|This review covers the 2nd revised version of the manuscript ACP-0218 |
“Influence of aerosol copper on HO2 uptake: A novel parameterized equation” by Huang et al
This manuscript reports an explicit model of uptake of HO2 radicals to deliquesced inorganic aerosol particles to partially reconcile previous inconsistencies among experimental measurements and parameterizations. This includes a detailed treatment of the aqueous phase chemistry of HO2 and superoxide with Cu ions, as well as considering effects of Setchenov salting and ionic strength. The model is also used to interpret data from a field campaign. The previous review rounds already covered the general aspects regarding scientific relevance and significance the topic, which are undoubted. In response to previous comments related to a dataset also included to fit a parameterization, the authors of that dataset have joined the revised version of the paper and this version now includes an updated description of experimental details and revised data analysis related to that dataset. In response to another review, the authors have expanded the model description and the discussion related to sensitivity and uncertainty. Overall, this work provides valuable new information, especially the fact that taking into account the properties of the concentrated solutions of deliquesced aerosol particles allows to calculate HO2 uptake coefficients using known and well documented aqueous peroxy radical chemistry involving Cu ions. While the model includes some complexity by involving links to nitrogen oxides and and sulfur, it lacks inclusion of the interaction with the Fe(II)/Fe(III) redox couple, which usually is associated with the presence of Cu in atmospheric particles and may have important impacts. Nevertheless, the manuscript provides progress in understanding uptake of HO2 to aerosol particles.
In spite of this being the 2nd revised version of this manuscript, still a number of deficiencies exist that should be addressed. In the comment below, I list the line numbers of the pdf file of the revised manuscript version. In principle, these concerns are rather minor in character, but still numerous. They should be addressed before the manuscript may be accepted for ACP.
1) Language: the new text additions are sometimes misleading, mostly due to deficiencies in the English language. This should be fixed by a thorough work-through by the authors.
2) Abstract line 24: the IUPAC website provides a recommendation for a number of different deliquesced aerosol systems, not only cloud droplets. See also comment further below on the same aspect in the manuscript.
3) Line 26: not sure whether it is useful to have the parameterization in the abstract without explanation of symbols.
4) Line 27: The Wangdu campaign is not something the reader understands without introduction. Either explain in more detail, ‘data from a campaign in the Wangdu region’, or just ‘data from a field campaign’.
5) Lines 42-45: these sentences should be split up, and the reason for the lower reactivity in absence of transition metals should be explained, including the self reaction of HO2 that has been parameterized by Thornton et al. In addition, a language issue here: The impact of HO2 uptake is not depending on a parameterization. It is the model output, or the calculated response of some parameters to HO2 uptake that changes. This is different.
6) Line 73: symbols used in eq. (1) and (2) need to be explained
7) Same paragraph: it does not become clear enough how the uptake coefficient was retrieved from the model output. This should be briefly described here.
8) Line 106: some numbers should be given here. What is the ratio between H^cc and H_0 over the RH range considered in this study? Same sentence: why does H^cc depend strongly on the Cu concentration? The dominant solute is ammonium sulfate, which should be the main driver of salting and ionic strength, isn’t it.
9) Line 115: Two things here: the solubility of Fe is not a defined quantity, the authors may refer to the solubility of Fe-containing minerals. There are literature reports about what fraction of Fe is typically in dissolved form. In addition, there is a language issue here (many similar cases throughout the manuscript), the sentence reads like the solubility of Fe is related to its ratio to Cu, which is certainly not true.
10) Line 118: Neglecting the presence of Fe should lead to a stronger caveat for this work. The authors cite the Mao et al. (2013) work a few times, which clearly indicates a strong impact of the Cu/Fe ratio on the fate of HO2 products in the aqueous phase. Fe may reduce the contribution of recycling of peroxy radicals to lower the effective HO2 loss rate, and Fe might be relevant for the interpretation of the effect of HO2 uptake on the HOx budget in conjunction with the field data.
11) Line 135: in eq. (10) and (11), the subscript ‘equ’ is misleading, since it should refer to ‘effective’ and not ‘equilibrium’ or ‘equation’. So ‘eff’ would be better.
12) Line 148: this paragraph is not sufficiently clear. k_eff seems to be the apparent first order loss rate coefficient of HO2 in the aqueous phase (not involving solubility). Typical numbers should be provided; otherwise it would not be understandable, why a 1 M Cu(II) solution would not lead to a very short reacto-diffusive length and thus strong concentration gradients in HO2. Also the reasons for the apparently low values should be explained, as this must result from complex recycling occurring, since the first reaction of Cu(II) with HO2 is very fast.
13) Line 158: the value of the accommodation coefficient is not related to the amount of Cu. The amount of Cu may control the uptake regime, with large amounts leading to uptake becoming accommodation limited. But the value of alpha is independent of Cu, unless it is involved in the process of surface to bulk transfer of HO2. Are the authors confusing accommodation coefficient with uptake coefficient? Also Table 1 is misleading, as the data in column seem to be uptake coefficient, as indicated in the Table caption, but inconsistent with the column header.
14) Line 162: probably related to the previous, the high accommodation coefficient for HO2 does not automatically mean it has a high loss rate. It only means that gamma may get large, if a strong sink is available in the condensed phase.
15) Line 164: language: The MARK model probably does not make the selection of alpha, but the authors selected it.
16) Line 167: as mentioned before, please clarify the column header of the table and the caption (alpha or gamma)
17) Line 172: first sentence: language!
18) Line 178: SMPS = Scanning Mobility Particle Sizer or Scanning Mobility Particle Spectrometer
19) Line 179: aerosol particles were produced using a …
20) Line 198 and following: while the model is indeed used to explore the RH dependence of HO2 uptake and associated aqueous phase chemistry, the comparison to experimental data is not really covering a substantial RH range. So that is essentially limited to the effect of the Cu(II) concentration.
21) Line 230: HO2 uptake at low Cu content: the figure should be plotted in log y-scale to demonstrate that the remaining uptake at low Cu content is driven by self reaction of HO2 (should be second order in HO2).
22) Line 231: What does the sentence ‘The threshold is also consist in …’ mean? What are droplets?
23) Line 233: I understand that increasing Cu content drives uptake to the accommodation limit; but why should that be determined by the solubility?
24) Line 254: the Thornton et al. (2008) parameterization was developed for deliquesced aerosol particles, not cloud droplets. This repetitively comes up below again.
25) Line 262 and following: as mentioned above, the Thornton et al. and IUPAC recommendation has not been suggested for dilute aqueous droplets but actually for deliquesced aerosol particles. As discussed in Thornton et al. but also in the comments accompanying the IUPAC recommendations, the fact that k_TMI is lower than the actual known rate coefficient of HO2 or O2- with Cu(II), is assumed to likely result from the combined effects of solute strength effects. It is indeed the added value of this work to make this aspect more quantitative. The authors could emphasize this somewhat more to detail the individual contributions of Setchenov salting, of ionic strength and of the recycling efficiency among the cupper and peroxy species to the reduction of the effective rate coefficient. The advantage of eq 15 is that it correctly represents the transition between the reacto-diffusive regime (when k_eff is higher) to the homogeneous bulk reaction regime covered by the parameterization suggested in this work (eqs 18-21)
26) Line 310 and following: the authors should clearly state that this parameterization is only reasonable as long k_eff remains sufficiently small, such that no HO2 gradients within particles develop. While they seem to show that this is valid with the mechanism involving Cu only, it is not granted that this is still true when for instance including Fe ions in the mechanism which could leading to an increasing sink for HO2 if the recycling efficiency is shut off; similar effects may occur in presence of organics.