Budget of nitrous acid (HONO) and its impacts on atmospheric oxidation capacity 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-2021-178

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https://doi.org/10.5194/acp-2021-178

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03 May 2021

Review status: a revised version of this preprint is currently under review for the journal ACP.

Budget of nitrous acid (HONO) and its impacts on atmospheric oxidation capacity at an urban site in the fall season of Guangzhou, China

Yihang Yu^1,2,, Peng Cheng^1,2,, Huirong Li^1,2, Wenda Yang^1,2, Baobin Han^1,2, Wei Song³, Weiwei Hu³, Xinming Wang³, Bin Yuan^4,5, Min Shao^4,5, Zhijiong Huang⁴, Zhen Li⁴, Junyu Zheng^4,5, Haichao Wang⁶, and Xiaofang Yu^1,2 Yihang Yu et al.

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¹Institute of Mass Spectrometry and Atmospheric Environment, Jinan University, Guangzhou 510632, China
²Guangdong Provincial Engineering Research Center for Online Source Apportionment System of Air Pollution, Guangzhou 510632, China
³State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
⁴Institute for Environmental and Climate Research, Jinan University, Guangzhou 511443, China
⁵Guangdong-Hongkong-Macau Joint Laboratory of Collaborative Innovation for Environmental Quality, Guangzhou 511443, China
⁶School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai, China
These authors contributed equally to this work.

Received: 05 Mar 2021 – Accepted for review: 23 Mar 2021 – Discussion started: 03 May 2021

Abstract. Nitrous acid (HONO) can produce hydroxyl radicals (OH) by photolysis and plays an important role in atmospheric photochemistry. Over the years, high concentrations of HONO have been observed in the Pearl River Delta region (PRD) of China, which may be one reason for the elevated atmospheric oxidation capacity. A comprehensive atmospheric observation campaign was conducted at an urban site in Guangzhou from 27 September to 9 November 2018. During the period, HONO was measured from 0.02 to 4.43 ppbv with an average of 0.74 ± 0.70 ppbv. The emission ratios (HONO/NOx) of 0.9 ± 0.4 % were derived from 11 fresh plumes. The primary emission rates of HONO at night were calculated to be between 0.04 ± 0.02 ppbv h⁻¹ and 0.30 ± 0.15 ppbv h⁻¹ based on a high-resolution emission inventory. The HONO formation rate by the homogeneous reaction of OH + NO at night was 0.26 ± 0.08 ppbv h⁻¹, which can be seen as secondary results from primary emission. They were both much higher than the increase rate of HONO (0.02 ppbv h⁻¹) during night. Soil emission rate of HONO at night was calculated to be 0.019 ± 0.001 ppbv h⁻¹. Assuming dry deposition as the dominant removal process of HONO at night, and a deposition velocity of at least ~2.5 cm s⁻¹ is required to balance the direct emissions and OH + NO reaction. Correlation analysis shows that NH₃ and relative humidity (RH) may participate in the heterogeneous transformation from NO₂ to HONO at night. In the daytime, the average primary emission P_emis was 0.12 ± 0.01 ppbv h⁻¹, and the homogeneous reaction P_OH + NO was 0.79 ± 0.61 ppbv h⁻¹, larger than the unknown sources P_Unknown (0.65 ± 0.46 ppbv h⁻¹). These results suggest primary emissions as a key factor affecting HONO at our site, both during daytime and nighttime. Similar to previous studies, the daytime unknown source of HONO, P_Unknown, appeared to be related to the photo-enhanced conversion of NO₂. The daytime average OH production rates by photolysis of HONO was 3.7 × 10⁶ cm⁻³ s⁻¹, lower than that from O¹D + H₂O at 4.9 × 10⁶ cm⁻³ s⁻¹. Simulations of OH and O₃ with the Master Chemical Mechanism (MCM) box model suggested strong enhancement effect of HONO on OH and O₃ by 59 % and 68.8 %, respectively, showing a remarkable contribution of HONO to the atmospheric oxidation in the fall season of Guangzhou.

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Yihang Yu et al.

Status: final response (author comments only)

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

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Short summary

The research content of this paper is the budget of nitrous acid (HONO) and its impacts on atmospheric oxidation capacity at an urban site in Guangzhou of China. We conducted a comprehensive atmospheric observation at an urban site in Guangzhou in autumn of 2018. The results further showed that the direct and indirect contributions of primary emission to HONO are great at the site both during daytime and nighttime. And HONO contributed significantly to the atmospheric oxidation of Guangzhou.


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