This paper presents a sensitive technique for the detection of glyoxal and describes in-situ experiments with ancillary measurements of key precursor species involved in the glyoxal photochemical cycle. It is a complete paper that adds a great deal to the literature and our understanding of this species which has attracted a good deal of attention in the last decade based primarily on satellite observations and long-path DOAS measurements that have shown hard to explain high levels of this short-lived species in remote environments. It is well written and well researched. The paper presents data, courtesy of the technique that is capable of detecting glyoxal in the low pptv range, that appear to be in reasonable accord with our understanding of remote atmospheric chemical distributions and processes, although questions remain on sources of glyoxal in the absence of sunlight. 

The paper explores in detail sources and sinks of marine glyoxal. Sources not considered in as much detail in previous studies but considered here include terpenes and acetaldehyde. Results indicate that monoterpenes are unlikely to be responsible for a significant part of glyoxal formation but acetaldehyde may well be an important contributor.

It will be interesting to see the response from the satellite and DOAS communities and a side-by-side comparison of glyoxal measurements may be warranted at some point. 

 

P 7 line 12: what is trace heating tape?

P 7 line 28 and Figure 1. It would help the casual reader to put the pulse delay generator in the figure and to better explain the trigger and the pulse.

Table 1: Data all look reasonable except n-hexane is surprisingly high. My guess is that there may be an issue with the measurement although it this will not impact any conclusions in the paper so it is minor.

I’m not sure that using the MCM names for chemical species throughout the document is the best approach but leave it to authors’ discretion.

P 12 line 2: Put dates here for the two respective campaigns

P 12 line 11. I would change sentence for clarification to “The maximum glyoxal mixing ratio of 36.3 pptv was observed during the first campaign on 22 June 2014; however, the 24 hour….”

Page 13 line 18 – update to: https://www2.acom.ucar.edu/modeling/tropospheric-ultraviolet-and-visible-tuv-radiation-model

P 15 paragraph beginning with line 17: Maybe I missed it but do you discuss/speculate why the mixing ratios are higher during the second campaign? If not, this would be a good addition or if you don’t know of any reasons why then perhaps state that.

P16 line 13: ATom not AToM

P20 line 14. “moderately elevated” is a better description here then significantly elevated

More explanation needed for Figure 8. The Figure 9 caption implies that these are diurnal means – is that correct? I find it intractable to compare the diurnal means described in Figure 9 with those from a scatter plot in Figure 8.

Figure 10 – Cumulative production and loss rates would be preferable in my view. The yellow legend doesn’t show up well and see note on MCM names.

Figure S9 – label as a and b

Walker et al. made several months of in situ measurements of glyoxal at a remote site in the Cape Verde islands.  While glyoxal concentrations measured in this study were lower than those reported from other remote marine locations, a detailed model using the Master Chemical Mechanism significantly underpredicted the amount of glyoxal at the site.  The authors explored the sensitivity of glyoxal to several different processes in the model, pointing to future directions of research. 

This is an excellent, well written paper, and I recommend that it be published in Atmospheric Chemistry and Physics once the following few points are addressed. 

Page 1, lines 13-14: The modeled glyoxal seems rather insensitive to aerosol effects, especially compared to the effects of acetaldehyde or sesquiterpenes.  Later in the paper the authors say this (page 21, line33-34): “…the sensitivity of the modelled glyoxal to changes in the rate of aerosol uptake is not sufficient to reconcile the model with the observations.”  I would suggest changing the language in the abstract to be more consistent with the later text. 

Page 3 line 18-19: It would be good to also cite Lerot et al. 2021, who report glyoxal retrievals from TROPOMI, which like the other satellites also sees enhanced glyoxal over remote tropical oceans.  The authors discuss several reasons why this might be the case. 

Page 3, line 34-35: While 1.5e14 is the number from Lawson et al. (2015), it is a little confusing to compare a column measurement with an in situ measurement without also discussing the assumptions used to convert the in situ mixing ratio into a VCD.  Stating that the satellite columns indicated higher levels of glyoxal than the in situ measurements would be fine. 

Page 14, line 8: Are there any measurement of aerosol composition, either at Cape Verde Atmospheric Observatory or from the ATom campaign, that could be used to better inform the model?  Several of the references for the glyoxal uptake value (e.g. Volkamer 2007) are from studies in urban areas, where I would expect the aerosol to be mostly organic.  I’m not sure what effect the different ions in sea spray aerosol would have (e.g. Waxman et al. 2015), and a “real” number is better than a made up on, but it should be noted that an uptake coefficient for urban aerosol may not be representative of marine aerosol. 

The yellow font, and to a lesser extent the yellow traces, used in Figures 10, 12, and S10 is rather hard to read.  A darker shade of yellow for at least the legend would help. 

I’m not sure what the ACP style guide says, but in Tables 1 and 2 I would use a lowercase “i” and “n” to abbreviate iso-butane and n-butane (and the other VOCs where this applies), to avoid confusion with nitrogen and iodine.

Figures 5, 7, S1, and S2: Figure 5 uses day of month, while the other figures use Julian Day.  It would be easier for the reader if a consistent date format, preferably that which was used in Figure 5, was used for all these figures.  Alternatively, dashed vertical lines on the Julian Day plots to indicate the first day of each month would work. 

References:

Lerot et al. Glyoxal tropospheric column retrievals from TROPOMI- multi-satellite intercomparison and ground-based validation, Atmos. Meas. Tech., 14, 7775–7807, https://doi.org/10.5194/amt-14-7775-2021, 2021. 

Waxman et al., Glyoxal and Methylglyoxal Setschenow Salting Constants in Sulfate, Nitrate, and Chloride Solutions: Measurements and Gibbs Energies, Environ. Sci. Technol., 49, 19, 11500–11508, https://doi.org/10.1021/acs.est.5b02782, 2015.