|I have reviewed the manuscript in view of the interactive public discussion, looking at the reports of the reviewers, the responses and the revised MS and supplement. I am happy that the authors have in general responded satisfactorily to the reviews, and made appropriate changes to the MS. There are a few areas though where some further clarifications are needed.|
The lack of NO measurements during the campaign, and the probable local sources of NOx (soil, traffic) causing the stated deviation from steady-state (also from large concentrations of peroxy radicals), means that alternate methods of estimating NO during these periods, e.g. from NO2 measurements, seemingly are not easy to implement. This is clearly stated in the paper. Therefore focussing on the periods were both NO and NO2 measurements are available is sensible and being careful that average diel profiles of model calculations and therefore conclusions regarding the level of agreement are only generated from periods where NO data are available to constrain the model. There are NO data though for significant periods, which makes the model comparison a useful and valid thing to do. Figure 1 and Figure S3 certainly supports the idea that NO is not in photostationary state with NO2, and there are local sources (soil, traffic) – which is now clearly stated in the revised MS.
Page 7 (revised MS) lines 1-6, it is not clear why just HO2* was measured? I realise this means that a comparison with the ethane PERCA instrument (some of HO2+RO2) could be performed (the latter measuring HO2+mainly isoprene RO2 which forms an interference in HO2), and this was the subject of a previous publication, but was the NO ever switched to a low value some of the time to enable HO2 measurements periodically to compare with modelled HO2?
The OH interference, although sometimes significant, has been characterised for this instrument, and so there should be confidence that the OH measurements from OHchem are accurate. Also, it is fairly likely that HO2* is the sum of HO2 and some fraction of isoprene peroxy radicals (which is characterised in the lab and with this RO2 being the main RO2 species). From this point of view the measurement time-series of radicals of OH and HO2* is valuable, and as the data have not been over interpreted and sensible comparisons with the model have been made (i.e. where NO measurements are available to constrain the model), then this is a worthwhile contribution.
Some subjective phrases have been made more quantitative. Please check this has been done for all phrases like “good” and “better” are made more quantitative.
The correction factor of 1.4 for the OH reactivity is interesting (and determined from lab studies of known sinks) – was this seen in the intercomparison also during the ambient measurements?
In response to reviewer 2’s comment regarding P10 L5, it is stated that “the correlation in the present study was not statistically significant”.
Can this be clarified, e.g. in terms of a correlation coefficient r or something similar, otherwise again the statement is a little subjective?
There is a statement in the conclusions about the level of OH before the interference is subtracted (OHwave going up to around 9x10(6)). There ought to be something similar in the abstract. The abstract has the level of OH stated after interference subtracted (4x10(6)) but it is not possible currently to gauge the level of interference from the abstract, and this is something that ought to be there.
The following comments use Page – line number (e.g. 2-17) for the revised MS.
4-25. Bottorff et al., 2015; manuscript in preparation?? Do you mean 2020?
17-9. For the “similar concentrations of OH were observed at this site in 2017...” were these concentrations after interference subtracted - just clarify.
Figure S8 left hand side panel - shows a budget calculation of OH production, but uses HO2* rather than HO2 to calculate HO2+NO. Could there be a statement (in the caption or the main text) saying what roughly the HO2*/HO2 ratio and hence what the degree of overestimation is of HO2+NO, as there are already missing sources of OH (comparing to the loss rate OHxk(OH)) even using HO2*+NO.