|Review of Le Breton et al.'s resubmitted manuscript (acp-2018-9-manuscript-version3.pdf)|
I thank the authors for having taken many of my suggestions in consideration and having made many appropriate changes. This manuscript will require further edits before it can be considered for publication as the present version seemed to have been submitted in a bit of a rush. It would have been nice to see a marked-up version of the paper that indicated what precise changes were made (there was a marked-up version but of another paper attached to the author-response-version2.pdf file).
1 - Many of the "measurements" (HCl, Cl2, ClONO2, HOCl, ClO, OClO, CMBO, IOPEX, ...) continue to be presented with units of mixing ratios, even though the instruments making these measurements have not been properly calibrated for these compounds.
For example, the authors stated in their response letter that "Le Breton et al 2017 showed that inorganic halides have a similar sensitivity. Furthermore, the comparable sensitivity for chloroacetic acid and ClNO2 emphasis a similar sensitivity for chloride containing species when applying the iodide ionisation". It is unclear if this paragraph refers to the 2017a or 2017b paper and what is meant by the terms "similar" and "comparable", since these are qualitative, not quantitative descriptors.
It'd be OK if the data were presented as raw data with units of Hz or counts and the subsequent discussion of the data (including that of Cl VOCs) adjusted accordingly. The use of concentration or mixing ratio units, however, is not justified.
2 - The calculations of Cl and OH concentrations using simplistic steady state equations remain questionable. For starters, some of the input parameters are highly uncertain (see point #1 above). There are also some pretty coarse assumptions being made about the Cl and OH hydrocarbon reactivity at this site.
Yes, one can calculate concentrations using rough approximations and steady state assumptions ("In previous work (Bannan et al., 2015) we have shown that it is possible to calculate Cl atom concentrations using simply state expressions"). However, simply having these type of calculations before does not mean that this approach gives accurate results (as the text in the supplemental suggests, see specific comment below).
Furthermore, just because a past steady state calculation "agreed well with the MCM" does not mean it will agree for other data sets (not sure what "agreeing well" even means in this context). Besides, chlorine chemistry is a recent addition to the MCM and is likely still incomplete, such that it hardly constitutes a gold standard.
The manuscript text should be revised further to reflect these uncertainties.
3 - some "action" items identified by the authors themselves have not been completed - for example, reactions are still not numbered consecutively (see specific comments below), entire paragraphs appear without sub- and superscripts, the references continue to have doi's missing, plus there are still numerous grammatical errors that need be corrected.
pg 1 / lines 29 and 30. Use of pptV and ppt is inconsistent. The acronym ppt should be defined as it could be misunderstood as part per thousand.
pg 2 / line 29. "The liberated chlorine will predominantly react with VOCs"
This should be one of the reactions listed above. Cl will more quickly abstract a hydrogen from a hydrocarbon (or add to a double bond) than react with O3.
pg 2 / line 31 HO2, RO2 etc. should have subscripts
pg 3 / line 7 - "The oxidation mechanism of saturated hydrocarbon (R12-R13) is"
pg 4 / lines 7-12 A major factor (which should be acknowledged here) is the availability of aerosol chloride, which varies considerably between measurement locations.
pg 5 / lines 6-9. Thanks for adding more detail on the photolysis rate measurements. Please state whether this a commercial instrument, how and when last it was calibrated, and clarify the statement "The photolysis rate of any given species was calculated by normalizing to the cross section and quantum yields" as this doesn't sound right.
pg 5 / lines 10-20 Numerous grammatical errors in this paragraph. Please correct.
Please state the uncertainty of the PTR-MS data.
line 38. N2 should have a subscript (twice).
pg 6 / lines 3-21 Numerous sub- and superscripts missing in this paragraph
line 25-26 " but is a commonly applied method within the CIMS community "
This is bad practice and done only by certain groups - most in the community strive to calibrate their instruments properly. Please remove this phrase.
pg 8 / line 5 "The high level of agreement". Be quantitative and state the level of agreement
pg 8 / lines 8-15. A comparison of N2O5 mixing ratios would be more meaningful if the production rate of NO3 (i.e., NO2 & O3 concentration) and the steady state lifetime of N2O5 were provided for context.
pg 8 / lines 19-. Seems like the authors gave up on sub- and superscripts entirely at this point.
pg 11/ line 28-35. There are 15 reactions (numbered R1-R15) listed in the introduction on page 2 and 3. Some of the same reactions stated again but numbered differently here.
pg 12 / line 7 " Steady state calculations reveal a sharp rise of chlorine atoms produced at sunrise peaking at 1.6x105 molecules cm3 around 7 am which then gradually decreases, contributing to Cl atom production until 2 pm (Figure 7a). "
The text gives a chlorine atom concentration, whereas Fig. 7a only gives the production rate. Consider showing the magnitude of the assumed sink (chlorine reactivity in 1/s, next to OH reactivity in 1/s) in the supplemental and adding a disclaimer, such as "If a Cl sink as shown in Fig. X and a steady state w.r.t. production and loss are assumed, ..."
pg 15 / line 21 "enabling an average daytime peak mixing ratio of chlorine atoms of 1.6x105 molecules cm-3." Stating a number for actual Cl atom concentration is not justified here given the large uncertainty with the magnitude of the hydrocarbon sink for Cl.
Supplemental / pg 1 / line 20 " it has been shown that using these emissions it is possible to estimate the Cl atom production" The statement, as written, suggests that such calculations give accurate results. This has not been shown. Please rephrase.