Budget of nitrous acid (HONO) at an urban site in the fall season of Guangzhou, China

Yu, Yihang; Cheng, Peng; Li, Huirong; Yang, Wenda; Han, Baobin; Song, Wei; Hu, Weiwei; Wang, Xinming; Yuan, Bin; Shao, Min; Huang, Zhijiong; Li, Zhen; Zheng, Junyu; Wang, Haichao; Yu, Xiaofang

doi:https://doi.org/10.5194/acp-22-8951-2022

Articles | Volume 22, issue 13

https://doi.org/10.5194/acp-22-8951-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-22-8951-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 22, issue 13

Research article

|

12 Jul 2022

Research article |

| 12 Jul 2022

Budget of nitrous acid (HONO) at an urban site in the fall season of Guangzhou, China

Yihang Yu, Peng Cheng, Huirong Li, Wenda Yang, Baobin Han, Wei Song, Weiwei Hu, Xinming Wang, Bin Yuan, Min Shao, Zhijiong Huang, Zhen Li, Junyu Zheng, Haichao Wang, and Xiaofang Yu

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Final revised paper (published on 12 Jul 2022)
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Preprint (discussion started on 03 May 2021)
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Interactive discussion

Status: closed

RC1:
'Comment on acp-2021-178', Anonymous Referee #1, 21 Jun 2021

This paper presents a detailed analysis of the HONO budget in the Pearl River Delta region of China. The paper is well written, the data and the analysis are well presented. The subject is fit for publication in ACP and I would recommend the paper is accepted after the authors have addressed the following concerns.

Major Comments

1) Please add more information about the box-model. The MCM is not a model, it is just the chemical mechanism used in a model. Which software/modelling tool was used? Which VOCs were included? How was photolysis calculated for the non-measured photolysis rates? Were other processes (heterogeneous, deposition, etc..) included?

2) The molybdenum converter used to measure NOx is subject to known interferences by other NOy species. Since a large part of the analysis in this paper relies heavily on NO and NO2 data, this issue cannot be neglected. I would expect the interference to be significant under the urban conditions considered here. The authors should address this issue and examine how the results of the studies are affected by it.

3) I think the discussion in section 3.2 needs to be improved. First the observed HONO production rate should be presented and shown (how was it calculated, which are the mean values, etc..). This will make the following calculations easier to understand. Besides that, I have two main comments regarding this section.

One, the authors infer that a large missing sink of HONO is required to explain the observations (lines 274-275). However, their calculation of HONO primary emissions relies on emission inventories that are likely not very accurate. The possiiblity that HONO primary emissions are overestimated in the emission inventories cannot be neglected and needs to be discussed.

Two, the authors are deriving a primary emission rate of 0.04 ppb/h or more (line 272), a soil emission rare of 0.02 ppb/h (line 297) and a net production via OH+NO of 0.26 ppb/h (line 314), while the average observed HONO production rate is 0.02 ppb/h (line 271). From this an unknown sink of 0.25 ppb/h is inferred. First of all, in order to close the budget, the unknown sink should be 0.30 ppb/h (unless you mean that 0.05 ppb/h is lost via deposition, it is not clear from section 3.2.4). More importantly, the discussion in section 3.2.3 implies an additional, non quantified source due to NO2 reaction on surfaces. so the unknown sink is actually a lower limit (but see also the previous comment, regarding possible overestimation of primary emissions). These calculations should be make clearer, maybe with an extra "summary" subsection at the end of section 3.2.

4) In section 3.4, I would suggest that if VOC data are available, than ozonolysis of alkenes should be added here. Several studies have suggested that these process may be important in urban conditions. In fact, why not use the model results from section 3.5 to calculate the OH production pathways? It would be more comprehensive than what is shown in figure 9.

Minor Comments

lines 169-171: what does it mean that "the boundary layer diurnal cycle has been modified"? And what are the "solar altitude" and the "photolysis rate correction coefficient"?

figure 3: a blue line with pink shading is confusing. It would be better to use a shade of blue. Also why not add the results obtained with the other two methods? It may be interesting to compare them.

figure 5: I would not consider the correlation netween HONO and NO, "a good correlation". In fact it is not even linear, meaning it doesn't really provide evidence that OH+NO is a major pathway.

figure 6: can you explain why you are averaging only the top five HONO/NO2 values?

Citation: https://doi.org/10.5194/acp-2021-178-RC1
- AC1: 'Reply on RC1', YiHang Yu, 09 Aug 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-178/acp-2021-178-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-178-AC1
RC2:
'Comment on acp-2021-178', Anonymous Referee #2, 25 Jun 2021
In this work, data collected from the Pearl River Delta in China have been used to explore potential nitrous acid (HONO) sources and their impacts on the production of hydroxyl radical (OH) and photochemical ozone (O3). The Authors perform a large number of calculations that are replicated from a variety of other publications to assess sources and sinks of HONO for their observational dataset. Despite the results of these calculations being grossly erroneous (e.g. direct emissions calculated exceeding the observations by over an order of magnitude), limited to single value comparisons (e.g. average accumulation rates calculated and observed), and using clearly erroneous assumptions (e.g. 10^6 molec cm-3 of OH at night) the Authors press on to calculate a radical budget and impact on O3 chemistry in the PRD. Overall, this work does not demonstrate any progress in our understanding of the impacts of HONO on oxidation chemistry due to fundamentally flawed data interpretation. The extreme mismatches between the calculated HONO sources and those observed are never depicted and raise serious questions regarding quality control of this work. Given that the topic of HONO sources and sinks is only the first part of this manuscript, it is not possible to consider the remainder of this work that draws on this analysis to try and improve understanding of oxidation chemistry and radical budgets. As this manuscript currently stands, it is unsuitable for publication in ACP and requires extensive re-work.

Below is an incomplete list of outstanding issues that require addressing, which may not yield an acceptable manuscript once completed, as the issues impacting this work are pervasive and beyond the scope of the requirements of peer review. The Authors are encouraged to significantly revisit the contents of the manuscript and independently ascertain that the work presents valid findings and communicates a complete understanding of the chemistry explored. As it currently stands, the manuscript replicates the prior work of others without careful reflection on whether the findings are consistent with the established knowledge of the related atmospheric chemistry.

Major issues:

The introduction of the manuscript is unorganized and simply lists topics in nearly random order (e.g. the sources and sinks of HONO). There are basic concepts of atmospheric chemistry that do not seem to be correctly understood (e.g. microbial production of HONO is not a heterogenous reaction). There is extensive discussion of mechanisms that have been thoroughly refuted (e.g. two photon excitation of NO2 followed by reaction of the excited state with water or termolecular reactions with NH3) which are presented as topics of open debate. The Authors should significantly rework the introduction for clarity, but also with a focus on having it reflect the contents of the work being done in the manuscript. Very little text presents the outstanding issue of poor air quality and oxidation chemistry in the PRD, despite significant work having been done in this area over the past 10 years. As it currently stands, the introduction is only weakly motivating this work and can be significantly improved.

This manuscript uses the performed HONO measurements extensively. The Authors’ data is collected using a custom-built instrument that uses similar principles to the LOPAP. No prior work demonstrating the accuracy, precision, reliability through intercomparison, etc are made. Instead the Authors cite the manuscripts that established the commercial LOPAP instrumentation as though they apply to their apparatus. It is not clear if the presented QA/QC values were determined from data collected during this study or from statements others have made in the literature.

Direct emissions of HONO calculations are grossly incorrect. The Authors present several methods from the literature that have been used previously, none of which give a reasonable result when they compare to their observations (e.g. they calculate direct emission rates of 0.3 ppbv hr-1 versus 0.02 observed). Despite having CO measurements, they do not draw on these to arrive at more reasonable estimate and belabour a number of other ways to estimate the direct HONO emission values. While one can appreciate the work done to arrive at an unexpected finding, the results conflicting with the observations in such an extreme way require some significant reflection on the state of understanding of direct HONO sources and why the established literature approaches fail to reach reasonable results with this observational dataset. Instead of taking the opportunity to make a meaningful contribution in this respect, the Authors simply press forward with further calculations on HONO sources and sinks. The absence of a temporally-resolved intercomparison between the measured and calculated direct HONO emission sources in a figure raises serious concerns. The Authors state that the site is more impacted by direct emissions than previously considered, but this result comes from a calculation that does not compare within the same order of magnitude of the observations.

Soil emissions of HONO are not justified and rely on a set of assumptions that are not justified (e.g. boundary layer height and surrounding landscape properties) and are quite clearly in error. The HONO production rates calculated again exceed those observed significantly, raising many questions around attention to the validity of data interpretation in this manuscript.

The use of a static OH value of 10^6 at night based on one measurement. Again, the result of the calculation differs from the observations (and again only comparing single values instead of temporally-resolved data) by over an order of magnitude.

Deposition losses of HONO rely on reasonable production terms. Since the production terms have major errors, and this calculation propagates those, the result cannot be correct. Further considerations for this section are the large body of work that has investigated the reactive uptake coefficients for HONO on surfaces, from which dry deposition velocities can be approximated, in order to make literature comparisons that are much more recent and detailed.

The daytime HONO budget compounds all of these errors further and the manuscript henceforth cannot be seen as scientifically reliable for further evaluation.
Citation: https://doi.org/10.5194/acp-2021-178-RC2
- AC2: 'Reply on RC2', YiHang Yu, 09 Aug 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-178/acp-2021-178-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-178-AC2
- AC3: 'Reply on RC2', Peng Cheng, 10 Aug 2021
  
  We thank the reviewer for taking the time to review our paper. It is always constructive in a peer-review process to directly point out any concerns and issues from the reviewer’s perspective and urge us as authors to provide reasons and clarifications whenever necessary.
  It is true that there have been a large body of literatures on the HONO sources and sinks, and many theories and findings have emerged from these studies, many of which are conflicting between each other. As a ripple effect, more confusions and new challenges emerged for studies that followed. As a participant in numerous HONO studies (e.g., Su et al., 2008, Su et al., 2011, Cheng et al., 2013, Yang et al., 2017, Tian et al., 2018), the corresponding author of this paper sincerely shares the reviewer’s understanding that it takes patient efforts to make any progress in this small yet important field of atmospheric chemistry, due to the lack of information on many processes being considered in the HONO budget. As such, it is crucial to acknowledge all kinds of uncertainties and be transparent on assumptions and caveats in the process of conducting measurements and calculations, in order to provide useful and accurate information and findings for future studies to rely on to make any further meaningful progress. Bearing these considerations in mind, we tried to consider all possible varying and uncertain factors as far as we deem appropriate and tried to be conservative in our estimates of the terms.
  For instance, direct emission is one of the main points of our paper that this source of HONO and its uncertainties need to be further investigated in future studies. We considered multiple methods and estimated a range of possible direct emission rates; we used two emission inventories to account for the uncertainty in this kind of input data in estimating emissions. Yet it is inevitable for such an effort to lead to a lengthy and in some cases tedious documentation of all the methods adopted and all the outcomes derived, which might become a cause of confusions. We hope to do whatever we can to make our paper clear, accurate, and scientifically sound. To answer the reviewer’s question why the direct emissions calculated exceeding the observations by over an order of magnitude, it is because the two terms are related but not consistent, since the latter is a result of many processes, e.g., emission, reaction, transport, etc. For example, NOx level often decreases at daytime, during which the emission rates of NOx are obviously greater than the observed growth rates. To answer another question why the comparison between P_emis and observed HONO was limited to averaged values, it is because averaging can smooth out the influence of fluctuation and uncertainties in various influencing factors (transport, dilution, OH, etc.). Because of the long lifetime of HONO at night, and effect of transport and large uncertainties in the dilution/diffusion conditions, a temporally-resolved budget appears desirable but would not be meaningful given all uncertainties. Otherwise, the assumption of the nocturnal OH concentration to be 1.0 × 10⁶ cm⁻³appears problematic but is possible in the PRD region. Sensitivity tests also showed limited impact from this assumption on our conclusion about HONO.
  In light of the reviewer’s comments, we have re-examined and revised our paper for better clarity, accuracy, and completeness toward a good reception of our paper by a broad range of a readers of ACP. Indeed, addressing those critical comments from the reviewer turned out very useful for us to improve our paper. We welcome the reviewer to review our responses and revisions, and provide any further comments and discussions, if any, with the goal of reaching a comprehensive and objective assessment of the scientific contributions made by our paper.
  
  Citation: https://doi.org/10.5194/acp-2021-178-AC3

Peer review completion

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

AR by Yihang Yu on behalf of the Authors (11 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (20 Aug 2021) by Steven Brown

RR by Anonymous Referee #1 (24 Sep 2021)

RR by Anonymous Referee #2 (18 Oct 2021)

Suggestions for revision or reasons for rejection

Overview

The Authors have made several changes to this manuscript to address some of the comments raised by the Reviewers yet did not fully address those made by both Reviewers in the first round of revisions. Additions of measurement intercomparison and QA/QC for the HONO measurements, as well as the inclusion of alkene and O3 chemistry are excellent and the Authors are thanked for these additions. The initial changes are a good start to reach an acceptable manuscript for acceptance into ACP, yet major revisions are still required. The Authors have requested specific and detailed constructive criticisms. Until the initial major issues raised are resolved, this Reviewer must point out that is not possible to assess the remainder of the work with constructive criticism as the existing flaws draw reasonable concern about the basis of this work and the potential over-interpretation of the dataset. In an effort to create a constructive dialogue towards a final accepted manuscript, one suggestion is that progress with the current work may be most easily accomplished if the Authors consider separating the HONO chemistry and budget from its impact on oxidants into a second manuscript. Once the major issues are resolved around the HONO budget, then the oxidant impacts work can be submitted separately for peer review.

Major Comments

Both Reviewers raised concerns around the HONO budget, focusing on nighttime observations alone, and requested better description of the accounting of sources and sinks. These were to be compared against the averaged nocturnal observations to demonstrate that the chemistry and transformations of HONO suggested by the Authors were reasonably approximating the observations. This was not completed and a figure depicting the rates of HONO generated by the sources and sinks, along with the resulting predicted HONO presented against the observations, was not made. The validity of the HONO budget remains questionable until this is presented and discussed with comparison to other reports, such as those extensively cited by the Authors in their response and more broadly for polluted urban environments. The Authors contend that comparing a single average value is sufficient for understanding HONO chemistry in the PRD as opposed to temporally-resolved diurnal data in order to minimize the impacts of fluctuations of other factors such as transport and dilution. The Reviewer finds such an argument to be of high concern for a dataset that spans several weeks of observations in a manuscript being considered in an atmospheric chemistry journal. The measurements are available to the Authors to do such a temporally-resolved representation of HONO nighttime chemistry and there is ample precedent in the literature demonstrating how to construct one (e.g. see the extensive work published by either Stutz or Kleffmann). Such an analysis has been a standard approach for assessing the impacts of HONO accumulated at night on oxidation chemistry the following day, which the Authors cite in their response and should revisit to improve this manuscript.

A good example of remaining questions around the HONO budget comes from the major error in the deposition of HONO that was corrected by the Authors using reactive uptake coefficients in the literature. This major error was identified by both Reviewers and it is now contended by the Authors that this correction makes the production and loss terms explored in this work robust enough to represent the observations. This is not depicted, nor summarized, to provide a convincing argument that the chemical controls on HONO are understood well enough to extend the analysis to oxidant impacts. The work performed by the Authors for representing the HONO deposition is well done and needs to be extended to the inclusion of NO2 heterogenous hydrolysis and a full HONO budget analysis that is compared to the observations. This mechanism has been demonstrated for decades to be a major source of nocturnal HONO, such that this has become the textbook mechanism for secondary HONO chemical formation. In a concerning rebuttal regarding the conversion of NO2 on surfaces at night, the Authors contend that this does not need to be performed because their other mechanisms make enough HONO already. This is not a factual position to take, as Figure 5e looks nearly identical to that presented previously by Stutz et al. (2004) when exploring this mechanism (i.e. a conversion ratio that increases with RH above 70 %). The Authors need to draw from the work that includes reactive uptake coefficients for this conversion to be calculated (e.g. see the extensive modeling work from Stutz) and numerically demonstrate that all sources and sinks are reasonably represented. Given the long-standing history of the importance of NO2 heterogeneous hydrolysis in the literature, it is confusing to see soil emissions given a highly weighted role in this work. It is similarly confusing to see the error in the soil HONO emission estimate being so tightly constrained (+/- 0.001 ppb hr-1), given that clear observations of this process have predmoninantly been seen in lab samples. Those field observations that do exist remain limited to agricultural soils. The literature cited in the Authors’ response regarding the approach in constraining the soil emissions include a PhD thesis that has not been published in a peer-reviewed journal and a second article that indicates the approach is semiquantitative in the title. While it is commendable to bring the dimension of soil emissions into the analysis, it cannot be made at the expense of much more thoroughly described HONO production chemistry, and without the approach being very clearly presented in this manuscript (or its SI).

Finally, the order of magnitude uncertainty presented in the direct HONO emission rate and in the NO + OH production of HONO at night continue to raise major concern when the sources and sinks, along with propagated error, are not fully communicated. The Authors have made good progress towards being able to do this, and with the inclusion of NO2 hydrolysis, can likely complete the analysis successfully. The range of direct emissions and the NO+OH sensitivity study then need to be combined with that result. Having pieced the presented facts for the latter two mechanisms together, a resulting difference of P(HONO) at 0.02 ppbv hr-1 between the large sources and sinks (each up to an order of magnitude larger) to match the observations seems possibly insignificantly different from zero or an overall loss once the error is propagated. One way to negate this issue could be to continue with the conservative approach that the Authors state they are taking in exploring the dataset: i) put sources at the lower limit and sinks at the upper limit, and ii) vice versa. Then present the resultant HONO production rates at both boundaries against that which was observed, including propagation of the error at both limits. This should then be followed in the discussion by a synthesis of the most likely combination of production and loss processes, the average temporal representation of these, and a demonstration that this compares well to the measurements. The extension of such results to the impact on oxidation capacity can then use the upper and lower limits of the HONO budget to set more reasonable boundaries on the potential impact that it has on chemical processes at this location.

The Reviewer hopes that the Authors find these suggested revisions more constructive such that the manuscript can be improved to meet the expectations of publication in ACP. Further constructive criticism and suggestions to improve this manuscript are currently out of the scope of peer review, as the time required is too substantial given the current state of this work. Obtaining such a detailed review is the responsibility of the Authors by addressing the major concerns that currently exist. The Authors are strongly encouraged to again consider revising their introduction to improve the narrative being outlined, as it remains a collections of statements/facts obtained from the literature instead of synthesizing the state of knowledge such that outstanding gaps are clearly presented for the current work to address. The quality of writing in this section significantly lowers the impact of this work.

Hide

ED: Reconsider after major revisions (19 Oct 2021) by Steven Brown

AR by Yihang Yu on behalf of the Authors (15 Feb 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (20 Feb 2022) by Steven Brown

RR by Anonymous Referee #2 (23 May 2022)

Suggestions for revision or reasons for rejection

The Authors have done an excellent job revising the manuscript to reflect the state of knowledge on HONO sources and sinks, along with some analysis of its daytime sources. The extension to interpreting its impact on daytime oxidation was reasonably removed to narrow the scope of the manuscript, given the magnitude of uncertainties in the HONO analysis. The manuscript is now ready for publication in Atmospheric Chemistry and Physics, pending the following minor revisions.

Page 2, Line 50: Missing recent study by Pusede et al.(2015) that is more comprehensive than some of the works cited here for detailed explorations of formation mechanisms.

Page 2, Line 58: ‘fresh air masses and vehicle exhaust’ Should this be ‘fresh air masses mixed with vehicle exhaust’?

Page 2, Line 61: HONO/NOx ratios throughout the depth of the nocturnal boundary layer, as well as during the day were also presented by VandenBoer et al. (2013) from the NACHTT campaign and also by a pair of papers from (Kleffmann et al., 2003; Vogel et al., 2003).

Page 3, Lines 74-76: Here the Authors are citing HONO formation mechanisms that have been disproven in the literature, especially the excited NO2 and water vapor reaction (shown to be a two photon process, which is not atmospherically relevant). The homogeneous nucleation of NH3, NO2, and H2O has also not been found to be important because, like most atmospheric ternary reaction mechanisms, the probability of the reaction is so low that its rate of formation for HONO simply is not that important. Remove these. There is far more evidence for substantial HONO formation from heterogeneous reactions.

Page 3, Line 84: ‘appear to suggest’ should be either ‘appear’ or ‘suggest’. Pick one.

Page 3, Line 91: ‘reduction on reductive’ is redundant. Revise.

Page 3, Line 93: Missing the following work by (Aubin and Abbatt, 2007)

Page 5, Line 162: This is a Time-of-Flight Aerosol Mass Spectrometer. Please revise.

Page 9, Line 204: ‘0.2 % to 9.1 %’ is here in the discussion. Maybe it would be nice to give these same values/units on panel (c) in the figure?

Page 10, Figure 2: The caption needs to provide reference to each panel letter in the caption text. Please add. The notation on (c) needs to be changed to more intuitive values because 0.07 or 7% are easier to interpret than 70x10^-3.

Page 10, Lines 217-221: No need to letter the criteria. They are not separated and analyzed separately through a sensitivity analysis or anything later. Instead, present them in order as a list in paragraph format, with the semicolon separations retained.

Page 11, Line 223: Should ‘criteria’ be ‘all of the criteria’?

Page 11, Lines 225-226: Combine the two Hong Kong studies to simplify here ‘… measured in Hong Kong at 1.2% (Xu et al.) and 1.0% (Yun et al.)’

Page 11, Line 233: ‘simply’ is not needed. Remove.

Page 11, Line 237: Should ‘exclude’ be ‘estimate’ or ‘take into account’ here?

Page 12, Lines 275-276: This section is much improved and I really enjoyed reading this, following all of the motivations and comparison! Thank you!

Page 13, Line 290: ‘overall…’ These are estimates. Make that clear. The alpha value above this is hard to follow through the calculation too. Why is it not presented like the other equations in the manuscript? Is the original source of this equation (Oswald et al., 2013)? Or is this equation something that is derived here? Please clarify and make a bit of a justification of why this is a valid parameterization (i.e. what has it been checked against in terms of generating a reasonable result?)

Page 13, Line 292: ‘principle’ can be deleted.

Page 13, Lines 306-307: Do all of these also assume a constant nocturnal OH value? If any actually measured OH, it is worth mentioning that fact, as it lends greater support to your approximation.

Page 14, Figure 3: Also missing cross-referencing of panel letters in the written caption. Please add the link between caption text and panel letters.

Page 15, Lines 335-336: Another very nice analysis and concluding statement!

Page 16: Line 383: ‘and uptake on aerosols’ is not supported by the measurements that exist of particulate nitrite. First principles application of equilibrium thermodynamic partitioning of HONO into PM2.5 inorganic aerosol has demonstrated this is highly unfavourable (VandenBoer et al., 2014), which was show also by experiment on acidic surfaces representative of this aerosol (Kleffmann et al., 1998). The only loss to aerosol for HONO that has been shown to date, supported by sporadic measurements in the literature, is the reactive uptake of HONO on mineral carbonates in dust.

Page 17, Line 400: This was determined by quantitative calculation in (VandenBoer et al., 2013).

Page 18, Lines 429-430: Wonderful conclusion. The sensitivity analysis here is identifying exactly where improvements are required and also highlighting the complexity of this chemistry.

Page 18, Line 434: Delete ‘toward’.

Page 18, Line 438: Should ‘unlikely more’ be ‘substantially less’?

Page 18, Line 439: Should ‘of’ be ‘and’? Also, should ‘largely’ be ‘entirely’? Since deposition is deliberately used to balance the remainder of the observations, the suggested phrasing would be more accurate. Please clarify.

Page 19, Line 452: Should ‘evoking’ be ‘exploring’?

Page 20, Line 479: ‘low wind speed’ Please give the value you used to create your threshold here.

Page 21, Line 500: ‘high NOx’ This is a long list. Why nothing from the polluted cities studied in North America to create a more complete global perspective? Substantial and pioneering work on HONO chemistry in high NOx areas has been done in locations such as Houston, Los Angeles, and the Uintah Basin, for example.

Page 22, Figure 6: Watch the x- and y-axis formatting. They do not look consistent with those throughout the rest of the manuscript.

Minor Revisions in Supplement:

Page 2, Line 34: At what concentrations of nitrite was the calibration performed? And what range of HONO mixing ratios do those calibration points correspond to? Please add to improve quality of analytical description.

Page 2, Line 36: ‘measurement’ is redundant. Delete.

Page 2, Line 37: ‘This 5 pptv…’ This is not how precision is defined in analytical chemistry. It can be determined by the variance observed from replicate analyses of a sample (e.g. the relative standard deviation in your calibration curve slope OR the relative standard deviation of the signal from a single standard that lies in the middle of your observation range for ambient HONO). Better yet would be to present the precision at the low range of the observations and also at the high range, as these tend to be higher and lower, respectively.

Page 2, Line 38: ‘resolution’ should be ‘response’ which is the term used below.

Page 2, Lines 47-48: The slope of the intercomparison is just as important as the correlation coefficient. Your slope suggests that the custom-build LOPAP measures systematically more HONO (~12%) than the commercial LOPAP. Did you correct your observational dataset for this? If not, you need to state this clearly in the manuscript and that it could add further uncertainty to the analysis.

Page 4, Figure S2: Is the linear fit including consideration of the uncertainties in both measurements? A standard linear regression will not return an appropriate linear equation for an intercomparison if not taken into account. If another fitting approach has been used, please give the details/name of the method used so this is clear. Also, the number of points used in the intercomparison is given as N=150, but at what time resolution is this being done? 15 minutes? That seems at odds with the 8 days stated when both instruments were making measurements, which would yield 192 comparison points at 1 hour time resolution. Please clarify.

Aubin, D. G. and Abbatt, J. P. D.: Interaction of NO2 with hydrocarbon soot: Focus on HONO yield, surface modification, and mechanism, J. Phys. Chem. A, 111(28), 6263–6273, doi:10.1021/jp068884h, 2007.

Kleffmann, J., Becker, K. H. and Wiesen, P.: Heterogeneous NO2 conversion processes on acid surfaces: possible atmospheric implications, Atmos. Environ., 32(16), 2721–2729, 1998.

Kleffmann, J., Kurtenbach, R., Lörzer, J., Wiesen, P., Kalthoff, N., Vogel, B. and Vogel, H.: Measured and simulated vertical profiles of nitrous acid - Part I: Field measurements, Atmos. Environ., 37(21), 2949–2955, doi:10.1016/S1352-2310(03)00242-5, 2003.

Oswald, R., Behrendt, T., Ermel, M., Wu, D., Su, H., Cheng, Y., Breuninger, C., Moravek, A., Mougin, E., Delon, C., Loubet, B., Pommerening-Röser, A., Sörgel, M., Pöschl, U., Hoffmann, T., Andreae, M. O., Meixner, F. X. and Trebs, I.: HONO emissions from soil bacteria as a major source of atmospheric reactive nitrogen, Science (80-. )., 341(6151), 1233–1235, doi:10.1126/science.1242266, 2013.

Pusede, S. E., VandenBoer, T. C., Murphy, J. G., Markovic, M. Z., Young, C. J., Veres, P. R., Roberts, J. M., Washenfelder, R. A., Brown, S. S., Ren, X., Tsai, C., Stutz, J., Brune, W. H., Browne, E. C., Wooldridge, P. J., Graham, A. R., Weber, R., Goldstein, A. H., Dusanter, S., Griffith, S. M., Stevens, P. S., Lefer, B. L. and Cohen, R. C.: An atmospheric constraint on the NO2 dependence of daytime near-surface nitrous acid (HONO), Environ. Sci. Technol., 49(21), 12774–12781, doi:10.1021/acs.est.5b02511, 2015.

VandenBoer, T. C., Brown, S. S., Murphy, J. G., Keene, W. C., Young, C. J., Pszenny, A. A. P., Kim, S., Warneke, C., de Gouw, J. A., Maben, J. R., Wagner, N. L., Riedel, T. P., Thornton, J. A., Wolfe, D. E., Dubé, W. P., Öztürk, F., Brock, C. A., Grossberg, N., Lefer, B., Lerner, B., Middlebrook, A. M. and Roberts, J. M.: Understanding the role of the ground surface in HONO vertical structure: High resolution vertical profiles during NACHTT-11, J. Geophys. Res. Atmos., 118(17), 10,155-10,171, doi:10.1002/jgrd.50721, 2013.

VandenBoer, T. C., Markovic, M. Z., Sanders, J. E., Ren, X., Pusede, S. E., Browne, E. C., Cohen, R. C., Zhang, L., Thomas, J., Brune, W. H. and Murphy, J. G.: Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010, J. Geophys. Res. Atmos., 119, 9093–9106, doi:10.1002/2013JD020971, 2014.

Vogel, B., Vogel, H., Kleffmann, J. and Kurtenbach, R.: Measured and simulated vertical profiles of nitrous acid - Part II. Model simulations and indications for a photolytic source, Atmos. Environ., 37(21), 2957–2966, doi:10.1016/S1352-2310(03)00243-7, 2003.

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ED: Publish subject to minor revisions (review by editor) (30 May 2022) by Steven Brown

The referee has made a number of suggestions for further, minor revisions. Please respond to these comments prior to publication. Comments are copied below for your reference.

The Authors have done an excellent job revising the manuscript to reflect the state of knowledge on HONO sources and sinks, along with some analysis of its daytime sources. The extension to interpreting its impact on daytime oxidation was reasonably removed to narrow the scope of the manuscript, given the magnitude of uncertainties in the HONO analysis. The manuscript is now ready for publication in Atmospheric Chemistry and Physics, pending the following minor revisions.

Page 2, Line 50: Missing recent study by Pusede et al.(2015) that is more comprehensive than some of the works cited here for detailed explorations of formation mechanisms.

Page 2, Line 58: ‘fresh air masses and vehicle exhaust’ Should this be ‘fresh air masses mixed with vehicle exhaust’?

Page 2, Line 61: HONO/NOx ratios throughout the depth of the nocturnal boundary layer, as well as during the day were also presented by VandenBoer et al. (2013) from the NACHTT campaign and also by a pair of papers from (Kleffmann et al., 2003; Vogel et al., 2003).

Page 3, Lines 74-76: Here the Authors are citing HONO formation mechanisms that have been disproven in the literature, especially the excited NO2 and water vapor reaction (shown to be a two photon process, which is not atmospherically relevant). The homogeneous nucleation of NH3, NO2, and H2O has also not been found to be important because, like most atmospheric ternary reaction mechanisms, the probability of the reaction is so low that its rate of formation for HONO simply is not that important. Remove these. There is far more evidence for substantial HONO formation from heterogeneous reactions.

Page 3, Line 84: ‘appear to suggest’ should be either ‘appear’ or ‘suggest’. Pick one.

Page 3, Line 91: ‘reduction on reductive’ is redundant. Revise.

Page 3, Line 93: Missing the following work by (Aubin and Abbatt, 2007)

Page 5, Line 162: This is a Time-of-Flight Aerosol Mass Spectrometer. Please revise.

Page 9, Line 204: ‘0.2 % to 9.1 %’ is here in the discussion. Maybe it would be nice to give these same values/units on panel (c) in the figure?

Page 10, Figure 2: The caption needs to provide reference to each panel letter in the caption text. Please add. The notation on (c) needs to be changed to more intuitive values because 0.07 or 7% are easier to interpret than 70x10^-3.

Page 10, Lines 217-221: No need to letter the criteria. They are not separated and analyzed separately through a sensitivity analysis or anything later. Instead, present them in order as a list in paragraph format, with the semicolon separations retained.

Page 11, Line 223: Should ‘criteria’ be ‘all of the criteria’?

Page 11, Lines 225-226: Combine the two Hong Kong studies to simplify here ‘… measured in Hong Kong at 1.2% (Xu et al.) and 1.0% (Yun et al.)’

Page 11, Line 233: ‘simply’ is not needed. Remove.

Page 11, Line 237: Should ‘exclude’ be ‘estimate’ or ‘take into account’ here?

Page 12, Lines 275-276: This section is much improved and I really enjoyed reading this, following all of the motivations and comparison! Thank you!

Page 13, Line 290: ‘overall…’ These are estimates. Make that clear. The alpha value above this is hard to follow through the calculation too. Why is it not presented like the other equations in the manuscript? Is the original source of this equation (Oswald et al., 2013)? Or is this equation something that is derived here? Please clarify and make a bit of a justification of why this is a valid parameterization (i.e. what has it been checked against in terms of generating a reasonable result?)

Page 13, Line 292: ‘principle’ can be deleted.

Page 13, Lines 306-307: Do all of these also assume a constant nocturnal OH value? If any actually measured OH, it is worth mentioning that fact, as it lends greater support to your approximation.

Page 14, Figure 3: Also missing cross-referencing of panel letters in the written caption. Please add the link between caption text and panel letters.

Page 15, Lines 335-336: Another very nice analysis and concluding statement!

Page 16: Line 383: ‘and uptake on aerosols’ is not supported by the measurements that exist of particulate nitrite. First principles application of equilibrium thermodynamic partitioning of HONO into PM2.5 inorganic aerosol has demonstrated this is highly unfavourable (VandenBoer et al., 2014), which was show also by experiment on acidic surfaces representative of this aerosol (Kleffmann et al., 1998). The only loss to aerosol for HONO that has been shown to date, supported by sporadic measurements in the literature, is the reactive uptake of HONO on mineral carbonates in dust.

Page 17, Line 400: This was determined by quantitative calculation in (VandenBoer et al., 2013).

Page 18, Lines 429-430: Wonderful conclusion. The sensitivity analysis here is identifying exactly where improvements are required and also highlighting the complexity of this chemistry.

Page 18, Line 434: Delete ‘toward’.

Page 18, Line 438: Should ‘unlikely more’ be ‘substantially less’?

Page 18, Line 439: Should ‘of’ be ‘and’? Also, should ‘largely’ be ‘entirely’? Since deposition is deliberately used to balance the remainder of the observations, the suggested phrasing would be more accurate. Please clarify.

Page 19, Line 452: Should ‘evoking’ be ‘exploring’?

Page 20, Line 479: ‘low wind speed’ Please give the value you used to create your threshold here.

Page 21, Line 500: ‘high NOx’ This is a long list. Why nothing from the polluted cities studied in North America to create a more complete global perspective? Substantial and pioneering work on HONO chemistry in high NOx areas has been done in locations such as Houston, Los Angeles, and the Uintah Basin, for example.

Page 22, Figure 6: Watch the x- and y-axis formatting. They do not look consistent with those throughout the rest of the manuscript.

Minor Revisions in Supplement:

Page 2, Line 34: At what concentrations of nitrite was the calibration performed? And what range of HONO mixing ratios do those calibration points correspond to? Please add to improve quality of analytical description.

Page 2, Line 36: ‘measurement’ is redundant. Delete.

Page 2, Line 37: ‘This 5 pptv…’ This is not how precision is defined in analytical chemistry. It can be determined by the variance observed from replicate analyses of a sample (e.g. the relative standard deviation in your calibration curve slope OR the relative standard deviation of the signal from a single standard that lies in the middle of your observation range for ambient HONO). Better yet would be to present the precision at the low range of the observations and also at the high range, as these tend to be higher and lower, respectively.

Page 2, Line 38: ‘resolution’ should be ‘response’ which is the term used below.

Page 2, Lines 47-48: The slope of the intercomparison is just as important as the correlation coefficient. Your slope suggests that the custom-build LOPAP measures systematically more HONO (~12%) than the commercial LOPAP. Did you correct your observational dataset for this? If not, you need to state this clearly in the manuscript and that it could add further uncertainty to the analysis.

Page 4, Figure S2: Is the linear fit including consideration of the uncertainties in both measurements? A standard linear regression will not return an appropriate linear equation for an intercomparison if not taken into account. If another fitting approach has been used, please give the details/name of the method used so this is clear. Also, the number of points used in the intercomparison is given as N=150, but at what time resolution is this being done? 15 minutes? That seems at odds with the 8 days stated when both instruments were making measurements, which would yield 192 comparison points at 1 hour time resolution. Please clarify.

Aubin, D. G. and Abbatt, J. P. D.: Interaction of NO2 with hydrocarbon soot: Focus on HONO yield, surface modification, and mechanism, J. Phys. Chem. A, 111(28), 6263–6273, doi:10.1021/jp068884h, 2007.

Kleffmann, J., Becker, K. H. and Wiesen, P.: Heterogeneous NO2 conversion processes on acid surfaces: possible atmospheric implications, Atmos. Environ., 32(16), 2721–2729, 1998.

Kleffmann, J., Kurtenbach, R., Lörzer, J., Wiesen, P., Kalthoff, N., Vogel, B. and Vogel, H.: Measured and simulated vertical profiles of nitrous acid - Part I: Field measurements, Atmos. Environ., 37(21), 2949–2955, doi:10.1016/S1352-2310(03)00242-5, 2003.

Oswald, R., Behrendt, T., Ermel, M., Wu, D., Su, H., Cheng, Y., Breuninger, C., Moravek, A., Mougin, E., Delon, C., Loubet, B., Pommerening-Röser, A., Sörgel, M., Pöschl, U., Hoffmann, T., Andreae, M. O., Meixner, F. X. and Trebs, I.: HONO emissions from soil bacteria as a major source of atmospheric reactive nitrogen, Science (80-. )., 341(6151), 1233–1235, doi:10.1126/science.1242266, 2013.

Pusede, S. E., VandenBoer, T. C., Murphy, J. G., Markovic, M. Z., Young, C. J., Veres, P. R., Roberts, J. M., Washenfelder, R. A., Brown, S. S., Ren, X., Tsai, C., Stutz, J., Brune, W. H., Browne, E. C., Wooldridge, P. J., Graham, A. R., Weber, R., Goldstein, A. H., Dusanter, S., Griffith, S. M., Stevens, P. S., Lefer, B. L. and Cohen, R. C.: An atmospheric constraint on the NO2 dependence of daytime near-surface nitrous acid (HONO), Environ. Sci. Technol., 49(21), 12774–12781, doi:10.1021/acs.est.5b02511, 2015.

VandenBoer, T. C., Brown, S. S., Murphy, J. G., Keene, W. C., Young, C. J., Pszenny, A. A. P., Kim, S., Warneke, C., de Gouw, J. A., Maben, J. R., Wagner, N. L., Riedel, T. P., Thornton, J. A., Wolfe, D. E., Dubé, W. P., Öztürk, F., Brock, C. A., Grossberg, N., Lefer, B., Lerner, B., Middlebrook, A. M. and Roberts, J. M.: Understanding the role of the ground surface in HONO vertical structure: High resolution vertical profiles during NACHTT-11, J. Geophys. Res. Atmos., 118(17), 10,155-10,171, doi:10.1002/jgrd.50721, 2013.

VandenBoer, T. C., Markovic, M. Z., Sanders, J. E., Ren, X., Pusede, S. E., Browne, E. C., Cohen, R. C., Zhang, L., Thomas, J., Brune, W. H. and Murphy, J. G.: Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010, J. Geophys. Res. Atmos., 119, 9093–9106, doi:10.1002/2013JD020971, 2014.

Vogel, B., Vogel, H., Kleffmann, J. and Kurtenbach, R.: Measured and simulated vertical profiles of nitrous acid - Part II. Model simulations and indications for a photolytic source, Atmos. Environ., 37(21), 2957–2966, doi:10.1016/S1352-2310(03)00243-7, 2003.

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AR by Yihang Yu on behalf of the Authors (09 Jun 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 Jun 2022) by Steven Brown

AR by Yihang Yu on behalf of the Authors (10 Jun 2022) Author's response Manuscript

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Short summary

We have investigated the budget of HONO at an urban site in Guangzhou. Budget and comprehensive uncertainty analysis suggest that at such locations as ours, HONO direct emissions and NO + OH can become comparable or even surpass other HONO sources that typically receive greater attention and interest, such as the NO₂ heterogeneous source and the unknown daytime photolytic source. Our findings emphasize the need to reduce the uncertainties of both conventional and novel HONO sources and sinks.