|In the first review I asked the authors to provide additional evidence on the contribution of anthropogenic nssSO4, mainly due to the fact that the authors claimed the anthropogenic contribution accounted for more than half the nssSO4 at times (Biological nssSO4 accounted for as little as 39% therefore anthropogenic nssSO4 accounted for the remaining 61%). The method used by the authors to estimate the contribution of anthropogenic nssSO4 relies on the use of the MSA to nssSO4 ratio. I disagree with the accuracy of this method based on publications showing the lack of correlation due to long range transport of particles and previously mentioned limitations of the method due to the conversion of MSA to nssSO4 (the latter has been incorporated in the text by the authors). While the authors have indicated that the method can be useful as a qualitative indicator, they have provided quantitative results. The significant contribution of anthropogenic nssSO4 indicated in this manuscript is a major claim if accurate. The accuracy of this method is further questionable because of the applied MSA/nssSO4 ratio, which is derived from Palmer station measurements. While this location is geographically closest these ratios have been shown to range .065 to .508 (Savoie and prospero 1989, Arimoto et al 1996, ) and the contrast in the biological activity in the Asmundsen Sea and Palmer Station with the Southern Ocean would suggest a single value is not a good representation especially if providing quantitative results. The complications of calculating the non-biogenic contribution of nssSO4 is also discussed in Legrand et al 2017, Legrand and Pasteur 1998 and Piel et al. 2006. Finally, the negative values of anthropogenic nssSO4 mentioned are also indicative of the limitations in the accuracy of the method. Therefore, I find it necessary to provide more evidence due to the limitations in using the MSA/nssSO4 as an indicator, or the removal of quantitative results on the contribution of anthropogenic nssSO4. |
The text below is each response to the reviewer responses to Reviewer #1.
1. No there were not anthropogenic nssSO4 concentrations in Hudson et al 1998, however, anthropogenic nssSO4 concentrations is not the only way of determining if the marine boundary layer is influenced by anthropogenic pollutants. Hudson et al. 1998 looked at particle concentration and back trajectories to identify cases that were influenced by continental/anthropogenic particles and found only the closest measurements (north of -44 S) were influenced by continental/anthropogenic particles. The cases with backtrajectories over the continent consistently contained higher particle concentrations.
It is impossible to follow the back trajectories from their starting point. There are too many for a single plot. There are only a few trajectories that have intercepted with continental sources. Do these correspond to cases with higher anthropogenic nssSO4? I suspect the main anthropogenic source would be from over continents. If the authors have other references to suggest there are other major sources of anthropogenic nssSO4 in the Southern Ocean, then they should be included in the text.
2. I do not understand your reasoning on why nssSO4 in the Amundsen Sea was formed by the condensation of DMS oxidation products onto existing particles. Sanchez et al. 2018 identified two sulfate particle types from single particle measurements and showed one, which contained mostly sulfate mass, corresponded to new particle formation in the free troposphere that was then entrained into the boundary layer. The second particle type was less than 50% sulfate and did not correspond to entrainment of particles from the free troposphere indicating it was likely from the condensation of DMS oxidation products onto pre-existing particles. I understand that new particle formation in the free troposphere is out of the scope of this paper, but with the measurements in this manuscript identifying whether nssSO4 is from DMS oxidation products condensed onto existing particles or new particle formation does not seem possible. My original reference to Sanchez et al. 2018 was intended to show that MSA and DMS do not necessarily correlate strongly with nssSO4 because of the possibility of long range transport of nssSO4 to regions where MSA and DMS are low in concentration (Korhonen et al. 2008; Woodhouse et al. 2010) and physical processes, such as the mixing of DMS and particles across the boundary layer inversion, are necessary for the formation of nssSO4 particles. Therefore, the lack of a correlation of MSA and nssSO4 should not be considered an indicator of anthropogenic influence. At least not without another anthropogenic tracer.
3. I am not sure what part of my response the authors thought was an opinion. NO3 is produced in clouds (Hegg and Hobbs 1988) and can have small marine sources (Luo et al 2018). Here are a few publications that show nitrate formation in clouds. Sulfate can also be produced in clouds (Hegg and Hobbs 1988). Therefore, your correlation of NO3 with nssSO4 in the remote southern ocean is much more likely due to cloud processing and not anthropogenic sources. It is not reasonable to use nitrate “solely as an indicator of anthropogenic contribution in this study” if the values are so low that they do not appear to be affected by anthropogenic sources. There are two spikes, A1 and A13, that could possibly be associated with anthropogenic sources. Maybe those cases have back trajectories over continents?
4. I did not misunderstand your manuscript. I understood 39%- 139% is biologically derived contribution of nss-sulfate, but this number also indicates the anthropogenic contribution was up to 61% (when the biological contribution is only 39%) and therefore, at times, more than half of the nssSO4 is anthroprogenic in the Amundsen Sea. And even higher in the southern ocean (76%). Again stronger evidence is needed to make such a claim. This may simply not be possible with the measurements made in this experiment. An anthropogenic tracer that is also formed naturally in the remote southern ocean is necessary for such an analysis.
Arimoto, R., Duce, R.A., Savoie, D.L., Prospero, J.M., Talbot, R., Cullen, J.D., Tomza, U.,Lewis, N.F., Ray, B.J., 1996. Relationships among aerosol constituents from Asiaand the North Pacific during PEM-West A. J. Geophys. Res. 101, 2011e2023.
Hegg, D. A., and Hobbs, P. V. ( 1988), Comparisons of sulfate and nitrate production in clouds on the mid‐Atlantic and Pacific Northwest coasts of the United States, J. Atmos. Chem., 7, 325– 333, doi:10.1007/BF00058708.
Korhonen, H., Carslaw, K. S., Spracklen, D. V., Mann, G. W. & Woodhouse, M. T. Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote Southern Hemisphere oceans: A global model study. Journal of Geophysical Research-Atmospheres 113, doi:10.1029/2007jd009718 (2008).
Legrand, M., Pasteur, E.C., 1998. Methane sulfonic acid to non-sea-salt sulfate ratioin coastal Antarctic aerosol and surface snow. J. Geophys. Res. 103,10991e11006.
Luo, L., Kao, S.-J., Bao, H., Xiao, H., Xiao, H., Yao, X., Gao, H., Li, J., and Lu, Y.: Sources of reactive nitrogen in marine aerosol over the Northwest Pacific Ocean in spring, Atmos. Chem. Phys., 18, 6207–6222, https://doi.org/10.5194/acp-18-6207-2018, 2018
Savoie, D.L., Prospero, J.M., 1989. Comparison of oceanic and continental sources ofnon-sea-salt sulphate over the Pacific Ocean. Nature 339, 685e687.
Woodhouse, M. T. et al. Low sensitivity of cloud condensation nuclei to changes in the sea-air flux of dimethyl-sulphide. Atmospheric Chemistry and Physics 10, 7545-7559, doi:10.5194/acp-10-7545-2010 (2010).