Tsona and co-workers report their results of a theoretical investigations aiming the study the role of pyruvic acid (PA) in the atmospheric formation of sulfuric acid along with the formation of clusters between pyruvic acid, sulfuric acid and ammonia, as precursors of aerosols, an issue which is interesting for atmospheric purposes. This article has two parts. The first one dealing with the role of pyruvic acid as catalyst in the hydrolysis of SO3, and the second one focused on the thermodynamic and the formation paths of the clusters.  In my opinion, the issue has potential for publication in Atmospheric Chemistry and Physics. However, I have several points to address the authors.

Regarding the role of pyruvic acid as catalyst in the hydrolysis of SO3.

• As a general point, the authors take as zero of energies the energy of the separate species, namely SO3 + H2O + PA (or SO3 + 2H2O). Although numerically this correct, in my opinion this may led to confusion, as one would infer three particle collisions along the processes.
• In Figure 1 the authors report their results on the gas phase water hydrolysis of SO3, and the relative energies are collected in the supplementary information. Although the authors mention previous works regarding this issue, no comparison has been done with results from the literature
• Regarding the reaction mechanism, there are two ways in which the hydrolysis can take place.The first one, involve the reaction of SO3···H2O with PA and the second one the reaction of PA···H2O + SO3. Along the text the authors suggest that preferably proceeds by reaction of PA···H2O + SO3. From Figure 2, it seems that the authors consider only the reaction of SO3··H2O with the most stable PA  (PAtc) whereas the reaction of the naked SO3 takes place with the PAtl and Pact complexes. In my opinion the authors miss some preliminary steps in the reaction mechanisms, which may have effect on the kinetics. a) The SO3···H2O can react with both, PAtl and Pact so that these processes should be considered. b) The PAtl···H2O and Pact···H2O complexes are held together be two hydrogen bonds, one between the acidic hydrogen of PA and the oxygen atom of water, and the other between with the carbonyl oxygen of PA with one hydrogen atom of H2O. In both cases the complexes form a ring structure which should be broken to from the RCtl and RCct complexes. This requires the existence of a transition state that should be taken into account. Moreover, for instance, the pre-reactive complexes PA···H2O··SO3 (namely RC’s) arising from PA···H2O + SO3 could also decompose, without energy barrier, into PA + SO3···H2O which should be considered.
• It is not clear how the authors have considered all these reactions weighted according the concentration of the different reactants (naked or forming complexes with water). The authors do not report values of the kinetic/equilibrium constants (for instance for the rection of the reactant complexes) at different temperatures to support the discussion in the paragraph 215. This information is necessary and the values of the bimolecular rate constants should be detailed. By the same way, the discussion in paragraph 240 should be supported by the specific values of concentrations and relative humidity.

Regarding the cluster formation:

• My main point here is why the authors do not have included any water molecule in the formation of clusters. It is well known that sulfuric acid is fully hydrated at ground level in the atmosphere (see references in line 490 and ff among others) so that water should play a role in the formation of these clusters. This issue must be discussed.
• Another point is how the authors have chosen the structures to calculate the clusters. Have they performed a previous scan or a dynamic calculation to select them?

The manuscript titled "Pyruvic acid, an efficient catalyst in SO3 hydrolysis and effective clustering agent in sulfuric acid-based new particle formation" by Tsona et al. discusses the role of pyruvic acid as a catalyst in SO3 hydrolysis. The investigation of whether pyruvic acid can take part in cluster formation brings an interesting addition to the article. The paper is well written and it fits to the scope of the journal.

Specific comments:

1. I would like to see a section in the Methodology on how the cluster conformers were found for the ACDC calculations. Presumably the clusters containing only sulfuric acid and ammonia can be found somewhere in the literature, but how about the clusters containing pyruvic acid? I can see that for the hydrolysis calculations, different conformers of the pyruvic acid were used, but the clusters should have a lot of conformers due to the higher number of molecules as well as the 4 different pyruvic acid conformers, which should be considered in finding the lowest energy cluster.

2. In Fig. 5 (bottom panel), can you discuss why are the enhancement factors of 238 K at 3 and 4*10^9 PA molecules/cm-3 the same, when all other points seem to have the same linear trend with the same slope?

3. Did you run any simulation wher you would have an initial concentration of PASA, formed during the SA formation? Would this have any effect on the cluster formation results or would the PA just evaporate from the cluster, once more SA and NH3 is added through collisions?

Technical corrections:

line 21: "The enhancing effect of PA of examined by evaluating the ratio of the ternary..." There is some typo here.

line 55, 222, 245: giving -> given

line 58: acid in the troposphere -> acids in the troposphere

line 85: You mention only zero-point energies, though later you use also Gibbs free energies. Did you get them also from this calculation?

line 88: "internal reaction coordinate" should be "intrinsic reaction coordinate" for IRC.

line 134: Is there a typo in the birth-death equation, the concentration term is missing from end (the cluster evaporation sink term).

line 156: "second molecule" -> "second H2O molecule"

line 167, 213: "decomposition" is an odd choice of word to describe the formation of a larger molecule from smaller species.

line 172: "previous results studies" a typo?

line 184: "their energies" specify here and in the supplement, what energies they are. Electronic or something else?

line 184: "relati"->"relative"

line 185: "that in during"->"that during"

line 251: Should this be Table S2? That's where the formation energies are. Also, the table caption has one extra superscript in "mol"

line 252: "The binding of PA to SA exhibits similar strength within 1 kcal mol-1 to the binding between two SA molecules" If this means that the energies of PASA and SA2 are within 1 kcal/mol, it should say "within 1.5 kcal/mol".

line 294: "(1 tam)"->"(1 atm)"

line 343: Is there some author missing from "NTT and analysed"?