General Comments

The authors present their work discussing the need for fine spatial and temporal pH data, thus requiring the high resolution measurements they made on two transect cruises. However, then ultimately conclude that less resolved traditional research cruises are able to capture the pH variability we need. The authors mention the use of these fine scale measurements in coastal and dynamic environments, which makes sense. But the authors have failed to convey what is the need for the same fine scale measurements in the less dynamic open ocean environments, and what the measurements would be used for. Many processes are at play, but what is the goal of disentangling these processes in the open ocean?  The authors should try to convey their message without devaluing their work in the conclusions.

With regards to scientific significance, the authors present underway pH measurements that are not very common to my knowledge. Importantly, they calibrate their underway measurements with discrete measurements. The authors highlight the use of underway pH measurements, though ultimately conclude they are not necessary to be this fine scale, somewhat mitigating the significance of their work. With regards to scientific quality, I do appreciate the details the authors present regarding their underway pH measurements and associated discrete measurements. However, their corrections and offsets are quite large. They compare very small changes in pH to determine which influences (temperature, biological activity, or water masses) are dominant, but their changes are much smaller than the offsets they present. This leads me to question if these small changes are really meaningful within all the noise. With regards to presentation quality, the authors generally present their work in a clear manner. The results section discussing the influences to pH is a bit confusing to follow. I will discuss more about this in the specific comments section below.

I am concerned with the results of this manuscript, which compare observed (corrected) underway pH values with pH calculated in a variety of manners. The authors suggest very small residuals, but this does not account for the uncertainty in their underway pH measurements. Specifically, in Fig. 4 and 5, almost all of the various pH calculations fall within the pH uncertainty. So I am not sure specific conclusions can be made from this pH data.

Specific Comments

The authors indicate they are looking at fine scales of up to 100 km2 and days across ocean basins (line 106). However, later in the manuscript they begin discussing pH changes on the order of seconds to minutes (lines 402-418). The controls of pH are being convoluted between larger scale processes and split-second thermodynamics. In reality, the instant thermodynamics of the carbon system are not controlling the pH at longer time scales. The whole thing is quite confusing.

Lines 140-156: What are the precision and accuracy estimates for the pH optode? A lot of the discussion is based on looking at small changes in pH, but there is no mention for how accurate the pH optode is by itself.

Lines 181-186: Why was the two point calibration conducted so far from the pH points of interest? All surface ocean pH will be around 8.1 (+/- 0.1 roughly). So why was one of the calibration points at pH 2-4 and 10-11? Also, how accurate is this calibration? The authors also mention making a pH offset adjustment with a CRM, though CRMs only give certified values for TA and DIC. The authors should give more details of this process.

Fig. 3b: Why did the raw pH measurements shift from being always below corrected values to always above corrected values?

Lines 259-261: The reader needs more information here about how pH was calculated from TA and DIC. You give more details later in the manuscript, but it needs to be mentioned when calculations are first introduced.

Line 277: The constants of Lueker et al. (2000) are most commonly used in “best-practice” calculations of open ocean conditions. I am curious by your choice to use Sulpis et al. (2020) instead, as this set of constants has been shown to perform worse than Lueker with regards to internal consistency analyses. I suspect your readers will have the same question. Can you please justify your choice or switch to using the Lueker constants?

Lines 280-287: For the derived pH parameters, which TA and DIC are you using? The underway measurements or the derived TA and calculated DIC (from pH and derived TA)? You need to be clearer. This process seems convoluted. You already used pH to calculate DIC and are now using DIC to calculate pH. I’m not sure this is a sound rationale. Also, your pH_TA,fCO2 is calculated using fCO2 that was already calculated using pH. Instead, could you use inputs of measured underway pH and derived TA as your inputs, and then simply adjust your output conditions to be the temperature and salinity changes you are after?

Lines 305-323: Again, I am curious what the uncertainty of the actual optode is. This needs to be considered as an uncertainty contribution.

Lines 310-323: Have you considered that the uncertainty in pH_TA,DIC calculations may not be random? There is a known pH-dependent pH offset (see Williams et al. 2017) where the error in calculated pH from TA and DIC is dependent on the pH. It seems you may be underestimating the uncertainty in pH_TA,DIC.

Lines 333-344: This information is more introductory than methods.

Lines 402-418: There are a lot of technical details here about essentially what happens to a CO2 molecule in the span of seconds to minutes. However, the timescales the authors say they are working with is on the order of days. What is the relevance of these split-second reactions? The temperature is not changing fast enough to observe changes in the dissociation of the carbon species at this scale. You may see temperature changes between day and night, but not within seconds to minutes.

Lines 415: These waters don’t have a higher heat capacity, they are just insulated from the air.

Section 3.1: This section is quite difficult to follow. The authors are trying to discuss differences in pH between two basins, from day to night cycles, and throughout cycles 1-n during a cruise. Perhaps the authors should choose to discuss these types of comparisons individually, instead of all together. I think the more interesting results are between basins and between day and night. The specific cycles themselves are a bit too in the weeds.

Lines 452-464: This is again a lot of details about instantaneous thermodynamics. These processes are occurring on much quicker timescales than days, so I am not sure of its relevance to this manuscript. For open ocean pH, with all the uncertainties of measurements and changing dynamics, there is no way to observe these fine scale processes. I suggest the discussions of thermodynamics be made more concise or removed.

Section 3.2: Can the authors point to any studies discussing the generally expected levels of biological activities in these open ocean regions? They mention strong biological day-night signals, but would you actually expect to see much change in an open ocean area?

Lines 578-584 (and throughout results): There is a lot of background information that would be better suited for the introduction.

Lines 582-584: Now the authors are discussing time frames from the LGM, which is significantly longer than anything discussed in this paper. This seems irrelevant.

Technical Corrections

Line 19: Is it Dec 2019-Jan 2020? List both years since they are different.

Lines 32-35: These two sentences say the same thing, and could be combined for conciseness.

Line 40-41: “High resolution studies” of what exactly?

Line 87: To be clear, increased atmospheric CO2 only boosts oceanic CO2 uptake if the atmospheric CO2 > oceanic CO2. If atmospheric CO2 increases in a region, but is still less than oceanic CO2, the flux will still be towards the atmosphere.

Line 96: Might be helpful to include a range of latitudes for the “average North Atlantic latitude” since you mention a separate region above 55 N later in the sentence. Also, since you first mention N Atlantic has longer equilibration times, you should list the 18 month time-frame first for sentence structure.

Line 101: Surface temperatures?

Line 117: Give date ranges for both cruises for consistency.

Line 118: You need to define pH total (which should be denoted as pH_T) before including in the figure caption. You should also make it clear you are measuring pH_T throughout the manuscript.

Line 122: Cruises datasets should be cited or give appropriate links to the available data. Do the cruises have cruise reports you could link to?

Line 134: Again, listing of dates needs to be consistent for both cruises.

Line 135: “Discrete carbonate chemistry and nutrient samples” – you should list the parameters like you did for the first cruise in line 128 OR mention they collected the same samples as the cruise above. In general, try to make sure you have parallel structure in your sentences.

Line 141: Here is your first mention of pH on the total scale – you need to define what this is.

Fig. 2: What is the scale on the top and right sides of Fig. 3b?

Line 346: Missing end parentheses.

Fig. 4: The mean pH lines seem unnecessary, and they make it harder to see the other expected pH lines in the figures.

Fig. 4-5: You need to define what observed pH is (you do this in Fig. 6 but it needs to be when first used). Is this the underway pH corrected using the discrete TA and DIC data?

Fig. 6: You need to define what ∆pH is.

Line 378: Again, is pH_obs the corrected underway pH measurements? Try to make this clearer.

Line 467: Are you referring to Fig. 8?