Airborne remote sensing of nitrous acid in the troposphere: potential sources of excess HONO

Weyland, Benjamin; Rosanka, Simon; Taraborrelli, Domenico; Bohn, Birger; Zahn, Andreas; Obersteiner, Florian; Förster, Eric; Mertens, Mariano; Jöckel, Patrick; Ziereis, Helmut; Kaiser, Katharina; Fischer, Horst; Crowley, John N.; Wang, Nijing; Edtbauer, Achim; Williams, Jonathan; Andrés Hernández, Maria Dolores; Burrows, John P.; Kluge, Flora; Rotermund, Meike; Butz, Andre; Pfeilsticker, Klaus

doi:10.5194/egusphere-2025-5085

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https://doi.org/10.5194/egusphere-2025-5085

Preprints

23 Oct 2025

| 23 Oct 2025

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Airborne remote sensing of nitrous acid in the troposphere: potential sources of excess HONO

Benjamin Weyland, Simon Rosanka, Domenico Taraborrelli, Birger Bohn, Andreas Zahn, Florian Obersteiner, Eric Förster, Mariano Mertens, Patrick Jöckel, Helmut Ziereis, Katharina Kaiser, Horst Fischer, John N. Crowley, Nijing Wang, Achim Edtbauer, Jonathan Williams, Maria Dolores Andrés Hernández, John P. Burrows, Flora Kluge, Meike Rotermund, Andre Butz, and Klaus Pfeilsticker

Abstract. The photolysis of nitrous acid (HONO) produces hydroxyl radicals (OH), the most important cleaning agent of the troposphere. For decades, HONO has been measured in concentrations which exceed the photo-stationary concentration arising from its gas phase formation via the reaction NO + OH and destruction by photolysis. Several heterogeneous formation mechanisms as well as the photolysis of particulate nitrate have been proposed which may explain this excess HONO. This study reports on airborne remote sensing measurements of the mini-DOAS instrument over continental Europe, Southeast Asia, and the tropical Atlantic. The observations form a C-shaped profile in the troposphere with maximum volume mixing ratios of approximately 150 ppt in the planetary boundary layer, about 10 ppt in the free troposphere and up to 100 ppt in the tropical upper troposphere. These measurements of HONO throughout the troposphere exceed model predictions by up to an order of magnitude. Together with a host of other measured species and parameters, various formation mechanisms are explored to investigate in situ HONO sources. Although a precise formation mechanism in the polluted boundary layer remains elusive, the photolysis of particulate nitrate may explain excess HONO in the marine boundary layer. The excess HONO observed in the upper troposphere requires a gas phase source with a formation rate of up to 300 ppt h^-1. The possible role of peroxynitrous acid (HOONO), formed by the reactions NO + HO₂+ M and NO₂+ OH + M, and further oxidation by reactions with NO or O₃, is explored.

How to cite. Weyland, B., Rosanka, S., Taraborrelli, D., Bohn, B., Zahn, A., Obersteiner, F., Förster, E., Mertens, M., Jöckel, P., Ziereis, H., Kaiser, K., Fischer, H., Crowley, J. N., Wang, N., Edtbauer, A., Williams, J., Andrés Hernández, M. D., Burrows, J. P., Kluge, F., Rotermund, M., Butz, A., and Pfeilsticker, K.: Airborne remote sensing of nitrous acid in the troposphere: potential sources of excess HONO, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-5085, 2025.

Received: 15 Oct 2025 – Discussion started: 23 Oct 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics

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RC1: 'Comment on egusphere-2025-5085', Anonymous Referee #1, 05 Nov 2025 reply

In the study of Wyland et al. remote airborne passive DOAS measurements of nitrous acid (HONO) - an important source of the OH radical - are presented. The observations show higher concentrations in the boundary layer (BL), which decrease towards the free troposphere (FT), but again increase in the tropical upper troposphere (UT). All HONO concentrations exceed expected values by know gas phase sources and sinks, which is explained by particle nitrate photolysis in the lower atmosphere, while new HONO gas phase sources are proposed to explain excess HONO at lower temperatures in the UT.
Caused by its high importance for the oxidation capacity of the atmosphere and by the limited data on vertical gradients, especially at higher altitudes, the study is of high importance and should be published after my concerns (especially major concern 3) have been considered.

Mayor concerns:
First, I have to apologize that I am no expert in the passive DOAS technique used in the present study and thus my review will only focus on the sources/sinks/mechanisms/etc. of nitrous acid. I recommend that also an expert in the used experimental technique should review the present study. I thus also apologize for questions to the DOAS approach used, which may be trivial for the experts...

1) Gradients in the BL:
In Fig. 5 and 6 the vertical gradients of HONO are presented. While the observed decreasing HONO levels from the BL to the FT are expected (decreasing precursors like NO_x, nitrate) and are also qualitatively described by the models (Fig. 6), there are very unusual positive vertical gradients below 3 km (CAFE) and ca. 1 km (EMeRGe), for which the concentrations are decreasing towards the ground, see Fig. 5. Here all former gradient studies in the BL show continuously decreasing HONO levels from the main source region (ground: heterogeneous and direct emission sources) to higher altitude, which is in contrast to the present results, cf. all gradient studies e.g. on high towers, but also ground based MAX-DOAS data, the latter of which cover partially the same altitude range (ground to a few km altitude) as studied here. I am also missing these MAX-DOAS gradient results (e.g. Hendrick et al., 2014 (see reference list); Garcia-Nieto et al., Sci Total Environ, 643 (2018) 957-966; Ryan et al., 2018 (see reference list); Wang et al., 2019 (see reference list); Wang et al., AMT, 13 (2020) 5087-5116; He et al., Sci. Total Environ, 858 (2023) 159703; Xing et al., Sci Total Environ, 915 (2024) 169159) both, in the introduction, but also in the discussion sections.
Interestingly, positive gradients below 1 km are also predicted by the EMAC model at lower altitude (see e.g. Fig. 6b), which is also hard to understand. Since NO levels should maximize near to the ground (source region of NO_x from soils/combustion...), also modelled HONO should decrease with height, even when considering only gas phase reactions R1 to R3 in the models.
Also, in Figure 2 the “representative HONO profiles” (blue lines) are very unrealistic at <1 km altitude (decreasing to zero towards the ground…?), which is again in contrast to all previous observations.
Can you explain these unusual observations/predictions in the BL? Could the data retrieval be a reason, e.g. by using an invalid a priori profile (see Fig. 2)?

2) Increasing HONO levels in the UT:
In the present study very high HONO levels (up to 150 ppt) and positive HONO gradients were observed in the UT (see Fig. 5). The HONO levels also result in extremely high production rates of HONO/OH of up to 1.6 ppb/h (!) in UT, see line 489. These production rates are also very unusual and would be similar to urban ground level observations (close to the main source region, with precursor levels orders of magnitude higher compared to the UT…)? While also in the study by Heue et al., 2014 high HONO levels were observed in the UT (but on average only 37 ppt, the 160 ppt shown by the yellow bar in Fig. 5 were obtained only under some extreme assumptions), these were observations from thunderstorms, for which high levels of OH and NO are formed in the flash path and where the HONO observations were reasonably well explained by the gas phase reaction R1 (NO+OH). In contrast, in the present study the high levels seem to be a general feature of the UT, which cannot generally by explained by lightning (see short photolytic lifetime of HONO, many observations will be outside of thunderstorms).
However, before proposing any new HONO source reactions, as done in the present study, the experimental data should be checked again. Here the a priori profiles assumed seem to have a strong impact on the calculated HONO levels (see Fig. 2 for lower altitude). For the data retrieval, the authors assumed zero HONO levels in the stratosphere (see line 125-126), which may be not the case. Caused by high stratospheric ozone levels, high actinic flux and low pressure (less quenching of O(¹D)), high OH levels may result. Since there is also NO in the stratosphere, this may also lead to significant HONO levels by the gas phase source reaction R1 in the overlaying stratosphere. If there is also light absorption by this stratospheric HONO, the upper tropospheric HONO will be overestimated with the zero stratospheric HONO assumed in the a priori profile.
By ignoring potential HONO absorption in the overlaying atmosphere, could that be a reason for the increasing HONO in the upper troposphere (see "C-shape profile")? But again, I am not familiar with the used experimental technique and I recommend that this issue is also reviewed by an expert.
At the end, the proposed HONO source strength in the UT looks very unrealistic for me.

3) Correlation studies:
The authors try to identify potential source reactions by correlating the measured HONO levels against different precursors/measures (e.g. NO₂, HNO₃, radiation), which is too simple, although also often applied in other studies. The HONO levels are not representative for any production term (P(HONO)), which the authors are looking for. E.g. at low actinic flux (morning/evening/clouds), HONO may be much higher compared to high actinic flux conditions (noon) although the formation rates - e.g. by any NO₂ dark conversion source (e.g. R5), may be the same! Thus, simple correlations using only HONO concentrations are often misleading. For example, during nighttime ground HONO perfectly correlates with Radon, but they have no chemical link (this is an “auto-correlation” the authors also mentioned, which is triggered by the variation of the vertical mixing/BLH...). Or observed nighttime correlations of HONO (or HONO/NO₂) with RH in the BL are artificial (decreasing temperature = increasing RH, but also lead to a more stable atmosphere => higher S(ground)/V => faster heterogeneous formation...).
Since OH/J(HONO)/NO/[HONO] data are available (to calculate the PSS, [HONO]excess and P(HONO)excess), please correlate P(HONO)excess (= the “missing” source) against different expected precursors/measures, e.g.:
-NO₂
-NO₂*J(NO₂)
-HNO₃/nitrate
-HNO₃/nitrate*J(HNO₃)
-S(particle)/V
-S(particle)/V*NO₂
-S(particle)/V*NO₂*J(NO₂)
-...

4) Definition of the EF (eq. (2) and (3)):
The EF to describe HONO formation by the nitrate photolysis should be better defined. In Eq. (2) only HONO formation from nitrate photolysis is considered (J(NO₃^-=>HONO), but not the total photolysis frequency of particle nitrate (J(NO₃^-)) as specified, which in reality will also lead to NO_xformation (main product channel of the nitrate photolysis). I.e. if only considering the total loss of nitrate, as written here, the EF would be in reality (with HONO and NO_x formation...) even higher! Define here that only the HONO channel is considered for nitrate photolysis... Or define and use also the HONO-yield besides the total J(NO₃^-)... Also, in Eq. (3) only the HONO levels exceeding the PSS (eq. 1) should be considered, i.e. [HONO]excess... Otherwise the EF is overestimated.
These issues get visible e.g. in Fig. 7, for which the grey data (very small EFs around 1 using gas phase HNO₃ data as nitrate...) seems to indicate that besides gas phase photolysis of HNO₃ no other source would be necessary to explain all HONO formation at flight height... I.e. this would imply: “mission accomplished, all HONO sources identified...”. However, as the quantum yield for HONO formation by HNO₃(g) photoylsis is <<1 (the main channel is NO₂+OH), equation (2) should be more precisely defined (s. above, it should be J(NO₃^-=>HONO)). Since HNO₃ photolysis in the gas phase will not mainly lead to HONO, the EF for the grey data should be much higher! E.g. if a quantum yield of HONO formation by HNO₃-photolysis of 0.01 is assumed (HNO₃+hn=>HONO+O), the EFs should be 100x higher than those shown in the figure, i.e. there is still an unexplained HONO source... Also, in lines 387-389, it is not the low photolysis frequency of HNO₃, but the very low quantum yield for HONO formation during gas phase photolysis, for which NO₂ is the main product! If HNO₃ photolysis were a pure HONO source, the observations could be well explained even when using the low photolysis frequency of HNO₃(g), see grey data in Fig. 7.

Specific concerns:
The following concerns are listed in the order how they appear in the manuscript.

Line 3-4:
It should be “for example” (and not “as well as”) and delete “particulate”, as a) nitrate photolysis is also a heterogeneous process (“as well as”) and b) takes place on particles and on ground surfaces (for near ground observations nitrate photolysis on ground surfaces will be even more important, see e.g. doi: 10.1029/2003GL018620 or doi: 10.1038/NGEO1164).
In addition, the most important “heterogeneous formation mechanisms” are also photochemical reactions, e.g. NO₂+org+hn... Thus, write this sentence more general.

Line 17, Perner and Platt (1979):
The first detection of HONO in the atmosphere was by Nash, 1974 (https://doi.org/10.3402/tellusa.v26i1-2.9768), see “Since its discovery…”.

Line 20-21:
HONO is not only the most important OH source for polluted conditions, but also in remote regions, see many studies under polar conditions, where also “production of OH from the photolysis of HONO may outpace OH production from the reaction of O(¹D) with H₂O“, see e.g. doi: 10.1029/2011JD016643.

Line 38:
Delete again “particulate” as especially for “near-surface measurements” also photolysis of nitrate on ground surfaces gets more important than particle nitrate photolysis, see references above (concern line 3-4).

Line 43, Ma et al (2013b):
The first reference Ma et al., should be (2013a).

Line 44-47:
For the topic of this section (“heterogeneous reactions involving NO_x on macroscopic surfaces”) the used references are partially not suitable. Delete the references on nitrate photolysis here (Scharko et al. (2014), Wang et al. (2015), Laufs and Kleffmann (2016), Benedict et al. (2017), Ye et al. (2017)) as this is explained in the next paragraph. You may also shift those references to this section and may add here some others on heterogeneous NO₂ reactions.

Lin 50-51:
The soil production of HONO should be described more generally, e.g. “by soil bacterial processes”, as nitrite/HONO is also formed from ammonia oxidation (not only nitrate reduction).

Line 51, Song et al. (2023b):
The first reference Song et al. should be (2023a).

Line 53, Ma et al. (2013a):
The second reference Ma et al., should be (2013b).

Line 59:
Delete “inorganic”, as most photosensitized NO₂ mechanisms are “organic”, e.g. on polyaromatic species, like polyphenols, humic acid, etc., see Stemmler et al., George et al..

Paragraph, line 62-70:
I am missing all ground MAX-DOAS studies, which also measure HONO up to some km altitude. The strong negative gradients observed in all these MAX-DOAS studies agree well with all gradient studies on high towers, but are in contradiction to those shown in the present study, see major concerns.

Lines 75-77:
This is reasonable only if dense wildfire plumes are excluded. Here HONO can be transported over longer distances/altitudes, caused by the lower photolysis frequencies in/below dense plumes and by strong convective vertical transport during wildfires.

Line 106, Nussbaumer et al. (2021b):
The first reference Nussbaumer et al. should be (2021a)

Lie 115-118:
Just as a comment, I agree to the used reference by Stutz et al. (2000). There are several former studies which confirm these cross sections, which are also the generally accepted values in the community.

Line 307:
You may add “see Fig. 6” behind the brackets, as the statement is confirmed in Fig. 6.

Line 311:
These “some tens of ppts within the MBL” are in contrast to the max few ppts observed near to sea level, e.g. on Cape Verde island (same region as CAFE), see e.g. Fig. 3a in Crilley et al. (2021) (https://doi.org/10.5194/acp-21-18213-2021, between <1 and 3 ppt…). Do you have any explanation for the much higher levels in the BL, also with respect of the expected negative vertical gradients, seen in all gradient studies, i.e. at a few hundred meter altitude (lower altitudes range of the present study) HONO levels should be even lower than these few ppts?

Line 316:
Add “and a Zeppelin” behind “aircraft”.

Fig. 5:
In the legend, it should be Ye et al., (2016a). In addition, please also check the BLH, which is typically between 1-2 km during daytime, but not at 3 km as shown here?

Line 336-338:
If wrong assumptions are used only in the evaluation of HONO, this conclusion is not justified. E.g. stratospheric HCHO levels may be indeed close to zero, while HONO levels not... I.e. there could be some non-considered light absorption by HONO in stratospheric air, leading to an overestimation of HONO at flight height, see major concern...
But I agree (below lines 339-340), reactions R1 to R3 may be not sufficient to explain atmospheric HONO levels... However, I expect that these additional HONO sources, for which particle nitrate photolysis is yet the main proposed source at higher altitude is decreasing with altitude (see gradients in particle nitrate...).

Line 398-401:
In dense biomass plumes (“…influenced by biomass burning…“) the actinic flux may be much lower than measured outside of the plume and considered in a model. If J(HONO) is lower, HONO levels during daytime may be also higher than expected by the model.

Line 407-409:
Neither d[HONO]/dt(total) - what is written here - nor the concentration (see below) should be used, but d[HONO]/dt(excess), i.e. the missing P(HONO) to explain HONO exceeding the PSS, see major concerns.

Line 409-410:
I cannot follow this statement? Which “auto-correlation” do you mean? If HONO is formed by any dark source (e.g. by reaction (5)), P(HONO)excess would be constant (although the excess HONO over the PSS would vary during daytime, with a minimum at noon...) and not correlating with J(HONO) - certainly, only if soot/organics and NO₂ levels (see reaction (5)) are constant... In contrast, only if HONO is formed by any photochemical source (e.g. particle nitrate photolysis), P(HONO)excess would correlate with J(HNO₃)*[nitrate]... Because of the different diurnal shapes of J(HONO) and J(HNO₃), the correlation of P(HONO)excess would be higher with J(HNO₃) compared to J(HONO) in this case... When calculating P(HONO)excess, J(HONO) is already considered and there is no “auto-correlation”... See major concern, do not use simple HONO levels.

Line 416-417:
In polluted air, almost all pollutants will increase compared to background air (see “auto-correlation”). That is why simple correlations of concentrations can lead to artificial conclusions, s. major concerns.

Line 418-419:
First, better use P(HONO)extra against NO₂, see above... Second, if the excess HONO is formed e.g. by particle nitrate photolysis (the most probable source at higher altitudes), its correlation with [NO₂] would be much weaker compared to [nitrate] (and even better with [nitrate]*J(nitrate)…), see different lifetimes of NO₂/HNO₃/nitrate...

Line 420-422:
Please reanalyze the correlations by using P(HONO)extra, see above... Trace gas levels will correlate by definition, see HONO and Radon, but that is meaningless. In contrast, a correlation of (P(HONO)excess with any precursor of 0.7 would be great for field data!

Line 424-425:
Photolysis of nitro-phenols is not a main proposed HONO source, see Bejan et al. (2007) and certainly not at the higher altitudes studied here, caused by the expected low levels of nitrophenols.

Line 426-428:
This is certainly a weak point, because OH data is necessary to calculate the PSS and P(HONO)extra. Thus, I would only concentrate on the CAFE data in the present study.

Line 434:
It is trivial that HONO will correlate e.g. with HCHO (both are high/low in polluted/clean air masses...), see highest correlation during the EMeRGe campaigns, but they most probably have no chemical link... Do not use the HONO mixing ratios, see above...

Lines 439ff:
Please reformulate after reasonable correlations (using P(HONO)extra...)) have been formed...

Lines 449-454:
HONO formation by NO₂ conversion on particles is typically of low importance, as the uptake kinetics of NO₂ on any particle surface and the particle S/V ratios are much too low to explain any significant HONO formation compared to ground surface NO₂ conversion or nitrate photolysis. During nighttime/daytime values of gamma(NO₂) only as high as some times 10^-6/10^-5 were measured on any realistic surfaces. Also, the exception NO₂+soot (here gamma values up to 10^-1 were reported) cannot explain any significant HONO formation, as the uptake kinetics quickly slow down by passivation of the soot surface. And even for any photoenhanced NO₂+soot reaction (see footnote V in Table A1) gamma values <10^-6 were observed in Monge et al. (2010), which are also too low. For the photocatalytic conversion of NO₂ on dust/TiO₂ the low TiO₂ photocatalyst content in dust (only a few %) has to be considered, for which gamma values also only in the range 10^-5 are expected and measured, see e.g. doi:10.1029/2007GL032006 (only for pure TiO₂ it could reach 10^-4). These values, together with the low S(particle)/V cannot explain any significant HONO formation.
In contrast, most studies which propose a particle NO₂ conversion source use simple correlation analysis as done here (see correlation of HONO and Radon...). From these studies, values of gamma(NO₂)SS of typically >10^-4 were inferred, which have never been observed in any realistic lab study on any realistic surface... Also, all model studies which use realistic uptake kinetics show a minor contribution by NO₂+particles.

Line 450:
First reference Zheng et al. should be (2020a)...

Line 458-459:
For HONO formation on soot neither the mass nor the number is a suitable measure, but the surface area. If SMPS data is available take the S/V ratio.

Line 472:
Second reference Zheng et al. should be (2020b)...

Line 483-486:
Please revalue this statement with the reanalyzed correlations, see above.

Line 490-492:
Exactly! As HONO strongly decrease with altitude (see e.g. all MAX-DOAS studies), P(HONO) should be much lower in the FT compared to polluted near ground conditions (see cited references)! I.e., these references proof that there is no agreement.

Line 601:
The second reference Nussbaumer et al. should be (2021b)

Table A1:
General:
- Please use consistent numbering of all reactions, here and throughout the text:
E.g. Here reaction no. 1 is similar to R1 in the text, while the others are different. This is confusing. Name them here also by (RX, RY, ...)
- In addition, some of the reactions are not up to date and/or may be deleted for simplicity, e.g.:
- Reaction 3 was excluded by Ye et al., see also footnote (iii);
- Reaction 4 was a multiphoton excitation process at high photon flux density of the laser used in Li et al., which will not happen at solar actinic fluxes, see also footnote (iv);
- In addition to the deactivation issue of reaction 5, even for any photoenhancement of NO₂+soot (see footnote (v)) the observed uptake coefficients (Monge et al.) are too low (<10^-6) to be of importance. However, reaction 5 is not only limited to soot as specified here, but was observed on many organic substrates, like polyphenols, PAHs, organic grime, etc. see e.g. studies by Markus Ammann's and Christian George's groups... Thus, another reaction NO2+organic(het.) may be a more general ground surface nighttime source of HONO;
- The shown mechanism of reaction (6) is not possible, as uptake coefficients of N₂O₄(g) (see second reaction (6)) of larger than one are necessary to explain NO₂ experiments/field observations at low ppb levels of NO₂ (= sub ppq levels of N₂O₄(g)...). This reaction works well at the high ppm levels used in Finlayson-Pitts et al., but not at low atmospheric ppb levels. Observed uptake coefficients of NO₂ on water, humid Teflon surfaces, etc. are typically <10^-7 at ppb levels, which cannot explain HONO formation in the atmosphere. Since HONO formation by NO₂ better follows <1. order kinetics (see increasing HONO/NO_x with decreasing NOx levels in the atmosphere, see also decreasing HONO yields with increasing NO₂ in lab studies), reaction (v) on organics (not soot, but e.g. organic grime, see above) is a better candidate for nighttime HONO formation, also because of the much higher uptake coefficients for NO₂+org. in lab studies (in the 10^-6 range) compared to NO₂+H₂O.
- Reaction 7 is the photocatalytic reaction of NO₂ on photocatalysts like TiO₂ (often Anatase) which shows a much faster uptake kinetics than 10^-8, observed for silica particles (see footnote vii). The reaction of NO2+silica is better represented by reaction 6, but definitely not by reaction 7. For the photocatalytic reaction on TiO₂, NO₂ uptake coefficients of up to some 10^-4 were observed (see e.g. https://doi.org/10.1039/B609005B or see Dyson et al. in the reference list), which decrease to values of 10^-5 to 10^-6 at low % content of TiO₂ in dust (see e.g. doi:10.1029/2007GL032006). For strong dust events this could be a candidate for HONO formation at flight height, e.g. during CAFE.
- Reaction 8 is indeed slow for pure HNO₃ adsorbed on clean surfaces, but may get important in the presence of organics/photosensitizers, see e.g. https://doi.org/10.1038/s41598-018-37973-x, https://doi.org/10.1029/2022GL098035.

Specific Table A1:
Reaction (6): delete the “J.” in the reference Finlayson-Pitts et al. (2003).

Footnote (iv): The reference Crowley (2018) is missing in the reference list?

Footnote (viii): The second reference Song et al. should be (2023b).

Figure A3:
Not [HONO] but P(HONO)excess calculated from [HONO] exceeding the PSS should be considered, see major concern. If NO/OH/J(HONO) data are available...

References:

General comments to the reference format style:
- besides the volume, also the issue should be listed, e.g. Acker et al: 33(2) and many others;
- for some references besides the normal http:// address / doi-number also an “_eprint” address is listed (e.g. Acker et al., Alicke et al., ...), which is unusual? Please follow the recommended reference style and if this is indeed recommended by the journal, it is missing for many other references (e.g. Akimoto et al., Alvarez, ...);
- also, for most references the “publisher” is listed besides the journal's name, which is also unusual. If indeed recommended, this information is missing for many other references (Acker et al., Alicke et al., Alvarez, Bogumil et al., ...), unify;
- please subscript all numbers in chemical formulas, e.g. Acker et al., the “2” in “HNO2”. Only for a few references the chemical formula are correctly written, e.g. Dillon et al., Dorich et al., Hendrick et al., Hrdina et al., Marno et al., ... ;
- for some references the http-links are not working, although the dois are correct? See Bogumil et al., Huang et al., Jacob et al., Kleffmann et al. 2003, Li, X. et al., 2012, Ma et al., 2013a, Mark et al., Reisinger, Weger et al.;
-delete blanks: Cheng et al: e1601 530, George et al.: NO 2, Jiang et al.: 12 115–12 131, Finlayson-Pitts et al.: NO 2, Kalberer et al: 13 825–13 832, Kluge et al., 2020: 12 363–12 389, Laufs and Kleffmann: HNO 3, Lu et al.: 114 002, Mao et al.: e2021GL095 740, Martins-Costa et al.: 20 937–20 941, Nussbaumer et al., 2023: 12 651–12 669, Sullivan et al.: HNO 3 and 30 537–30 539, Romer et al.: 13 738–13 746, Rotermund et al.: 15 375–15 407, Ryan et al.: 13 969–13 985, Scharko et al.: 11 991–12 001, Schulz et al.: 14 979–15 001, Song et al., 2023b: 15 733–15 747, Stutz et al., 2000: 14 585–14 592, Tao et al.,: 11 729–11 746, Thalman et al.: O 2 –O 2, 15 371–15 381, Wang et al., 2025: 121 094, Wang et al., 2020: 10 807–10 829, Weger et al.: 17 545–17 572, Xu et al.: 10 557–10 570, Zhang et al., 2021: 15 809–15 826, Zhu et al., 2019: 13 067–13 078, Zhu et al., 2003: 10 667–10 677.

Specific comments references:
Acker et al.: paper no. L02809 is missing;

Alicke et al.: paper no. 8247 is missing, delete PHO 3–1–PHO3–17;

Alvarez: paper no. A183 is missing, delete “number: 0”;

Amedro et al: it should be “OH+NO₂”, delete “OH & thinsp; + & thinsp;NO2” (LaTeX used….?);

Bejan et al.: Abd El Aal, Y.;

Order the two Butkovskaya references chronologically (first 2005 and then 2007...);

Chatfield: doi number is missing: https://doi.org/10.1029/94GL02659;

Crowley et al. (2025): Please list all authors, see e.g. Andres Hernandez et al., where also more than 10 authors are listed... Unify style… The doi number is missing: https://doi.org/10.1039/D5EA00006H ;

Deutschmann et al.: Puķīte is not correctly written;

Dörisch et al: m/z 62;

George et al.: Add missing authors (more than 10 also in other references...); add Doi number: https://doi.org/10.5194/acp-23-7799-2023;

Graham et al.: add Doi number: https://doi.org/10.1016/0009-2614(77)80387-4;

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Finlayson-Pitts et al.: Finlayson-Pitts, B. J., Wingen, L. M., Sumner, A. L., and Ramazan K. A.;

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Kritten: add “Dissertation, Ruperto Carola University of Heidelberg, Germany”;

Lammel and Cape: 361-369;

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Ndour et al., paper no. is missing: L05812;

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Citation: https://doi.org/10.5194/egusphere-2025-5085-RC1

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Short summary

The destruction by sunlight of nitrous acid (HONO) produces the so-called detergent of the atmosphere. HONO has been measured in concentrations which exceed predictions based on known chemistry for decades. Several reactions have been proposed which may explain this excess HONO. This study reports on airborne measurements of HONO; the observations exceed predictions and form a C-shaped profile in the troposphere. Together with a host of other measurements, various reactions are investigated.


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