This manuscript utilizes an excellent dataset and derives many conclusions, particularly highlighting the discrepancies between measurements and poorly constrained inventories in these regions.
The measurements section requires some extra detail for clarity, and the derivation of the time-averaged CO2 and CH4 data for direct correlation with NMHC GC data needs explanation.
For the modelling some more consideration needs to be given to the predominance of the waste and agricultural sources along a narrow coastal strip of the Arabian Peninsula and closer to the ship track, while many O & G emissions, particularly oil extraction activities, are further inland.
2.2 Measurements Line 125 – water vapour contents were corrected – up to what % was recorded at the sub-tropical latitudes? Is the correction linear to this level?
2.2 Line 129 – Where do these calibration gases come from and which instrument is capable of measuring CH4 and CO in ppb to 3 decimal places? Don’t give these to more decimal places than the specified instrument precision.
2.2 Line 132 - Precisions for G2401 are based on temperature-controlled lab conditions, not a moving platform at different temps. What was the precision for the on-ship calibration runs.
Line 190 – Is this each region as defined in Fig. 1?
Section 3.2 – Through to the title of this section the only mention is of alkanes, then in this paragraph it suddenly switches to NMHC. Please be consistent, or clarify that you are referring to the same dataset for these measurements.
Fig. 5 – 4 main categories, but only 3 trend lines that don’t necessarily seem to relate to the data. Which category is the lime-green trend line referring to? Needs more explanation in the caption. This figure compares ethane from GC measurements of NMHC with CH4, presumably from Picarro measurements. How were the values of CH4 derived to make these direct correlations?
Fig.6 – Same comment as above. How are the averaged CO2 and CH4 values derived for comparison with the NMHC measurements?
3.4 Were ship engines turned off for the whole time in the harbours as no obvious evidence for filtered data?
Line 339 – Kuwait city has no natural gas network, but numerous large landfill sites. Inventory emissions from O & G activity are far-removed from the coastal region. Is being so close to big sources a good comparison with modelled inventory data? These sources would potentially be within the same 01 x 0.1° model box as the receptor point.
3.5 What about photochemical ages for the times spent in harbours? Does this suggest an even smaller source footprint than 38 km? Harbours are often in industrial areas, potentially with storage of fossil fuels and very local generation of emissions that strongly influence the observed peaks.
Line 425 – As pointed out the wind fields are at approximately 100 x 100 km scale. The winds in the north part of the Arabian Gulf are highly variable. On land the winds are dominantly from the NW. In the Gulf they often blow from the SE and these sea breezes lead to unpredictable emission dispersion along the coastal strip that may be difficult for model simulation. Less than 5 km inland the wind can be 180° different to along the coastal strip. HySplit simulations, for example, are unable to pick up these sea breezes.
Fig. 8 – Need to reposition geographic labels that don’t fit into the boxes. Lot of detail so probably better as a full-page figure.
Line 354 – should be naphtha.
Line 405 – paragraph is about CH4, but refers to Fig. 9 that is about CO2
Important study presenting CH4 and CO2 data for a key oil and gas production region – where there exists limited measurement-based characterization of emissions.
The authors present a thorough characterization of CH4 and CO2 enhancements, and simulate potential sources of emissions. The paper would be benefit from a discussion on what would be needed as next steps in terms of fully quantifying emission rates (not only characterizing the mixing ratios), this is important as towards the end the authors ask if they can verify inventories.
Line 3: change the term ‘atmospheric distribution’ or expand descrption of this term.
Line 25: Highlight short-term potency of methane. Importance to illustrate the difference in their climate impact.
Lines 40-43: Important to highlight that the fossil fuel sector haskey mitigation opportunities, likely more cost-effective than the other sectors (e.g. waste or ag). You could cite IEA methane tracker work and opportunity to reduce a significant fraction of emissions at net zero cost. I would suggest the IEA data instead of the MARCOGAZ report.
Line 46. Alvarez et al is relevant here but this is only for the US. Similarly, I would suggest rephrasing that methane emissions occur throughout the oil and gas supply chain. Alvarez et al. also shows that in the US majority of emissions are for upstream sector.
Line 55. Any reason for not mentioning Hmiel et al. here?
Suggest revising consistent use of significant figures throughout the manuscript.
Line 69. For fields discovered in Levantine Sea, are they already producing? If so, is production significant?
Line 70. I assume that these emissions estimates are from EDGAR, please mention explicitly. Can you also include estimate from UNFCCC. For this, you can use Scarpelli et al.
Line 95. In terms of referencing Yacovitch et al. Can you expand on why you do not attempt to estimate emission rates?
Lines 160-170 I suggest that you also compare your inventory results to the Scarpelli et al. gridded inventory, as this is based on UNFCCC data.
Also, do you adjust EDGAR to any changes in production during the time of the study?
Figure 4- Include units of radiative heat.
Line 220 – I would caution (or request expanding) discussion on using flaring as proxy for extraction and production sites. While this can be true, gas production fields tend to have less flaring. At the same time, oil production fields could be venting gas instead of flaring.
Line 239 – Emissions could also be related to venting, not necessarily fugitive emissions. Also, what about the correlation between CH4 and CO2 to check for combustion sources (not only correlation with NMHCs).
Line 255: methane to ethane ratio depends on gas composition, but also on source of emissions. Ratio would be different if emissions are happening at the wellhead, at a storage tank, or after a processing plant.
Line 260: For natural gas the ratio is expected to be 0.86 (indicated as horizontal line in Fig. 6). Can you expand on why is this expected for natural gas? Is this for natural gas production stage? What about other stages of the supply chain? And different gas compositions?
Line 320; Can you expand on known source locations? Is this only based on EDGAR? I would expect high density of oil and gas infrastructure relative to granularity of EDGAR inventory. Also, can you expand on how episodic emissions could be impacting your simulations (i.e. impact of super-emitters)?
Lines 340-345: It would be useful to expand on potential differences in methane emissions between oil vs gas production (or combined production). It could be hypothesized that gas fields (where natural gas in main product) could have lower emissions that oil fields (where associated gas is a co-product, often not captured).
Can you also expand on influence of onshore infrastructure (processing gas from the offshore platforms) vs emissions from offshore infrastructure?
Please see Supplement file
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