4 in the troposphere?
Line 195: Are the lifetimes of X and CO similar? If not, does this introduce errors in this method due to different transport ranges?
Line 198: I recommend adding the definition of the emission ratio here as well.
Line 202: Is this equation used for all measurements and if yes, why were the air masses classified into the four categories before? Or is it only used to distinguish the sources of ozone in air masses defined as “mixed pollution air”?
Figure 2: This manuscript has many Figures. I recommend choosing one or two months (for example February and August) and showing the lightning intensity and the fire radiation power in one Figure. The remaining panels can be shown in a Supplement. This makes the comparison of the location of maximum lightning and maximum fire activity easier and reduces the number of Figures.
Line 291: Do the coordinates describe a box and therefore also include continental regions? If yes, does it maybe make more sense to only consider the maritime regions? It could help to add an outline of the discussed area in the Supplement or as a subpanel in Figure 3.
Figure 3: Is the SD by itself really meaningful in regard to the uncertainty of the reanalysis products / satellite observations? Maybe it would make more sense to look at the SD relative to the O3 measurement (SD/O3value*100%). I also recommend adding the mean monthly O3 in the Figure.
Line 332: How was this altitude range chosen? The chemical composition at 6km vs 12km is very different. While upper tropospheric ozone is strongly impacted by deep convective updraft as well as lightning activity, the free troposphere is rather decoupled from the surface and is not impacted by lightning. I recommend looking at the free troposphere, e.g. 3 - 10km, and the upper troposphere, e.g. >10 km, separately.
Section 3.2: This section makes me wonder why the authors limit themselves to one of the ATom deployments. The four deployments cover the same area during all seasons and could provide some more insights into the discrepancies with the reanalysis products.
Figure 6 + 7: These Figures (including their description in the text) are a repetition of Figures 4 and 5 and I recommend deleting them or moving them to the Supplement.
Lines 400 ff.: It is difficult to see the differences and similarities between the in situ observations and the satellite / reanalysis products just by the color. Maybe instead (or additionally) a vertical profile with averaged ozone could be used (Altitude as y-axis and Ozone mixing ratio as x-axis). Or an additional column could be added to the Figure showing the difference between the ATom2 data and the other products in each panel.
Line 407: “in the” is repeated.
Table 2: Why was only the latitude band between 10°N and 30°N investigated?
Figure 9: Similar to O3, it is difficult to rank the agreement between the ATom2 data and the other products just by color. I recommend using a more quantitative method.
Line 466: “not depicted by any”?
Figure 10: See comments above.
Lines 482: In the upper troposphere, lightning NOx is the most important source of O3. Could it make more sense to define lightning as an individual category?
Line 503: Can you briefly explain how “the probability of boundary layer influences” is obtained?
Lines 523: The results for CAMS and MERRA-2 could be added to the Supplement.
Figure 14: How was this plot generated? Some of the colored points were previously categorized as e.g. urban influence and not as mixed pollution. Please clarify this. What are the gray data points? Maybe instead of the absolute values, the relative contribution to the overall O3 could be shown.
Figure 15: I recommend moving this Figure to the Supplement.
Line 637: This hypothesis could be tested by calculating ozone loss terms along the ATom2 flight track.
Lines 657 f.: This sounds like the reanalysis products only show overestimations for urban air masses? I understood earlier that the downward motions in the Hadley cell are overestimated in the reanalysis products leading to an overestimation in ozone. Could you please clarify this?
Citation: https://doi.org/10.5194/egusphere-2024-3758-RC1
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