the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Feb 2025
Influence of oceanic ventilation and terrestrial transport on the atmospheric volatile chlorinated hydrocarbons over the Western Pacific
Abstract. Volatile chlorinated hydrocarbons (VCHCs), key ozone-depleting substances and greenhouse gases, depend on oceanic emission and uptake for their atmospheric budget. However, data on VCHCs in the Western Pacific remain limited. This study investigated the distribution and sources of VCHCs (CHCl3, C2HCl3, CCl4, and CH3CCl3) in the Western Pacific during 2019–2020. Elevated seawater concentrations of CHCl3 and C2HCl3 in the Kuroshio-Oyashio Extension (KOE) were driven by mesoscale eddies enhancing primary productivity, while CCl4 and CH3CCl3 concentrations were mainly influenced by atmospheric inputs. Atmospheric concentrations of VCHCs decreased from coastal to open ocean areas, with terrestrial air masses from Eastern Asia contributing significantly. Additionally, atmospheric CHCl3 and C2HCl3 concentrations were positively correlated with Chl-a in the KOE region. These findings suggested that both atmospheric transport from the continent and ocean emissions could influence CHCl3 and C2HCl3 levels. However, analysis of sea-to-air fluxes and saturation anomalies showed that atmospheric transport primarily influenced atmospheric CHCl3 and C2HCl3 concentrations. The estimated sea-to-air flux indicated that the Western Pacific acted as a source for CHCl3 and C2HCl3 but a sink for CCl4 and CH3CCl3, with the potential to absorb 17 ± 2 % of CCl4 emissions from Eastern China, 7 ± 5 % from Eastern Asia, and 3 ± 1 % of global emissions. Additionally, this region accounted for 8 ± 4 % of the global oceanic absorption of CCl4. These findings underscored the Western Pacific’s key role in regulating atmospheric CCl4 concentrations and mitigating its accumulation in Eastern Asia, providing essential data for global VCHCs emission and uptake estimates.
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RC1: 'Comment on egusphere-2025-251', Anonymous Referee #1, 05 Jul 2025
Ocean plays an important role on the biogeochemical cycle of volatile chlorinated hydrocarbons. However, due to scarce data, our understanding is limited about Western Pacific. Quantitative analysis of marine CCl4 emission or sink help us to narrow or even close the gap of CCl4, thus the topic of this paper is very important.
This manuscript presents an interesting data set of measurements of CHCl3, C2HCl3, CCl4 and CH3CCl3 from both atmosphere and sea water, and present sea-to-air fluxes in globally important region where in addition very little data has been published previously. The manuscript is well structured and generally appropriately written. However, there are a number of problems mainly connected to the data analysis and interpretation that should be addressed first.
First of all, the variation of atmospheric CCl4 is with 10% and the atmospheric concentration of CH3CCl3 and C2HCl3 in only several ppts, so that the quality of sample analysis is essential for this study. The authors presented precision for CCl4 as 4% and precision of C2HCl3 is 3% in the supplement material. Generally, the precision will be much worse if the concentration is smaller. In this study, the concentration of C2HCl3 is almost two orders of magnitude lower compared to CCl4 concentration. I doubt why the precision of C2HCl3 are even better than CCl4. Can the author provide more detailed information how these precisions were achieved?
For the second, the calibration method described in lines 199-204. How much the uncertainties introduced by the dilution? For VCHCs at ppt level, normally calibration scales are applied in calibration to minimize the inconsistent between standards. I also doubt the comparison between this study and AGAGE background is misleading due to the calibration method.
For the third, the variation of atmospheric CCl4 in this study is similar to its analysis precision. So the authors need to prove the changes of atmospheric CCl4 is not caused by the measurement uncertainties and be aware not to over-interpretation the concentration differences.
For the fourth, in section 3.1, the authors ascribe the elevated concentrations of VCHCs to the influence of polluted air mass from mainland or east China may not correct. From Fig 3 and Fig 4, the trajectories of air mass with high observed concentration are generally from Siberia and Northeast China, and then pass Japan. However, from the recent publications, these regions are not the major source of VCHCs and there is no report of fluorine/chlorine chemistry located in these regions. It should be noted most of the trajectories in figure 4 did not cover North China or East China mentioned in the references.
Last but not least, both two surveys conducted in autumn and winter. Concerning the seasonal variation of marine microalgae and seawater temperature, wind speed, the sea-to-air flux obtained by this study might be bias from yearly average.
Citation: https://doi.org/10.5194/egusphere-2025-251-RC1 -
RC2: 'Comment on egusphere-2025-251', Anonymous Referee #2, 06 Jul 2025
General comments
This paper reports results of two measuring campaigns of selected volatile chlorinated hydrocarbons in the Western Pacific. Air and surface water samples were simultaneously taken to estimate the equilibrium deviation and to calculate fluxes. This is a challenging project that generated a substantial amount of data. It is interesting research, but the manuscript needs some further improvement.
Two sampling campaigns were organised. One from October 31, 2019, to December 1, 2019, and the second from October 3, 2019, to January 5, 2020. Both campaigns roughly cover a 2 - 3 months period in the same season. Therefore, seasonal effects are not included and so, the validity of the results as yearly averages is not guaranteed. In this sense ln. 716-717 in the conclusions should be interpreted with reticence.
The area covered is about 4500 km S-N and 4000 km EW at the equator. It would be valuable adding information on the number of sampling locations (they are indicated in Fig. 1 but not always very clear) and the number of samples per location. The distribution of the concentrations is graphically presented in fig. 3. It would be good adding statistical data on the concentration distributions e.g. as box plots.
Data for Henry’s law constants are taken from Schwardt (2021). Temperature dependence equations are used to correct for the seawater temperature. Schwardt used the EPICS method to determine air/water partitioning using deionised water. It is known that H also depends on salinity. Date from the literature show a 30 % of H for tetrachloromethane when salinity increases from 0 to 35 ppt at 25°C. Reported salinity of the samples in the study is roughly 35 ppt (Fig. 12). So, is salinity considered, if not what would be the effect on the calculated fluxes by increasing H values with 30 %?
The analytical methods are described. However, some detailed but relevant information is not reported: argumentation on why using GC-Ms for air samples and GC_ECD for water samples; type of GC column used for air samples; number of data points on which calibration curves and RSBs are calculated. In Table S3 information about MDL is given. The definition of MDL as used here is not reported. Is it the instrumental MDL or the overall method (including, sampling, transportation, storage sample preparation) MDL. RSDs on MDL are all below 10 % even 3 %. So, this looks more like instrumental MDL. The difference is important because this affects results of equation 9 and so the calculated fluxes.
Detailed comments
p.3 ln 51. My suggestion is avoiding subjective phrasing “very short-lived” (less than six months). Compared with CCl4 this lifetime is short but not in absolute terms. As an example, isoprene about 1 h; aromatic hydrocarbons in the order of days.
- 55 “disproportionately” compared with what?
Ln. 100-102. Li et al. (2024) Is this statement general or specific for Eastern China? The concentration distribution in fig. 4 in that reference paper (comparing sectors) is highly skewed, with mean values highly influenced by a few large concentrations. I suggest drawing the readers attention to this point to avoid pointing too fast to some specific sectors.
Ln. 202 Please add the number of concentration levels for the calibration curves.
Ln. 207 S3. Please add the number of data to calculate RSD. Which definition of MDL was used and check layout of the table.
Ln. 209. Were seawater samples taken always at the same depth? If so, which? A bit confusing that in this line there is (0-5m).
Ln. 216 For water sample analysis a GC-ECD method is used while for air it is GC-MS. Any specific reasons? For the water sample analysis, the type of GC column is specified but not for air samples. Was the same Gc column used. If not, please add information on the Gc column used for air sample analysis.
Ln. 205-252. I assume that Henry’s constant is dimensionless. Please specify. Why using concentrations in pmol/L which is equal to nmol/m3? I prefer the latter because then it fits better with the equation where F is given as nmol.m-2.d-1.
Fig. 3. A large amount of information is presented in this figure. Maybe it would be good mentioning the different concentration scales and units. CCl4 from 74-84 pptv: CH3CCl3 from 1.6-2.6 pptv. Because the same-coloured dot stands for quite different concentrations. Or even better to report boxplots for the concentration distributions e.g. as supplementary material.
Citation: https://doi.org/10.5194/egusphere-2025-251-RC2
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