Reviewer Summary

In this study the authors describe the results of 8 transit flights between Langley Research Center in Hampton VA and Bermuda as part of the ACTIVATE study in the northwestern Atlantic Ocean in 2022. Two planes flying at different altitudes tracked a path to Bermuda and back, with many on-board instruments to investigate different components of the gas and aerosol composition along this path. This study provides a great deal of insight into the characteristics of gas and aerosol composition within the marine boundary layer and the free troposphere. Overall, this is a well written and organized paper that provides significant value to the atmospheric science community and should be published after consideration of a few minor questions and comments.

Comments

Comment #1, Pg 10 Lines 210-221:

In this section you are discussing how sea salt can be mis-identified as dust-mix at RH < 75%. You state: “we presume this is the case during these flights as the RH in the BL was usually below 75% based on Falcon…”, so does that imply that you are assuming that the “dusty mix” (for at least the first four flights) is actually “dry” sea salt? And is the lack of “dusty-mix” on wetter days an argument in favor of the dusty-mix on the “dry” days being sea salt? Perhaps this section could be re-worded a bit to make it a little more clear what is happening and what conclusions you are drawing from these observations.

Comment #2, Pg 11 Lines 227-229

While there is definitely a decrease in the CH4 and CO concentrations from LARC to Bermuda, it doesn’t seem that large in either case. How do these numbers compare to a more “background” concentration of these gases in this region? Is the number observed at Bermuda representative of a clean background, or is it still elevated?

Comment #3 Pg 11 Lines 229-230

This is somewhat related to previous comment, but you mention that the concentrations “level off” around 900 km offshore, what concentration do they level off to? Is it the concentration observed at Bermuda?

Comment #4 Pg 13 Lines 254

You state that you used 70 of 86 measurements of the smoke plume and found that the CH4/CO ratio is low enough to suggest biomass burning. How was that number of measurements chosen, was it due to their CO concentration being greater than 168 ppm? If that is the case, why was 168 ppm chosen as the cut-off? Some additional information could be useful explaining why those measurements were chosen.

Comment #5 Pg 13 Lines 254-257

Related to the previous comment, do you have any comment on what the source of the potential biomass burning could be, especially since it seems to be a unique case in the flights reported in this study?

Comment #6 Pg 14 Lines 271

Here you mention the particle number concentration leveling off roughly 900 km offshore. Are these numbers leveling off at the measurement of 1150 cm-3 and do you feel they are representative of general background in the region, or something related to the outflow that had been followed on the flight?

Comment #7 Pg 23 Lines 407

I believe you mean “wet scavenging” instead of “wet savaging”

This study examined the evolution of trace gas and aerosol properties off the U.S. East Coast on a basis of eight transit flights. The results are fruitful and are beneficial to our understanding of the air pollutants and greenhouse gases transition between continental and marine environments. I suggest that it can be published in ACP after minor revisions.

1. Regarding the new particle formation, the Figure S9 might be better than Figure 10 to discuss the NPF events. Much higher N3-10 nm close to continental areas may indicate the NPF events typically take place for continental air mass-dominated cases, but not marine.
2. R160 and R179 in Figure 13, the AMS results showed much higher sulfate mass fraction. RF 161 in Figure 14, ammonium mass fraction was dominated between 600-800 km. There is a lack of explanation.
3. Typically, the particle hygroscopicity decreases with enhanced organic fraction. While, in Figure 17, some cases (e, f and i) showed a non-pronounced dependency between f(RH) and organic mass fraction. What are the reasons?
4. In Figure 19, there is spike of dnp/dlogDp below 10 nm. Is it an outlier?