|I thank the authors for the replies and the work they’ve put into it!|
Nevertheless, the case presented for the refractory OM is still not convincing. The explanation provided does not exactly rule out the possibility of the overload by the sea salt. Indeed, concentrations up to 100 µg m-3 or higher can be measured by the instrument, but these refer to flash vaporising species. So, in normal conditions, where mass concentration is dominated by the non-refractory flash vaporising species, the instrument can measure such high concentrations, but it has never been shown that it is similarly capable of measuring such high concentrations of slow vaporising species. Quite on contrary. The fact that background (filter) OM did not show significant changes over the course of the campaign (Figure R1) does not exactly point to the lack of the overload (sea salt m/z’s would be more applicable, like the background signal of NaCl at m/z 58 and Na at 23, to see the overload issue), but, actually, it shows that there was no significant amount of refractory OM. E.g. if refractory OM contribution is so much higher in the second part of the campaign would it not manifest in the higher background OM concentrations? But now, the OM behaves exactly as SO4 for the part when refractory OM is not anticipated and the part where it is?
Moreover, looking at figures 3, S3 and S13, I struggle to see the coherent picture. Why S3 has fewer points for filter than S13? E.g. S13 has filter OM data since 16th May, while in S3 filter points start after 19th? Big emphasis is attributed to the discrepancies between ACSM and Filter/DMPS at the end of the campaign, however, there are similar if not larger discrepancies between filter and ACSM OM at the beginning of the campaign (16th-21st May) and these are not reflected in the volume comparisons. The filter and ACSM volumes seem to agree for the period where Filter-ACSM OM disagree by a lot (16th-21st May). While looking at Figures 3 and S3, it seems that ACSM volume is larger than DMPS volume for the period where ACSM OM is much lower than the filter OM (19th-20th May)?
Also, the ‘oversampling’ (~19th May and ~25th-29th May) by ACSM is not discussed anywhere, which is now on the order of the ‘under-sampling’ later in the campaign (after the 1st June). The latter is attributed to the ‘refractory’ OM, so how do authors explain the former (oversampling)? Is that just uncertainty (both) due to density assumptions?
All these would make me very cautious in attributing the discrepancies between ACSM and DMPS to the refractory OM. It is a bit like cherry picking – authors attribute undersampling to refractory but ignore oversampling. Also, ignore the fact that high OM discrepancies between filter and ACSM still lead to good agreements in volumes at the first part of the campaign…
To me, the explanation of different sensitivity to sea salt for distinct water composition and different sea salt ions, would be much more feasible explanation than the refractory OM. The volume/volume discrepancies trends do not coincide with OM composition (neither from filters as in WSOC/WIOC trends nor PMF as in trends for different factor contributions), which really makes the refractory OM explanation quite unlikely. Also, the ssSO4 behaviour mimics that of ‘refractory’ OM, which points again to the sea salt rather than OM. More so that OM is more oxidised for the later part of the campaign.
On a minor note:
Figure S3 b) is difficult to read as points are not properly represented in the legend. Which ones are the ACSM and which ones are the Filter?
Also, density of 2.165 is applicable to sea salt, but not other inorganic species. E.g. NH4 (line 350 of the revised manuscript). Presence of NH4 would mean the presence of nss-SO4 in addition to ssSO4? Were these significant or on the level of noise? Do not include them in the calculations if the latter.
Reply No 9: 30% noise is very high; this is not typical for PMF. Does that ‘noise’ have meaningful time trend or mass spectrum, could it be some unresolved OM factor?
Reply 13: ‘In addition, during sampling the ToF-ACSM was run with a 2 min filter/8 minute sample. When sampling with long times between filters, any drift in sensitivity can result in a difference signal that is an artefact.’ – I would not expect any significant drift in sensitivity for 8 minute period? Where can it come from? Change in SEM, filament? They would not occur on the 8 minute time scale. Provide reference or do not refer to it in the manuscript.
Some typos and missing spaces should be corrected, especially for the newly added text.