|The authors have addressed most of the issues raised in the first round of reviewing, and the manuscript has improved significantly. There are, however, a few important points to clarify before the manuscript can be accepted for publication.|
The term “London van der Waals” is used throughout the manuscript to denote the enhancement of molecular collisions due to intermolecular interactions. While possibly the appropriate term for the large particles studied by Chan and Mozurkevich, the dominant interaction between two sulfuric acid molecules is certainly not the weak London van der Waals force (which scales as r^-6), but the interaction between the large (about 3 Debye) permanent dipole moments (which scales as r^-3). Thus, “London van der Waals” should, when discussing the specific case of sulfuric acid collisions, be replaced either by “dipole-dipole” or (should the authors prefer a more technical term), Keesom van der Waals (see for example https://en.wikipedia.org/wiki/Van_der_Waals_force). Dipole-dipole interaction is, however, more informative for large fraction of the readers.
The computational values for single hydrates from Ding et al (2003) listed in Table 2 are not directly comparable to the experiment-based values reported in this study and by Hanson and Lovejoy. Instead the experiment-based values represent ensemble averages over different the hydrate distribution of the evaporating cluster. I would recommend adding these averages to Table 2 to allow direct comparison, as they can easily be computed using the data of Ding et al. as explained for example by Paasonen et al (2012),
The authors should provide a plausible physico-chemical explanation for why they assume that the formation of H2SO4(NO3-) would proceed at the collision rate, while H2SO4(HSO4-) would not (despite a greater binding energy, a greater number of degrees of freedom, and the complete absence of any bond-breaking processes in the cluster formation reaction). Alternatively, they should at least state that there is no plausible mechanistic explanation for this assumption.
The large uncertainties in the literature estimates of collision frequencies should be clearly stated, especially since in their response the authors state that the collision rate between H2SO4 and HSO4- is a key parameter in their mass spectrometric data analysis. Presumably the collisions parameters used by Hanson and Lovejoy in their instruments data analysis also affect Hanson and Lovejoy dataset and thus the comparison of that data to the data reported in the present manuscript. The kinetic limit is also affected by the neutral–neutral collisions frequencies as well as the wall loss rates used in the model for the molecules and clusters. Since these parameters may have rather large uncertainties, comparing the measurement data to the calculated kinetic limit can not be used to draw solid conclusions about fundamental quantities such as collision or sticking frequencies.
The authors should make sure they use the terms ‘equilibrium’ and ‘steady-state’ consistently: in general steady-state in any time independent situation, whereas equilibrium refer to thermodynamic equilibrium in which there no net growth flux of clusters.
Table 3: The wall losses (and possibly also their size dependence) in the Hanson and Eisele flow tube experiment are likely to be considerably larger that in the CLOUD experiment. Has this been taken into account in the modeling? Also it should be noted that the vapor concentration in the Hanson and Eisele experiment are so high that the evaporation is likely to play a minor role and thus even a simpler model with evaporation neglected might describe this type of experiment rather well.
Page 15, line 20: ‘normalized’ should be replaced by ‘interpolated’
Page 15, lines 28-31: It is somewhat confusing that the authors first refer to the the quantum chemical calculations of Temelso et al. and Henschel et al. and then choose the quantum chemical data of Ding et al for their hydration model. Are the significant differences between the newer Temelso data set and the rather old Ding dataset? Why has the older Ding data been chosen?
Page 23, line 19: It should be stated that the data analysis concerning sulfuric acid-water clusters relies on the assumption that clusters containing more than two sulfuric acid molecules have negligible evaporation rates compared to the two acid clusters.
Page 24, lines 3-4: to make it clear that the model has been fitted to match the date, I would reformulate the sentence ‘Using the proposed model, measured dimer concentrations in the ternary system can be reproduced with a high accuracy for the conditions of this study’ as ’Using the proposed model with the evaporation rate of the sulfuric acid- ammonia as a fitting parameters, the measured dimer concentrations in the ternary system can be reproduced with a high accuracy for the conditions of this study’
Paasonen et al. (2012), On the formation of sulphuric acid - amine clusters in varying atmospheric conditions and its influence on atmospheric new particle formation.
Atmospheric Chemistry and Physics, Vol 12, pp 9113-9133.