Observations of tropospheric HONO are incompatible with understanding of atmospheric chemistry

Rowlinson, Matthew James; Carpenter, Lucy J.; Evans, Mat J.; Lee, James D.; Andersen, Simone; Sherwen, Tomas; Callaghan, Anna B.; Sommariva, Roberto; Bloss, William; Hou, Siqi; Crilley, Leigh R.; Pfeilsticker, Klaus; Weyland, Benjamin; Ryerson, Thomas B.; Veres, Patrick R.; Campuzano-Jost, Pedro; Guo, Hongyu; Nault, Benjamin A.; Jimenez, Jose L.; Fomba, Khanneh Wadinga

doi:10.5194/egusphere-2025-830

Preprints

https://doi.org/10.5194/egusphere-2025-830

Preprints

26 Feb 2025

| 26 Feb 2025

Observations of tropospheric HONO are incompatible with understanding of atmospheric chemistry

Matthew James Rowlinson, Lucy J. Carpenter, Mat J. Evans, James D. Lee, Simone Andersen, Tomas Sherwen, Anna B. Callaghan, Roberto Sommariva, William Bloss, Siqi Hou, Leigh R. Crilley, Klaus Pfeilsticker, Benjamin Weyland, Thomas B. Ryerson, Patrick R. Veres, Pedro Campuzano-Jost, Hongyu Guo, Benjamin A. Nault, Jose L. Jimenez, and Khanneh Wadinga Fomba

Abstract. Recent observations of nitrous acid (HONO) in the remote troposphere found much higher concentrations than can be explained through known sources, with important implications for air quality and climate. Laboratory evidence and modelling of field observations suggests that nitrate aerosol photolysis is the likely mechanism providing the additional HONO, offering a rapid route for recycling of NO_x from nitric acid (HNO₃). Previous studies of the global impact of this chemistry have used either very restricted HONO data or a “top-down” approach to parameterize the HONO source by reconciling simulated and observed NO_xconcentrations. Here, we use multiple, independent tropospheric HONO observations from different locations to parameterize nitrate photolysis, and evaluate its impacts on global atmospheric chemistry using GEOS-Chem. The simulations improve agreement between modelled and observed HONO concentrations relative to previous studies, decreasing the model bias by 5–20 %. The remaining (and large) underestimate of HONO in the model is due predominantly to an underestimate of total nitrate aerosol (-95 %) and is reduced to 20 % when accounting for low model nitrate. Despite the low bias in the model HONO, we find that nitrate aerosol photolysis leads to substantial global increases in NO_x, O₃ and OH concentrations, likely beyond the observational constraints. The additional source of NO_x (~48 Tg N yr^-1 globally) is comparable to total NO_xemissions from all sources (~55 Tg yr^-1). These HONO observations in the remote troposphere, thus imply a large uncertainty in the NO_x budget and an incomplete understanding of atmospheric chemistry.

How to cite. Rowlinson, M. J., Carpenter, L. J., Evans, M. J., Lee, J. D., Andersen, S., Sherwen, T., Callaghan, A. B., Sommariva, R., Bloss, W., Hou, S., Crilley, L. R., Pfeilsticker, K., Weyland, B., Ryerson, T. B., Veres, P. R., Campuzano-Jost, P., Guo, H., Nault, B. A., Jimenez, J. L., and Fomba, K. W.: Observations of tropospheric HONO are incompatible with understanding of atmospheric chemistry, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-830, 2025.

Received: 21 Feb 2025 – Discussion started: 26 Feb 2025

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

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Journal article(s) based on this preprint

27 Nov 2025

A nitrate photolysis source of tropospheric HONO is incompatible with current understanding of atmospheric chemistry

Matthew J. Rowlinson, Lucy J. Carpenter, Mat J. Evans, James D. Lee, Simone T. Andersen, Tomas Sherwen, Anna B. Callaghan, Roberto Sommariva, William Bloss, Siqi Hou, Leigh R. Crilley, Klaus Pfeilsticker, Benjamin Weyland, Thomas B. Ryerson, Patrick R. Veres, Pedro Campuzano-Jost, Hongyu Guo, Benjamin A. Nault, Jose L. Jimenez, and Khanneh Wadinga Fomba

Atmos. Chem. Phys., 25, 16945–16968, https://doi.org/10.5194/acp-25-16945-2025,https://doi.org/10.5194/acp-25-16945-2025, 2025

Short summary

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-830', Anonymous Referee #1, 20 Mar 2025

Rowlinson et al. present a new parameterization of particulate nitrate photolysis to explain high HONO concentrations observed in recent field campaigns conducted in less polluted regions. They include this parameterization in the GEOS-Chem model and compare simulations against HONO observations from additional campaigns, finding improved agreement compared to simulations without this source. However, including this new HONO source leads to large changes in the global NOx source and the O₃ and OH burdens in the model, which are inconsistent with observational constraints. The authors, therefore, conclude that these HONO measurements highlight a gap in our current understanding of atmospheric chemistry.
The study addresses an important issue: photolysis of HONO is a source of NOx and HOx, both key species in tropospheric chemistry. Observations in remote areas have reported unexpectedly high HONO concentrations, suggesting an unidentified source that could significantly influence NOx and HOx cycling. Some prior studies have suggested particulate nitrate photolysis as this source and have derived photolysis rates based on the inferred HONO source. This study expands on that approach using a broader observational dataset and evaluates its global implications.
Although the results are potentially impactful, there are some major issues to be addressed:
i) The study adopts a narrow framing of uncertainties surrounding HONO sources, assuming particulate nitrate photolysis as the sole explanation for observed HONO (cf. Eq. 1), without adequate justification. Notably, the role of heterogeneous NO₂ reactions on aerosols is not discussed, although this process is included in GEOS-Chem. What is the uncertainty associated with it? What other mechanisms have been proposed in the literature, and why are they insufficient to explain the observed HONO? Addressing these questions is necessary to substantiate the study’s conclusion that the HONO observations point to a fundamental gap in our understanding.
ii) The key result supporting this conclusion is the large effect of the proposed HONO source on the modeled burdens of O₃ and OH, which is pushed beyond observational bounds. This occurs even though the model underestimates particulate nitrate concentrations and potentially also the HONO source. The issue again is that the study assumes that the missing HONO source is only from particulate nitrate photolysis. If the missing reaction producing HONO consumes NOx (and HOx), then the impact on O₃ and OH would be smaller. Li et al. (2014; doi: 10.1126/science.1248999) hypothesized such a mechanism, and recent laboratory work by Song et al. (2023; doi: 10.1038/s41612-023-00357-8) provides empirical evidence for another such reaction. This alternative explanation seems more plausible and should be considered.
iii) The physical interpretation of the parameterization for HONO production from particulate nitrate photolysis is unclear.

From Eqs. 1–4, the HONO production rate is given by:

P(HONO) = (2/3) × J(HNO₃) × EF × [pNO₃]

where EF = C1 / (1 + C2 × [pNO₃]). At atmospherically relevant [pNO₃], C2 × [pNO₃] >> 1, which reduces the EF to:

EF ≈ (C1 / C2) / [pNO₃]

and thus,

P(HONO) ≈ (2/3) × J(HNO₃) × (C1 / C2)

This implies that the HONO production rate is effectively independent of [pNO₃], which is difficult to interpret physically if particulate nitrate photolysis is the missing HONO source.

Citation: https://doi.org/10.5194/egusphere-2025-830-RC1
RC2:
'Comment on egusphere-2025-830', Anonymous Referee #2, 04 Apr 2025
General Comments

This study presents a compelling investigation into the role of nitrate aerosol photolysis as a major source of nitrous acid (HONO) in the troposphere, and the role of HONO source in the distribution of NOx and OH. By integrating multi-platform observational data (aircraft, ship, and ground-based campaigns), the authors validate a novel parameterization for nitrate photolysis enhancement factors (EFs), which is first proposed based on laboratory studies. Taking advantage of this parameterization method, the work highlights a previously underestimated renoxification pathway that contributes significantly to reactive nitrogen recycling (~48 Tg N yr⁻¹), rivaling direct anthropogenic NOx emissions. While the author point out that these HONO measurements represent a key uncertainty in our current understanding of atmospheric chemistry, there are several major issues concerning the model, measurements and their comparsions.

Specific comments:
The two assumptions of the parameterization method are critical to the study’s conclusions but require rigorous uncertainty propagation analysis. Addressing these through sensitivity analyses, mechanistic refinements, and observational validation would strengthen the parameterization’s robustness and provide clearer boundaries for the proposed renoxification impacts. Attributing observed HONO enhancements exclusively to nitrate photolysis lacks comprehensive validation, as alternative NOₓ-related HONO sources could not contribute to NOₓ regeneration source. A fixed HONO/NO₂ production ratio, based on a median value reported in Ye et al., neglects a large span of it. Such ratio uncertainties directly affect NOₓ and O₃ simulations, just like EF and particulate nitrate.

Althought the “new” parameterization adapts the pNO3 dependence and fit the field data with a R²=0.41, the substantial divergence between fitted and observed EFs (spanning >1 order of magnitude) introduces critical uncertainties in HONO predictions.

It is a bit confusing that particulate nitrate is underestimated in Figure 2, but overestimated in Figure 6?

The accurate representation of particulate nitrate (pNO₃⁻) concentrations is paramount for reliable HONO source quantification in the model. A key limitation arises because even with the proposed parameterization, the model cannot reconcile observed HONO levels if pNO₃⁻ is misrepresented (e.g., the current -95% low bias in total nitrate). This interdependency introduces a fundamental uncertainty: errors in pNO₃⁻ simulation propagate directly into HONO and NOₓ predictions, undermining confidence in the inferred atmospheric chemistry impacts (e.g., O₃ and OH increases). While the study acknowledges this issue, it does not address strategies to improve pNO₃⁻ modeling (e.g., aerosol-phase thermodynamics)—a gap that weakens the mechanistic credibility of the conclusions.

Current model uncertainties in simulating particulate nitrate, HONO, and NOₓ arise not only from incomplete mechanistic understanding but also from critical observational data gaps. Addressing both mechanism weaknesses and these data gaps is essential to advance model.
Citation: https://doi.org/10.5194/egusphere-2025-830-RC2
AC1: 'Comment on egusphere-2025-830', Mathew Evans, 30 Jul 2025

Replies to reviewers

We thank the reviewers for their comments, which are addressed below. We combine responses from reviewers when they make the same point.

Reviewer 1:
i) The study adopts a narrow framing of uncertainties surrounding HONO sources, assuming particulate nitrate photolysis as the sole explanation for observed HONO (cf. Eq. 1), without adequate justification. Notably, the role of heterogeneous NO2 reactions on aerosols is not discussed, although this process is included in GEOS-Chem. What is the uncertainty associated with it? What other mechanisms have been proposed in the literature, and why are they insufficient to explain the observed HONO? Addressing these questions is necessary to substantiate the study’s conclusion that the HONO observations point to a fundamental gap in our understanding.
Reviewer 2:
a) The two assumptions of the parameterization method are critical to the study’s conclusions but require rigorous uncertainty propagation analysis. Addressing these through sensitivity analyses, mechanistic refinements, and observational validation would strengthen the parameterization’s robustness and provide clearer boundaries for the proposed renoxification impacts. Attributing observed HONO enhancements exclusively to nitrate photolysis lacks comprehensive validation, as alternative NOₓ-related HONO sources could not contribute to NOₓ regeneration source. A fixed HONO/NO2 production ratio, based on a median value reported in Ye et al., neglects a large span of it. Such ratio uncertainties directly affect NOₓ and O₃ simulations, just like EF and particulate nitrate.

We agree with both reviewers’ comments here. We have reframed the question to be a more explicit exploration of what would happen if nitrate photolysis is the sole missing HONO source. We update the title of the paper to reflect this (“A nitrate photolysis source of remote tropospheric HONO is incompatible with current understanding of atmospheric chemistry”). We have included further quantitative discussion of alternative HONO production sources in the text which allows us to provide additional evidence as to why a heterogeneous conversion of NO2 to HONO appears unlikely to be the source at a site such as Cabo Verde (see lines 69-101 of the updated paper). As the reviewer says heterogenous uptake of NO2 to form HONO is in the GEOS-Chem model but at the measured values of gamma this is a slow process (see the discussion around line 85) and leads to a diurnal cycle (high at night and low during the day) which is the opposite of what is observed.

Reviewer 1
ii) The key result supporting this conclusion is the large effect of the proposed HONO source on the modeled burdens of O3and OH, which is pushed beyond observational bounds. This occurs even though the model underestimates particulate nitrate concentrations and potentially also the HONO source. The issue again is that the study assumes that the missing HONO source is only from particulate nitrate photolysis. If the missing reaction producing HONO consumes NOx (and HOx), then the impact on O3 and OH would be smaller. Li et al. (2014; doi: 10.1126/science.1248999) hypothesized such a mechanism, and recent laboratory work by Song et al. (2023; doi: 10.1038/s41612-023-00357-8) provides empirical evidence for another such reaction. This alternative explanation seems more plausible and should be considered.

We include the two references suggested into the text of the paper as potential gas phase sources. We note that there was a formal reply to the Li et al. paper by Ye et al. (2015) who concluded the reaction HO2.HO2.H2O + NO2 → HONO + O2 + H2O is an unlikely source for HONO based on their data. The daylight Song et al., mechanism involves the photolytic conversion of HNO3 adsorbed onto the surface of the chamber which in many ways is similar to the nitrate photolysis process proposed in previous papers and implemented here, where the HNO3 is absorbed into the aerosol and then photolyses. The observations in remote locations show a day time maximum in HONO and so the night-time source suggested in Song et al., is unlikely to be applicable. Please see lines 95 to 101 of the paper for the additional text.
We have also included words in the paper’s discussion section to re-iterate that the nitrate photolysis process converts NOy to NOx and that a HONO source that is a conversion from a NOx would likely have smaller impacts on atmospheric composition. Please see lines 623 to 630 of the paper.

iii) The physical interpretation of the parameterization for HONO production from particulate nitrate photolysis is unclear.

From Eqs. 1–4, the HONO production rate is given by:

P(HONO) = (2/3) × J(HNO3) × EF × [pNO3]

where EF = C1 / (1 + C2 × [pNO3]). At atmospherically relevant [pNO3], C2 × [pNO3] >> 1, which reduces the EF to:

EF ≈ (C1 / C2) / [pNO3]

and thus,

P(HONO) ≈ (2/3) × J(HNO3) × (C1 / C2)

This implies that the HONO production rate is effectively independent of [pNO3], which is difficult to interpret physically if particulate nitrate photolysis is the missing HONO source.

We agree with the interpretation of the reviewer here and have included discussion in the text. Both our parameterization and that of Andersen are in this regime which is effectively independent of nitrate for most atmospheric conditions. See lines 317 to 326 of the updated text.

Reviewer 2

This study presents a compelling investigation into the role of nitrate aerosol photolysis as a major source of nitrous acid (HONO) in the troposphere, and the role of HONO source in the distribution of NOx and OH. By integrating multi-platform observational data (aircraft, ship, and ground-based campaigns), the authors validate a novel parameterization for nitrate photolysis enhancement factors (EFs), which is first proposed based on laboratory studies. Taking advantage of this parameterization method, the work highlights a previously underestimated renoxification pathway that contributes significantly to reactive nitrogen recycling (~48 Tg N yr⁻¹), rivaling direct anthropogenic NOx emissions. While the author point out that these HONO measurements represent a key uncertainty in our current understanding of atmospheric chemistry, there are several major issues concerning the model, measurements and their comparsions.

Specific comments:

1. Although the “new” parameterization adapts the pNO3 dependence and fit the field data with a R2=0.41, the substantial divergence between fitted and observed EFs (spanning >1 order of magnitude) introduces critical uncertainties in HONO predictions.
We have included further comments about the quality of the fit in the text of the paper and the need for future laboratory and field work to better constrain the EF function. Please see lines 332 to 341 of the updated text

2. It is a bit confusing that particulate nitrate is underestimated in Figure 2, but overestimated in Figure 6?
This reflects a general underestimate in total nitrate in the model, but an overestimate in fine mode nitrate. We have updated the captions in Figure 2 and Figure 6 to make this clearer and included a comment to this effect in the discussion (please see lines 632 to 635).

3. The accurate representation of particulate nitrate (pNO₃⁻) concentrations is paramount for reliable HONO source quantification in the model. A key limitation arises because even with the proposed parameterization, the model cannot reconcile observed HONO levels if pNO₃⁻ is misrepresented (e.g., the current -95% low bias in total nitrate). This interdependency introduces a fundamental uncertainty: errors in pNO₃⁻ simulation propagate directly into HONO and NOₓ predictions, undermining confidence in the inferred atmospheric chemistry impacts (e.g., O₃ and OH increases). While the study acknowledges this issue, it does not address strategies to improve pNO₃⁻ modeling (e.g., aerosol-phase thermodynamics)—a gap that weakens the mechanistic credibility of the conclusions.
We include new comments in the paper that outline the need to improve the representation of aerosol nitrate (both in the fine and coarse modes) in order to further improve our understanding of this process. Please see lines 400 and 401 and lines 632 to 635.

4. Current model uncertainties in simulating particulate nitrate, HONO, and NOₓ arise not only from incomplete mechanistic understanding but also from critical observational data gaps. Addressing both mechanism weaknesses and these data gaps is essential to advance model.
We include new comments in the paper discussion outlining the need for improved instrumentation to measure low concentrations of HONO and, given the current model simulation, where these measurements should be best undertaken. See lines 638 to 643 of the updated paper.

Citation: https://doi.org/10.5194/egusphere-2025-830-AC1

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-830', Anonymous Referee #1, 20 Mar 2025

Rowlinson et al. present a new parameterization of particulate nitrate photolysis to explain high HONO concentrations observed in recent field campaigns conducted in less polluted regions. They include this parameterization in the GEOS-Chem model and compare simulations against HONO observations from additional campaigns, finding improved agreement compared to simulations without this source. However, including this new HONO source leads to large changes in the global NOx source and the O₃ and OH burdens in the model, which are inconsistent with observational constraints. The authors, therefore, conclude that these HONO measurements highlight a gap in our current understanding of atmospheric chemistry.
The study addresses an important issue: photolysis of HONO is a source of NOx and HOx, both key species in tropospheric chemistry. Observations in remote areas have reported unexpectedly high HONO concentrations, suggesting an unidentified source that could significantly influence NOx and HOx cycling. Some prior studies have suggested particulate nitrate photolysis as this source and have derived photolysis rates based on the inferred HONO source. This study expands on that approach using a broader observational dataset and evaluates its global implications.
Although the results are potentially impactful, there are some major issues to be addressed:
i) The study adopts a narrow framing of uncertainties surrounding HONO sources, assuming particulate nitrate photolysis as the sole explanation for observed HONO (cf. Eq. 1), without adequate justification. Notably, the role of heterogeneous NO₂ reactions on aerosols is not discussed, although this process is included in GEOS-Chem. What is the uncertainty associated with it? What other mechanisms have been proposed in the literature, and why are they insufficient to explain the observed HONO? Addressing these questions is necessary to substantiate the study’s conclusion that the HONO observations point to a fundamental gap in our understanding.
ii) The key result supporting this conclusion is the large effect of the proposed HONO source on the modeled burdens of O₃ and OH, which is pushed beyond observational bounds. This occurs even though the model underestimates particulate nitrate concentrations and potentially also the HONO source. The issue again is that the study assumes that the missing HONO source is only from particulate nitrate photolysis. If the missing reaction producing HONO consumes NOx (and HOx), then the impact on O₃ and OH would be smaller. Li et al. (2014; doi: 10.1126/science.1248999) hypothesized such a mechanism, and recent laboratory work by Song et al. (2023; doi: 10.1038/s41612-023-00357-8) provides empirical evidence for another such reaction. This alternative explanation seems more plausible and should be considered.
iii) The physical interpretation of the parameterization for HONO production from particulate nitrate photolysis is unclear.

From Eqs. 1–4, the HONO production rate is given by:

P(HONO) = (2/3) × J(HNO₃) × EF × [pNO₃]

where EF = C1 / (1 + C2 × [pNO₃]). At atmospherically relevant [pNO₃], C2 × [pNO₃] >> 1, which reduces the EF to:

EF ≈ (C1 / C2) / [pNO₃]

and thus,

P(HONO) ≈ (2/3) × J(HNO₃) × (C1 / C2)

This implies that the HONO production rate is effectively independent of [pNO₃], which is difficult to interpret physically if particulate nitrate photolysis is the missing HONO source.

Citation: https://doi.org/10.5194/egusphere-2025-830-RC1
RC2:
'Comment on egusphere-2025-830', Anonymous Referee #2, 04 Apr 2025
General Comments

This study presents a compelling investigation into the role of nitrate aerosol photolysis as a major source of nitrous acid (HONO) in the troposphere, and the role of HONO source in the distribution of NOx and OH. By integrating multi-platform observational data (aircraft, ship, and ground-based campaigns), the authors validate a novel parameterization for nitrate photolysis enhancement factors (EFs), which is first proposed based on laboratory studies. Taking advantage of this parameterization method, the work highlights a previously underestimated renoxification pathway that contributes significantly to reactive nitrogen recycling (~48 Tg N yr⁻¹), rivaling direct anthropogenic NOx emissions. While the author point out that these HONO measurements represent a key uncertainty in our current understanding of atmospheric chemistry, there are several major issues concerning the model, measurements and their comparsions.

Specific comments:
The two assumptions of the parameterization method are critical to the study’s conclusions but require rigorous uncertainty propagation analysis. Addressing these through sensitivity analyses, mechanistic refinements, and observational validation would strengthen the parameterization’s robustness and provide clearer boundaries for the proposed renoxification impacts. Attributing observed HONO enhancements exclusively to nitrate photolysis lacks comprehensive validation, as alternative NOₓ-related HONO sources could not contribute to NOₓ regeneration source. A fixed HONO/NO₂ production ratio, based on a median value reported in Ye et al., neglects a large span of it. Such ratio uncertainties directly affect NOₓ and O₃ simulations, just like EF and particulate nitrate.

Althought the “new” parameterization adapts the pNO3 dependence and fit the field data with a R²=0.41, the substantial divergence between fitted and observed EFs (spanning >1 order of magnitude) introduces critical uncertainties in HONO predictions.

It is a bit confusing that particulate nitrate is underestimated in Figure 2, but overestimated in Figure 6?

The accurate representation of particulate nitrate (pNO₃⁻) concentrations is paramount for reliable HONO source quantification in the model. A key limitation arises because even with the proposed parameterization, the model cannot reconcile observed HONO levels if pNO₃⁻ is misrepresented (e.g., the current -95% low bias in total nitrate). This interdependency introduces a fundamental uncertainty: errors in pNO₃⁻ simulation propagate directly into HONO and NOₓ predictions, undermining confidence in the inferred atmospheric chemistry impacts (e.g., O₃ and OH increases). While the study acknowledges this issue, it does not address strategies to improve pNO₃⁻ modeling (e.g., aerosol-phase thermodynamics)—a gap that weakens the mechanistic credibility of the conclusions.

Current model uncertainties in simulating particulate nitrate, HONO, and NOₓ arise not only from incomplete mechanistic understanding but also from critical observational data gaps. Addressing both mechanism weaknesses and these data gaps is essential to advance model.
Citation: https://doi.org/10.5194/egusphere-2025-830-RC2
AC1: 'Comment on egusphere-2025-830', Mathew Evans, 30 Jul 2025

Replies to reviewers

We thank the reviewers for their comments, which are addressed below. We combine responses from reviewers when they make the same point.

Reviewer 1:
i) The study adopts a narrow framing of uncertainties surrounding HONO sources, assuming particulate nitrate photolysis as the sole explanation for observed HONO (cf. Eq. 1), without adequate justification. Notably, the role of heterogeneous NO2 reactions on aerosols is not discussed, although this process is included in GEOS-Chem. What is the uncertainty associated with it? What other mechanisms have been proposed in the literature, and why are they insufficient to explain the observed HONO? Addressing these questions is necessary to substantiate the study’s conclusion that the HONO observations point to a fundamental gap in our understanding.
Reviewer 2:
a) The two assumptions of the parameterization method are critical to the study’s conclusions but require rigorous uncertainty propagation analysis. Addressing these through sensitivity analyses, mechanistic refinements, and observational validation would strengthen the parameterization’s robustness and provide clearer boundaries for the proposed renoxification impacts. Attributing observed HONO enhancements exclusively to nitrate photolysis lacks comprehensive validation, as alternative NOₓ-related HONO sources could not contribute to NOₓ regeneration source. A fixed HONO/NO2 production ratio, based on a median value reported in Ye et al., neglects a large span of it. Such ratio uncertainties directly affect NOₓ and O₃ simulations, just like EF and particulate nitrate.

We agree with both reviewers’ comments here. We have reframed the question to be a more explicit exploration of what would happen if nitrate photolysis is the sole missing HONO source. We update the title of the paper to reflect this (“A nitrate photolysis source of remote tropospheric HONO is incompatible with current understanding of atmospheric chemistry”). We have included further quantitative discussion of alternative HONO production sources in the text which allows us to provide additional evidence as to why a heterogeneous conversion of NO2 to HONO appears unlikely to be the source at a site such as Cabo Verde (see lines 69-101 of the updated paper). As the reviewer says heterogenous uptake of NO2 to form HONO is in the GEOS-Chem model but at the measured values of gamma this is a slow process (see the discussion around line 85) and leads to a diurnal cycle (high at night and low during the day) which is the opposite of what is observed.

Reviewer 1
ii) The key result supporting this conclusion is the large effect of the proposed HONO source on the modeled burdens of O3and OH, which is pushed beyond observational bounds. This occurs even though the model underestimates particulate nitrate concentrations and potentially also the HONO source. The issue again is that the study assumes that the missing HONO source is only from particulate nitrate photolysis. If the missing reaction producing HONO consumes NOx (and HOx), then the impact on O3 and OH would be smaller. Li et al. (2014; doi: 10.1126/science.1248999) hypothesized such a mechanism, and recent laboratory work by Song et al. (2023; doi: 10.1038/s41612-023-00357-8) provides empirical evidence for another such reaction. This alternative explanation seems more plausible and should be considered.

We include the two references suggested into the text of the paper as potential gas phase sources. We note that there was a formal reply to the Li et al. paper by Ye et al. (2015) who concluded the reaction HO2.HO2.H2O + NO2 → HONO + O2 + H2O is an unlikely source for HONO based on their data. The daylight Song et al., mechanism involves the photolytic conversion of HNO3 adsorbed onto the surface of the chamber which in many ways is similar to the nitrate photolysis process proposed in previous papers and implemented here, where the HNO3 is absorbed into the aerosol and then photolyses. The observations in remote locations show a day time maximum in HONO and so the night-time source suggested in Song et al., is unlikely to be applicable. Please see lines 95 to 101 of the paper for the additional text.
We have also included words in the paper’s discussion section to re-iterate that the nitrate photolysis process converts NOy to NOx and that a HONO source that is a conversion from a NOx would likely have smaller impacts on atmospheric composition. Please see lines 623 to 630 of the paper.

iii) The physical interpretation of the parameterization for HONO production from particulate nitrate photolysis is unclear.

From Eqs. 1–4, the HONO production rate is given by:

P(HONO) = (2/3) × J(HNO3) × EF × [pNO3]

where EF = C1 / (1 + C2 × [pNO3]). At atmospherically relevant [pNO3], C2 × [pNO3] >> 1, which reduces the EF to:

EF ≈ (C1 / C2) / [pNO3]

and thus,

P(HONO) ≈ (2/3) × J(HNO3) × (C1 / C2)

This implies that the HONO production rate is effectively independent of [pNO3], which is difficult to interpret physically if particulate nitrate photolysis is the missing HONO source.

We agree with the interpretation of the reviewer here and have included discussion in the text. Both our parameterization and that of Andersen are in this regime which is effectively independent of nitrate for most atmospheric conditions. See lines 317 to 326 of the updated text.

Reviewer 2

This study presents a compelling investigation into the role of nitrate aerosol photolysis as a major source of nitrous acid (HONO) in the troposphere, and the role of HONO source in the distribution of NOx and OH. By integrating multi-platform observational data (aircraft, ship, and ground-based campaigns), the authors validate a novel parameterization for nitrate photolysis enhancement factors (EFs), which is first proposed based on laboratory studies. Taking advantage of this parameterization method, the work highlights a previously underestimated renoxification pathway that contributes significantly to reactive nitrogen recycling (~48 Tg N yr⁻¹), rivaling direct anthropogenic NOx emissions. While the author point out that these HONO measurements represent a key uncertainty in our current understanding of atmospheric chemistry, there are several major issues concerning the model, measurements and their comparsions.

Specific comments:

1. Although the “new” parameterization adapts the pNO3 dependence and fit the field data with a R2=0.41, the substantial divergence between fitted and observed EFs (spanning >1 order of magnitude) introduces critical uncertainties in HONO predictions.
We have included further comments about the quality of the fit in the text of the paper and the need for future laboratory and field work to better constrain the EF function. Please see lines 332 to 341 of the updated text

2. It is a bit confusing that particulate nitrate is underestimated in Figure 2, but overestimated in Figure 6?
This reflects a general underestimate in total nitrate in the model, but an overestimate in fine mode nitrate. We have updated the captions in Figure 2 and Figure 6 to make this clearer and included a comment to this effect in the discussion (please see lines 632 to 635).

3. The accurate representation of particulate nitrate (pNO₃⁻) concentrations is paramount for reliable HONO source quantification in the model. A key limitation arises because even with the proposed parameterization, the model cannot reconcile observed HONO levels if pNO₃⁻ is misrepresented (e.g., the current -95% low bias in total nitrate). This interdependency introduces a fundamental uncertainty: errors in pNO₃⁻ simulation propagate directly into HONO and NOₓ predictions, undermining confidence in the inferred atmospheric chemistry impacts (e.g., O₃ and OH increases). While the study acknowledges this issue, it does not address strategies to improve pNO₃⁻ modeling (e.g., aerosol-phase thermodynamics)—a gap that weakens the mechanistic credibility of the conclusions.
We include new comments in the paper that outline the need to improve the representation of aerosol nitrate (both in the fine and coarse modes) in order to further improve our understanding of this process. Please see lines 400 and 401 and lines 632 to 635.

4. Current model uncertainties in simulating particulate nitrate, HONO, and NOₓ arise not only from incomplete mechanistic understanding but also from critical observational data gaps. Addressing both mechanism weaknesses and these data gaps is essential to advance model.
We include new comments in the paper discussion outlining the need for improved instrumentation to measure low concentrations of HONO and, given the current model simulation, where these measurements should be best undertaken. See lines 638 to 643 of the updated paper.

Citation: https://doi.org/10.5194/egusphere-2025-830-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Mathew Evans on behalf of the Authors (02 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Aug 2025) by Markus Ammann

RR by Anonymous Referee #1 (25 Aug 2025)

RR by Anonymous Referee #2 (26 Aug 2025)

ED: Publish as is (26 Aug 2025) by Markus Ammann

AR by Mathew Evans on behalf of the Authors (14 Sep 2025)

Journal article(s) based on this preprint

27 Nov 2025

A nitrate photolysis source of tropospheric HONO is incompatible with current understanding of atmospheric chemistry

Atmos. Chem. Phys., 25, 16945–16968, https://doi.org/10.5194/acp-25-16945-2025,https://doi.org/10.5194/acp-25-16945-2025, 2025

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HONO is key to tropospheric chemistry. Observations show high HONO concentrations in remote air, possibly explained by nitrate aerosol photolysis. We use observational data to parameterize nitrate photolysis, evaluating simulated HONO against observations from multiple sources. We show improved agreement with observed HONO, but large overestimates in NO_x and O₃, beyond observational constraints. This implies a large uncertainty in the NO_x budget and our understanding of atmospheric chemistry.


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