Temple et al outline the status of the custom built University of York Laser Induced Fluorescence instrument for the detection of SO2, and its performance via an intercomparison with a commercial Pulsed Fluorescence instrument and a Chemical Ionisation Mass Spectrometer. The UoY SO2-LIF is shown to outperform the other instruments based on comparison to data from airborne field campaigns and they outline how they have further improved the performance of the instrument for field deployment. 



This is a nice and easy to read paper with a clear goal and clear conclusions. 



I found that it lacked a bit in the introduction to motivate what it is about SO2 that lends LIF as being such a useful approach to its detection, but aside from a few other typos or minor issues I see no problem with the publication of this. 

Line 13: add a word after “(PF)” to indicate that this is not a form of mass spectrometry. Perhaps “instrument”?

Line 17: ppb is used where as elsewhere “pptv” is used. Be consistent please.

Line 55: I think at the end of this paragraph a short new paragraph would be helpful outlining the properties of SO2 that make it suitable for detection – it’s cross section, stability etc. Here it would be helpful to explain how UV remote sensing instruments are not suitable for estimating remote SO2 abundances due to their low concentrations, for example, hence motivating the focus of this work.

Line 90: Reference Figure S4.

Line 183: “was” should be “were”.

Figure 10: Personally, I think it would be neater to have the gradient and R2 presented inside the plot like in Figure 8. Also, why is there no uncertainty on the CIMS-SO2:CO2 gradient?

Figure 11: ppb used in y axis label whereas ppbv mentioned in the text.

Line 404: Reference for for the ACSIS project would be helpful – and possibly a Figure in the supplement of where it was flying.

The manuscript by Temple et al. characterizes and deploys a custom-built laser-induced fluorescence (LIF) SO₂ instrument based on the design of Rollins et al. and Rickly et al. Measurements by the LIF instrument are subsequently compared against measurements of SO₂ from an iodide CIMS (I-CIMS) instrument and commercial pulsed fluorescence (PF) instrument all aboard aircraft during the ACRUISE-3 campaign over the marine atmosphere. In polluted environments, the SO₂ measurements from the LIF and PF instruments agreed within the errors of the instruments, though the LIF had a faster time response. Additionally, only the SO₂ LIF instrument was capable of detecting SO₂ mixing ratios between 70 and 400 pptv in clean, marine environments (i.e., LIF had a 3-sigma LoD of 70 pptv for a 10 sec integration time during the campaign). SO₂:CO₂ emission ratios were also calculated from the SO₂ data collected by all three instruments, and the authors showed that all the sampled ships were compliant with IMO2020 regulations.

Overall, this is a solid paper. I commend the authors for their work characterizing the SO₂ LIF instrument and their measurement protocol during the campaign (i.e., instrument sensitivities were remarkably stable).

This paper would be of interest to the readership of AMT. I recommend publication after attention to the following comments:

- Line 174: "and to capture instrument drift": Did the authors notice any consistent drift for the LIF instrument over the course of a given flight? If so, what is the possible cause?

- Line 181: "As a result of inconsistencies in the laser linewidth": How was this determined that the laser linewidth was the cause of variations in the sensitivity as opposed to errors in the calibration system, etc.? There is a lot of overlap in the error bars in Figs S1-S3, so it doesn't appear that the sensitivities are statistically different.

- Line 242: "larger interfering peak": What are some possible compounds that would correspond to the interfering peak in Figure 4(a) at m/z 190.899311? After seeing that figure, it is understandable why obtaining SO₂ from that m/z is so challenging.

- Figure 8: I assume these are York regression fits being reported? Also, could the authors report the error on the slope and intercept for the fits (particularly Fig 8a)?

- Lines 401-404: A comparison is made between SO₂ LIF data between the ACRUISE and ACSIS-7 campaigns, but no further analysis is done other than just showing the vertical profile of SO₂ in Fig S9. The authors should either develop this analysis further or consider cutting ACSIS-7 and Fig S9 from the manuscript.

- Figure S10: Why is this figure included if not mentioned in the main text?

Technical Corrections:

- Figure 4 was hard to read since it appeared more pixelated than other figures.