Review of “Role of Criegee intermediates in the formation of sulfuric acid at a Mediterranean (Cape Corsica) site under influence of biogenic emissions”

This study focuses on the investigation of the H₂SO₄ budget in a remote coastal environment by using measured H₂SO₄ and OH radical with a CIMS instrument. The study investigates whether SCI have an impact in the H₂SO₄ formation for this environment and finds that they contribute to less than 10% to the H₂SO₄ during day and up to 40% during the night. 50% of the H₂SO₄ observed during the night cannot be explained.

The paper is well written and well structured and makes use of the most up to date literature values for the SCI chemistry to assess their impact on the H₂SO₄ formation. It extensively discusses the relatively large uncertainties that pertains to both the SCI, the loss rate for H₂SO₄ as well as the uncertainties on the rate coefficient between OH and SO₂. The latter I find rather interesting as the discrepancies with available (and recommended) rates a re large and introduce a big bias when trying to investigate additional sources for the H₂SO₄.

I recommend publication after the following comments are considered.

Specific comments:

Check the formatting of the citations. Often it is formatted wrongly when in the text (especially in the SI).

Please add the page number to the SI.

Page 2, Line 45: I do not understand the sentence “…a noticeable fraction of nucleation mode particle’s growth…”

Page 3, Line70: together with the formation of a SCI there is always the formation of a carbonyl compound.

Page 4, Lines 102-103: There is experimental evidence that the formation of SO₃ from the reaction of SCI from β-pinene and SO₂ is pretty much immediate (Ahrens et al., 2014) so that I would think there is not much doubt about it also for larger SCI.

Page 4, Line 110: I would also add the reference to the paper by Novelli et al. (2017) which came up with a similar estimate from measured data.

Section 2.2.1. Please be careful in the definition of ROx. Is it OH+HO₂+RO₂ or is the OH contribution removed? Later on (page 6 line 176 and 181), I assume, ROx becomes RO₂ but that is a big difference. Was the HO₂ contribution removed from the ROx signal or this is a typo? If the CIMS can separate between HO₂ and RO₂ it would be interesting to shortly clarify this.

Page 9, Line 275: Isn’t the value of OH measured at night lower than the stated detection limit of 5x10⁵ cm^-3? How meaningful is the nightime analysis then?

Page 12, Line 318: It is stated that the reaction between OH and SO₂ under estimate the H₂SO₄ concentration at night. Although this is clearly visible in figure 2a, I am not so convinced it is clear from figure 1 where I have the feeling that overall the H₂SO₄ is well explained by the OH radical as in the night often the H₂SO₄ calculated from OH is missing or has some sharp low values which might bias the median profile shown in figure 2a. What is the cause for the sharp low values for the H₂SO₄ calculated from OH+SO₂ in figure 1?

Page 14, Line 352: which instead of what

Page 15, Figure 5a: Shouldn’t the difference between the measured H₂SO₄and the H₂SO₄calculated from the contribution of the OH radical always be higher or equal to the H₂SO₄calculated from the SCI? Also, it looks relatively stable over the whole day…more as if affected by some scaling factor than additional chemistry. Once considering all the different uncertainties I am not so sure that I would conclude the abstract stating that SCI are an important source of H₂SO₄in SCI rich environments. Also, as the reaction with acids (which is fast) is not included as a loss rate for the SCI, what described here is an upper limit.

Page 18, Lines 455-457: I am not sure I follow here. If the lower rate coefficient for OH + SO₂ as proposed by (Blitz et al., 2017a, 2017b), shouldn’t the contribution of SCI to the formation of H₂SO₄ increase substantially?

Reference

Ahrens, J., Carlsson, P. T., Hertl, N., Olzmann, M., Pfeifle, M., Wolf, J. L., and Zeuch, T.: Infrared detection of Criegee intermediates formed during the ozonolysis of beta-pinene and their reactivity towards sulfur dioxide, Angew Chem Int Ed Engl, 53, 715-719, doi:10.1002/anie.201307327, 2014.

Blitz, M. A., Salter, R. J., Heard, D. E., and Seakins, P. W.: An Experimental Study of the Kinetics of OH/OD(ν=1,2,3) + SO2: the Limiting High Pressure Rate Coefficients as a Function of Temperature, The Journal of Physical Chemistry A, doi:10.1021/acs.jpca.7b01294, 2017a.

Blitz, M. A., Salter, R. J., Heard, D. E., and Seakins, P. W.: An Experimental and Master Equation Study of the Kinetics of OH/D + SO2: The Limiting High Pressure Rate Coefficients, The Journal of Physical Chemistry A, doi:10.1021/acs.jpca.7b01295, 2017b.

Novelli, A., Hens, K., Tatum Ernest, C., Martinez, M., Nölscher, A. C., Sinha, V., Paasonen, P., Petäjä, T., Sipilä, M., Elste, T., Plass-Dülmer, C., Phillips, G. J., Kubistin, D., Williams, J., Vereecken, L., Lelieveld, J., and Harder, H.: Estimating the atmospheric concentration of Criegee intermediates and their possible interference in a FAGE-LIF instrument, Atmos. Chem. Phys., 17, 7807-7826, doi:10.5194/acp-17-7807-2017, 2017.

Kukui et al. investigate the potential contribution of Criegee intermediates to sulfuric acid formation in the western Mediterranean, via analysis of field measurements from the ChArMEx project. Two chemical pathways to sulfuric acid are considered: reaction of OH with SO2 (+ H2O + O2), and the reactions of stabilized (thermalized) Criegee intermediates (SCIs) with SO2. Using their measurements of OH and unsaturated VOCs, Kukui et al. estimate the contribution of each channel to measured sulfuric acid and determine that reaction of SCIs with SO2 contribute ~10% of daytime and ~40% of nighttime sulfuric acid. While this analysis closes the daytime budget for sulfuric acid, ~50% of the nighttime sulfuric acid concentration remains unaccounted for. The authors conclude that SCI chemistry may be an important nighttime sulfuric acid source for VOC-rich environments.

This is a timely and interesting study that is suitable for publication in ACP. Some comments, questions, and suggestions follow.

• The reaction of MACR-oxide with SO2 has also recently been measured (Lin et al., Chemistry Communications, 2021).
• To what extent do you anticipate that an inability to quantify all unsaturated VOCs with your field instrument could be contributing to the significant unexplained nighttime sulfuric acid? One way to explore this could be through comparison of calculated vs. measured OH reactivity. For example, if you compare measured [VOC] to measured OH reactivity, do you have significant underprediction of OH reactivity that could be rationalized by higher [unsaturated VOC] than you were able to quantify in the field?
• What parameters from the literature do you use to calculate the concentration of the water dimer vs. monomer?
• The layout of Figure 3 (positioning of inset figures) makes this figure confusing to look at.
• In section 4.2., the phrasing where you describe the work of Vereecken et al. (that uses the Blitz et al. rate constants) is somewhat confusing – please rephase for clarity.
• I would also suggest stating somewhere that there are, inevitably, large uncertainties in the rate coefficients for the reaction of SO2 with more complex SCI that have not yet been directly measured or calculated.
• In Table S5, it is noted that the experimental results of Caravan et al. suggest faster unimolecular decomposition of Z-MVK-oxide in better agreement with the results of Vereecken et al. This is not correct – Caravan et al. actually suggest that faster decay rate in their experiments could be due to internal excitation of anti-MVK-oxide and/or a low yield of stabilized anti-MVK-oxide – both of which are experimental factors.