Enhancing Potential of Trimethylamine Oxide on Atmospheric Particle Formation

Round 1

Reviewer 1 Report

Overall I found this manuscript to be very well written and very interesting to read. The methods used are appropriate and comprehensive.  My minor comments mainly concerns the atmospheric relevance of TMAO and how strong statement that can be made concerning the atmospheric relevance of TMAO at this point. I recommend that the manuscript should be accepted for publications when the authors have addressed my minor comments.

Abstract, line 7-8: “Cluster growth occurs close to the acid:base ratio of 1:1, which is

same than for other monoprotic bases.”

Do you mean: “Cluster growth occurs close to the acid:base ratio of 1:1, which is the same as for other monoprotic bases.” ?

Last sentence in the abstract: “Therefore, at relatively low concentrations and high temperatures, guanidine and trimethylamine oxide are likely to dominate particle formation events over amines.”

Can you make this strong statement without knowledge about the concentration levels? Has anyone measured trimethylamine oxide or guanidine in the atmosphere?

I would probably write: “Therefore, at relatively low concentrations and high temperatures, guanidine and trimethylamine oxide may dominate particle formation events over amines.”

Or clarify a bit more what you mean with this statement. Do you expect this to generally be the case in the atmosphere?

Introduction:

L17-18: ”Atmospheric aerosol particles are known to affect human health, and they remain as one of the leading uncertainties in global climate modeling and prediction.”

What “prediction” do you mean? Should it maybe be “global climate modeling predictions”?

L31-33: “Since the Earth’s atmosphere is oxidizing, amines are subject to oxidation, thus leading to oxidation products that might have significantly different roles than amines in particle formation”

Yes but will not the very short lifetime of amines in the gas-phase limit the gas-phase oxidation? In the experiments e.g. by Murphy et al. Atmos. Chem. Phys., 7, 2313–2337, 2007 which you refer to the TMA concentration (~100 ppb) is in excess compared to the acids (in this case HNO3) and therefore stay in the gas-phase for long time. But I expect that this will rarely be the case in the atmosphere.   

L47-48: “In addition to atmospheric oxidation processes, amine oxides might also be emitted directly into the atmosphere. TMAO is a significant nitrogen source for marine bacteria and it has been detected at nanomolar concentrations in oceanic surface waters”

This is interesting. Maybe TMAO can be released into the gas-phase when high pH ocean water droplets evaporates.     

L189-L190: “For instance, sulfuric acid and dimethylamine form particles with an acid:base ratio of 1:2 when particle size reaches 12 nm. In the same study, sulfuric acid and ammonia did not form fully neutralized particles smaller than 12 nm in diameter.”

Will not the acid:base also depend on the amount of acid and base in the gas-phase?

Figure 9. Why do you define the H2SO4 concentration in molecules cm-3 and the base concentrations in ppt? Would it not be more relevant to define base concentration levels in molecules cm-3 too?  

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled “Enhancing Potential of Trimethylamine Oxide on Atmospheric Particle Formation” by Myllys et al. computationally investigated the enhancing potential of TMAO compared to the previously studied guanidine and DMA. In addition, an experimental method was used to corroborate the computational results. A very thorough investigation was done by comparing multiple indicators of the enhancing potential instead of just the binding energies. In addition, the authors attempt to find a classification that would explain the enhancing potential of the different bases with sulfuric acid. The manuscript is well structured and easy to follow.

I have one major comment about the gas-phase basicity calculations. Even though it’s a minor part of the calculations, it’s important to the conclusions of the paper in trying to find a simple value that could be used to classify the enhancing potential of other bases.

The GB energy difference between TMAO and GUA is only 1 kcal/mol. Is it possible that the calculated values have some errors and the order in the energies of TMAO and GUA could be opposite? I’m assuming that they were calculated with the same level of theory as the cluster energies (since, there’s no reference for the energies). Since the GB calculation only needs the energies of the monomers, the single-point energies is be possible to compute with a more accurate method, for example CCSD(T)-F12. In addition, the GB values should be positive. Also, did you check if switching to proton affinities makes a difference in the order?

Some minor corrections:

- Line 37: You say “latter”, even though you only mention one compound (TMA).

- Line 134: Do you mean intramolecular bonds? Or intermolecular between DMA and what?

- Line 144: … the formation of 2SA2TMAO cluster from either 1SA2TMAO or 2SA1TMAO? Same for line 150.

- Figure 3: What are the free energies in this figure? The formation energy of the cluster from monomers?

- Line 219: “This indicates that (add: out of the three bases), GUA is the strongest…” to make it clear that you’re only comparing the three bases.

-Line 262: “the enhancing efficiency of TMAO and DMA changes with varying vapor concentrations.” should be “whether TMAO is more enhancing than DMA depends on the vapor concentrations” or something similar. Since you just said that the enhancing efficiency of TMAO doesn’t depend on the vapor concentration when [base]>0.1ppt.

- Line 360: Concentration of which vapor, acid or base?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf