The manuscript has improved and the authors did a better job in explaining similarities to previous work. However, I still have several comments before the manuscript may be accepted for publication.
The line numbers in my comments below refer to the marked-up manuscript that was attached to the response to the reviewers.
1) The abstract is rather lengthy, in particular in comparison to the introduction. I suggest shortening the abstract.
2) In general, the paper is very long and tedious to read. I suggest trying to shorten it where possible. I point out some repetitive paragraphs below.
3) While discussing the commonalities of CAPRAM and CLEPS, the authors make it sound as if there is only one set of rate constants for inorganic and many organic reactions. A look at the NIST data base ‘Solution Kinetics’ shows that for many reactions there are multiple values with sometimes great discrepancies. CAPRAM 2.4 was built and revised and includes recommended values. This should be noted here.
4) In some parts, it still seems to me that important previous work was not considered. In a detailed analysis, it has been shown that the role of radicals other than OH for oxidation of organics in the aqueous phase is minor [Ervens et al., 2003a]. This should be at least mentioned and the addition of these radicals to the current mechanism justified. Has an analysis be done that shows that any reaction rates with other radicals that exceed those of OH?
5) Doussin and Monod were by far not the first ones who published SARs for OH reactions with organics. Many of the data in their SAR were taken from the data set that was used earlier for similar SARs [Ervens et al., 2003b; Herrmann, 2003].
6) There are several SARs available for NO3 and SO4- reactions. They should be referred to and discussed in 3.4.2 etc [Herrmann, 2003]. These SARs should be used instead of omitting reactions for SO4- and NO3 since this leads to a bias in the treatment of organic sinks.
7) Now the authors point out at several places that the cloud period of 12 hours is not realistic. Based on these statements, the ‘test’ of the model (p. 1, l. 40) should be better justified. Why not using shorter cloud periods of one hour or less as it has been done in previous model studies? Such tests do not require any sophisticated microphysics since it can be handled just like the performed model run by switching on the cloud for a realistic time after some initialization period. Given that the cloud is unrealistic, the comparisons to ambient measurements (p. 17, l. 34) does not seem very meaningful.
8) Some more details on the O/C calculation (end of Section 6.3) should be given. The end result largely depends on the initial O/C ratio. It has been shown previously that the O/C ratio of background aerosol can be enhanced due to cloud processing. This is not a new finding. Given the unrealistic cloud here, the resulting value is less important than the trend which has been shown by [Schrödner et al., 2014]. I do not agree with the authors that this reference is not relevant in this context. Also the statement in the conclusion about O/C ratio (p. 19, l. 8) should be discussed in the context of previous studies that have shown the same trends [Daumit et al., 2013; Ervens et al., 2014].
Minor comments
p. 2, l. 14: ‘phase partitioning’ should be replaced by ‘phase separation’
p. 2, l. 28: It is not true that CAPRAM v.3 is the most cited aqueous phase mechanism. The earlier version 2.3 has about twice as many citations (Web of Science) [Herrmann et al., 2000]
p. 35: Define ‘dilute conditions’
p. 3, l. 17: MVK and MACR have Henry’s law constants of 6.5 M/atm and 41 M/atm. Under common fog or cloud conditions, their fraction in the aqueous phase is << 0.1% - even without salting-out effects.
p. 3, l. 31: I do not understand this. Even if all dissociated and hydrated forms are considered, I do not see how 87 chemical species yield 657 chemical forms. How can each species occur in about eight different forms (657 / 87).
p. 6, l. 30; and p. 8, l. 32: Using Bond Dissociation Energies (BDEs) does not rely on estimates, These Energies have been measured and are tabulated [Benson et al., 1968; Benson, 1976]
p. 13, l. 8-11: This new text should be rewritten: 1) The first part sounds odd. I suggest changing it to ‘We made sure that all species have am equivalent in the respective other phase, even if this species in that phase is not reactive.’ 2) The mass transfer coefficients do not determine ultimately where a species resides as they describe the kinetics of the uptake. Only the Henry’s law constant describes the thermodynamics and therefore where a species will reside.
p. 14, l. 12: ‘Kinetic parameterization’ of mass transfer or including chemical processes?
p. 14, l. 30: The mass transfer is not represented by a rate constant as this implies a chemical transformation. I suggest using ‘The mass transfer coefficients…’
p. 14, l. 31: Not all Henry’s law constants are estimated.
p. 15, l. 9: Specify whether dry or wet deposition or both.
p. 15, l. 18: Microphysics is not necessarily needed in order to simulate a more realistic cloud. In past models, reasonable approaches were taken by just switching on/off a cloud that is then present with constant LWC, drop size etc for a shorter period than 12 hours.
p. 16, l. 9, and p. 18, l. 22: Replace ‘dry’ by ‘cloud-free’. Real dry conditions (RH = 0%) are barely encountered in the atmosphere. What is the RH (water vapor concentration) during the gas-phase only runs?
p. 16, l. 30/31 and 34/35: These lines are repetitive and can be combined.
p. 16, l. 39: How is oxalic acid partitioning treated in the model? Oxalic acid has a fairly high vapor pressure. The only reason it remains in the aqueous/particle phase is the fact that is forms salts and stable complexes [Furukawa and Takahashi, 2011; Paris and Desboeufs, 2013].
p. 16, l. 39: What is meant by ‘oxidation sink for all species’? A reduction in the oxidation rates because DOC is added and therefore OH is reduced? Clarify.
p. 17, l. 1-12, and l. 26-29: These lines can be combined as they are repetitive.
p. 17, l. 17: What is meant by ‘pseudo steady state conditions’? Are they in thermodynamic equilibrium with their gas phase counterparts?
p. 18, l. 18: This statement is not true. The solubility of small functionalized organics often exceeds that one of their larger homologues (e.g. KH(glyoxal) = 10^5 M/atm vs KH(methylglyoxal) = 3000 M/atm).
p. 18, l. 36: Earlier it is said that photolysis processes are largely omitted due to the lack of data.
p. 19, l. 14/15: This text is confusing since it reads as if glyoxal etc are considered high molecular weight compound precursors.
p. 19, l. 17-19: This is not a conclusion of the current study.
p. 19, l. 38: Why are the mechanisms more useful for longer experiments? If all reactions are implemented correctly the time period or the number of phases (multiphase vs bulk aqueous phase) should not matter.
Technical comments
p. 1, l. 27: hydrosoluble should be replaced by water-soluble as it is used like that later on
p. 1, l. 39: ‘is’ --> ‘are’
p. 3, l. 21: charges missing on SO4· and Cl2·
p. 6, l. 3: replace ‘from’ by ‘by’
p. 6, l. 6: Add ‘constant’ (‘rate constant’)
p. 8, l. 30; p. 9, l. 5: reaction rates --> rate constants
p. 15, l. 5: ‘The simulation is located…’ sounds odd. Reword.
p. 15, l. 35: ‘oxidants’ --> ‘oxidant’
p. 16, l. 17 and 19: ‘They emit..’ and ‘We emit…’ is very colloquial. Reword.
p. 16, l. 21: hydrocarbons --> hydrocarbon
p. 17, l. 1: ‘sinks’ --> ‘sink’
p. 17, l. 16: ‘has’ --> ‘have’
p. 17, l. 37: Add ‘reaction’ (reactive sink reaction)
References
Benson, S. W., F. R. Cruickshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw, and R. Walsh (1968), Additivity Rules for the estimation of Thermochemical Properties.
Benson, S. W. (1976), Thermochemical Kinetics, 2nd ed., Wiley, New York.
Daumit, K. E., S. H. Kessler, and J. H. Kroll (2013), Average chemical properties and potential formation pathways of highly oxidized organic aerosol, Faraday Discussions, 165(0), 181-202, 10.1039/c3fd00045a.
Ervens, B., C. George, J. E. Williams, G. V. Buxton, G. A. Salmon, M. Bydder, F. Wilkinson, F. Dentener, P. Mirabel, R. Wolke, and H. Herrmann (2003a), CAPRAM2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application, J. Geophys. Res., 108(D14), 4426, doi: 10.1029/2002JD002202.
Ervens, B., S. Gligorovski, and H. Herrmann (2003b), Temperature dependent rate constants for hydroxyl radical reactions with organic compounds in aqueous solution, Phys. Chem. Chem. Phys., 5, 1811-1824.
Ervens, B., A. Sorooshian, Y. B. Lim, and B. J. Turpin (2014), Key parameters controlling OH-initiated formation of secondary organic aerosol in the aqueous phase (aqSOA), J. Geophys. Res. - Atmos., 119(7), 3997-4016, 10.1002/2013JD021021.
Furukawa, T., and Y. Takahashi (2011), Oxalate metal complexes in aerosol particles: implications for the hygroscopicity of oxalate-containing particles, Atmos. Chem. Phys., 11(9), 4289-4301, 10.5194/acp-11-4289-2011.
Herrmann, H., B. Ervens, H.-W. Jacobi, R. Wolke, P. Nowacki, and R. Zellner (2000), CAPRAM2.3: A Chemical Aqueous Phase Radical Mechanism for Tropospheric Chemistry, J. Atmos. Chem. , 36, 231-284.
Herrmann, H. (2003), Kinetics of aqueous phase reactions relevant for atmospheric chemistry, Chem. Rev., 103(12), 4691-4716.
Paris, R., and K. V. Desboeufs (2013), Effect of atmospheric organic complexation on iron-bearing dust solubility, Atmos. Chem. Phys., 13(9), 4895-4905, 10.5194/acp-13-4895-2013.
Schrödner, R., A. Tilgner, R. Wolke, and H. Herrmann (2014), Modeling the multiphase processing of an urban and a rural air mass with COSMO–MUSCAT, Urban Climate, 10, Part 4(0), 720-731, http://dx.doi.org/10.1016/j.uclim.2014.02.001. |
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