This paper reports on measured exchange fluxes of HONO (along with NO and NO₂) using the aerodynamic gradient method during the growth process of paddy fields in the Huaihe River Basin, China during 2021. Maximal NO and HONO emissions were observed during rotary tillage. The measurement methods and quality control schemes were published previously and are solid. The length (1.5 month) and representativeness of this dataset in terms of agricultural practices is certainly unique and worth publishing. The manuscript is well-thought-out, and the discussion of the results is very impressive and complete, thus being highly informative and relevant because nearly all previous publications on soil HONO emissions are included. I do have some points for improvement that are listed below. My main concern is the treatment of data with chemical flux divergence. This part must be improved. The manuscript should be edited by a native English speaker. Please find below my detailed comments that should be addressed before publication:

Detailed comments:

Lines 29-30: “… affect the chemistry of the troposphere…”

Line 30: “… the scarcity of in-situ data on soil-atmosphere HONO exchange fluxes has constrained…”

Line 39: “… HONO and NO fluxes…”

Line 49: “… and soil emissions for HONO production.”

Lines 110-112: this is a repetition and was written already before. Could be deleted or rephrased.

Line 150: “… in the ambient air were…”

Line 155: Sentence is misleading. The molybdenum converter is not required for NO measurements.

Lines 160-170: What was the length and the material of the inlet lines, and what was the residence time of the air within the tubing? HONO may still adsorb on surfaces. Was the intercomparison of the two instruments made with the same inlet length/type?

Line 178: the correct name is “von Kármán constant”.

Line 290-311: In fact, if a chemical divergence is present, the fluxes cannot be calculated. Please refer to the Damköhler numbers (Da) here and rephrase the text accordingly. First you talk about upward NO₂ fluxes due to chemistry and then downward NO₂ fluxes due to photolysis. This must be proven with some Da numbers and a flux estimate should not exist for these conditions.

Line 332: Could it also be possible that drying of soil in the morning (occurrence of optimum WFPS) was causing the HONO emissions (related to direct emission from ammonia oxidizing bacteria)?

Line 395: Could you please double check the units in equation S10? I am surprised that the ground HONO source is very low, especially in the afternoon.

Line 430: … not only from agricultural soils in China….

Line 434: Can you mention here how much higher (factor) the HONO fluxed were during tillage compared to other conditions?

Figure 4: size should be increased.

General comments

Meng et al conducted flux measurements of HONO, NO and NO2 over paddy fields. The measured period covered several agricultural activities. They found relatively higher HONO and NO emissions during the rotary tillage period but lower emissions during irrigation/fertilization periods. No2 flux was generally negative as soil represents a NO2 sink. Through correlation analysis, they found the co-existence of soil biogenic emissions and NO2 conversion which dominate the daytime HONO budget. HONO contribution to OH was also estimated.

This study was focused on an interesting topic and the dataset benefits our understanding of soil HONO emissions. Some descriptions are still not clear, which needs to be improved before publication.  

Major comments

• The conclusion that fertilizer doesn’t significantly affect HONO emissions is not convincing, mainly due to the short period of measurements. As reported in several studies listed below, peak HONO emissions after fertilization were observed in a wide range of 3-15 days after fertilization and could still sustain at a high level within 3 weeks after fertilization. Therefore, I would suggest the authors to be careful when drawing this conclusion.

https://acp.copernicus.org/articles/23/15733/2023/

https://pubs.acs.org/doi/10.1021/acs.est.4c01070

https://linkinghub.elsevier.com/retrieve/pii/S0048969720343965

https://www.sciencedirect.com/science/article/pii/S1352231018306599?via%3Dihub

• The description of the AG method is not clear enough. There is no information on how the 3D winds were measured and validated; the impact of sampling pressure change due to NOx switching was not discussed;
• Correlations between HONO flux and NO2*JNO2 were used to support light-induced NO2 conversion. As No2 flux was also measured here, it would be great to see the correlation between HONO flux and NO2 flux*JNO2.

Specific comments

L30-31, emission processes refer to mechanism?

L33, be careful with using “the first time” as there are already many flux measurements. I don’t think it is interesting to indicate the first measurement at a specific location. Otherwise, there will be too many first-time…

L39, “HONO and NO fluxes, and they exhibited” …

L41-42, maybe it’s better to name “soil release mechanisms from biological processes” as “soil biological emissions”?

L46, what’s “flux rates”. The unit of nmol m-2 s-1 was used in previous sentences.

Abstract: in general, I think the abstract needs to be improved to be more concise.

L60-74, emissions from the combustion process should also be included here

L164-165, When NOx analyzer is switching between the upper channel and the lower channel, gas flows as well as the gas pressure in both channels change. What’s the corresponding impact on BBCEAS measurements?

L170: Normally the sampling tube needs to be heated to above ambient temperature. 30 C seems not to satisfy this demand through the measurement period, see Figure 2.

L237, pls also indicate the period for each activity in Table 1

L242, the minor ticks for the x-axis in Figure 3 look weird

L250, what is the level of 80% WFPS in water holding capacity? Was there still liquid water over the paddy fields after irrigation and fertilization? See the lab work below where high HONO emissions were observed at high WHC.

https://pubs.acs.org/doi/10.1021/acs.est.4c01070

https://pubs.acs.org/doi/10.1021/acs.est.2c07793

https://onlinelibrary.wiley.com/doi/10.1029/2021JD036379

L278, pls indicate that 0.19 nmol m-2 s-1 is the measured value in this study

L283, Another point to be discussed. When soil is waterlogged, HONO can’t be evaporated from the soil due to its high water solubility.

L295, Table 2, there are quite a lot of flux measurements in below study

https://pubs.acs.org/doi/10.1021/acs.est.4c01070

L311: I don’t think the negative NO2 flux was due to NO2 photolysis

L332, maybe add the correlation between HONO flux and NO2 flux*JNO2

L395-398, those results largely rely on MLH estimations. Better to show how MLH was estimated.

L438, first, it was not a long-term measurement; second, there were several more studies measuring flux at high water content;

Supplementary

Aerosol surface area, Sa, was used to calculate Paerosol. Sa was stated to be calculated based on aerosol size distribution. However, there is no mention of how the aerosol size was measured.