General comments:

The authors of this manuscript set up a REA system for measuring the vertical fluxes of HONO based on development of a three-channel-LOPAP instrument for HONO measurements of the updraft, downdraft and chemical interferences as well as a software for controlling valves and measurement cycles. The system was well tested in both laboratory and field to be reliable for the measurements of HONO vertical fluxes. In general, this is a nice research work, which can direct researches to conduct field measurements of HONO vertical fluxes to comprehensively understand the atmospheric HONO sources in different areas.

Specific comments:

Are the fluxes measured through the REA method affected by the height above the ground? Which is the height proper for selection?

There are several prerequisites and assumptions for the REA method. How to verify the reliability of fluxes measured by the REA method?

How did you measure the sampling efficiency of 99.6% for HONO? Part of HONO signal from the Channel 2 might be attributed to breakthrough from the Channel 1, rather than the chemical interference.

Were the wind directions affected by the small cone at the inlet? Small turbulence of wind could occur around the cone, which may affect the vertical fluxes.

Technical corrections:

Line 292, I don’t know why the time correction is about 3 times of the time response.

Line 293, “,.” should be “,”.

Line 301, „P“ should be “P”.

Line 315-316, I don’t understand the meaning of this sentence. Why were the channels 1 and 2 divided by the half of the ratio, whereas channel 3 multiplied the ratio?

Line 329, the air flow rate of 3.7 l/min in here was inconsistent with that of 2.65 l/ min mentioned in line 180.

Line 365, the dilution seemed to vary with time, e.g., the dilution between 13:00 and 14:00 was less than a factor of 4, which may result in significant uncertainty of fluxes.

Lines 368-374, I don’t know why the different average ratios of channel 1 / channel 3 were present at different places for the same experiment despite of small difference. Line 369, “ratio (1/3)” should be “ratio (channel 1 / channel 3)”.

Lines 400-402, the detail information for maloperation of the instrument is not necessary.

Lines 429-431, Fig.5 seemed not support the conclusion.

Lines 431-433, the sentence is suggested to be moved before the sentence of “In addition..” for logical connection.

Lines 459-460, this sentence is suggested to be rephrased due to unclear description.

Line 466, the reasons for exclusion of the HONO sources other than the photosensitized conversion of NO2 on organic surfaces are not very convincing, because the authors didn’t consider the possible heterogeneous HONO formation in dew and ground surfaces during night, which may be high enough to explain the large difference between the negative flux at night and positive flux in daytime.

The actual mechanism for ground/surface source of HONO is still up for debate, and high-quality flux measurements will aid in answering this question. The work by von der Heyden et al, describes an adapted LOPAP system for determining HONO fluxes by REA. In my opinion, the authors have done an good job describing the instrument, calculations and have provided a number of lab and field tests to validate the instrument for measuring fluxes. I especially appreciated how the authors listed some of the problems encountered related to flow rates and inlet pressure between the different channels and the solutions in section 3.2. It was refreshing to see, as presenting this sort of information will help other solve similar problems in the future.

I would have liked to see more characterization or validation on the effect of having an 80cm inlet before the stripping coil on the measured HONO. HONO formation on inlet surfaces is well known, as the authors state in the text, and this inlet in my opinion reduces one of the advantages of a normal LOPAP system (i.e. the very short inlet prior to sampling). While the authors attempted to mitigate it by covering it in foil, I would have liked to have seen some experimental validation that this inlet wont lead to a bias in the measured HONO, rather than just stating that the small residence time means it won’t matter.

Overall, this manuscript is well written, clearly presented and will be of interest to many in the community.

Minor Comments

Line 25: As you state in section 5.3, much of your discussion on potential HONO sources is speculative. I am not sure you can claim these HONO formation mechanism are minor.

Line 140: there have been more recent instrument intercomparisons comparing HONO measurements  from the LOPAP to other instruments (Crilley et al., 2019; Pinto et al., 2014), which indicate that there can be significant variability in the reported measured values of HONO between the LOPAP and other established techniques . This should be indicated here.

Figure 1: Why does stripping coil for channel 3 only have one coil? Wouldn’t it be better if you also quantified the interferences in this coil as well?  

Line 311-319: I struggled to follow the explanation for the corrections applied for dilution during REA measurements. This may be better presented as equation. Furthermore, it wasn’t clear to me channel 3 was treated differently to channel 1 and 2.

Line 319: what is HONO sampling efficiency of the stripping coils? I couldn’t find that value in the text.

References:

Crilley, L. R., Kramer, L. J., Ouyang, B., Duan, J., Zhang, W., Tong, S., Ge, M., Tang, K., Qin, M., Xie, P., Shaw, M. D., Lewis, A. C., Mehra, A., Bannan, T. J., Worrall, S. D., Priestley, M., Bacak, A., Coe, H., Allan, J., Percival, C. J., Popoola, O. A. M., Jones, R. L. and Bloss, W. J.: Intercomparison of nitrous acid (HONO) measurement techniques in a megacity (Beijing), Atmos. Meas. Tech., 12(12), doi:10.5194/amt-12-6449-2019, 2019.

Pinto, J. P., Dibb, J., Lee, B. H., Rappenglück, B., Wood, E. C., Levy, M., Zhang, R., Lefer, B., Ren, X. and Stutz, J.: Intercomparison of field measurements of nitrous acid (HONO) during the SHARP campaign, J. Geophys. Res. Atmos., 119(9), 5583–5601, 2014.