the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Dec 2024
Evaluating Nitrogen Oxide and α-pinene Oxidation Chemistry: Insights from Oxygen and Nitrogen Stable Isotopes
Abstract. The chemical interaction between nitrogen oxides (NOx = NO + NO2) and α-pinene plays a critical role in air quality and climate. However, uncertainties remain regarding their coupling in NOx loss, renoxification, and oxidation chemistry. To address these gaps, we conducted controlled chamber experiments, analyzing nitric acid (HNO3), NO2, and particulate nitrate (pNO3) for their oxygen and nitrogen stable isotope variations (Δ17O, δ18O, and δ15N). A strong linear relationship between δ18O and Δ17O across experiments revealed contributions of oxygen from ozone (O3) and atmospheric oxygen (O2) in forming reactive radicals. The δ15N values followed the order δ15N(pNO3) < NO2 < HNO3, reflecting isotope fractionation during NOx oxidation. A new chemical mechanism accurately predicted aerosol precursor decay and simulated Δ17O and δ15N values. Simulations showed NOx photochemical cycling and pNO3 formation, primarily from organic nitrate, with Δ17O(NO2) simulations achieving a root mean square error (RMSE) of 1.7 ‰. Improved δ15N(NO2) and pNO3 simulations used a nitrogen isotope fractionation factor (15α) of 0.997 for NO2 + OH reactions. However, modeling Δ17O and δ15N of HNO3 proved challenging, likely due to sampling artifacts. This study provides insights into Δ17O transfer dynamics, nitrogen isotope fractionation, and the role of NOx-BVOC chemistry in air quality, highlighting the potential of Δ17O and δ15N as tools for evaluating complex atmospheric processes.
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RC1: 'Comment on egusphere-2024-3860', Matthew Johnson, 20 Jan 2025
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General Comments
The goal of this paper is to use analysis of the relative abundance of stable isotopes of N and O to plumb the interactions coupled NOx and alpha pinene oxidation. It is an ambitious goal and a powerful tool, and the authors are able to derive some important and unique results. The authors use a sophisticated atmospheric chamber and isotopic measurement methods and protocols that have been developed, painstakingly, over many years. There can be a gap between the isotope and aerosol communities and one concern is that the results be presented in a way that allows both groups to understand the work and its implications. Here I am mainly thinking of researchers outside the isotope community. In addition, the style should be adjusted to present quantitative results in place of broad-brush qualitative descriptions. A final concern is that modeling is used to derive the results and additional work should be done to describe the impacts of the many necessary assumptions. This can be done by paying attention to error budgets and sensitivity analysis, and by validating the model through comparison to experimental and field results. Overall I like the paper very much and recommend publication after revision to address these points.Specific Comments
The prose flows, there are many details, and it is entirely possible for the reader to get lost in the trees and fail to appreciate the landscape. I suggest being sure to add text to frame or give context, for example by adding transitions to introduce sections and show how they fit into the bigger picture. One example is that Section 3 dives straight into Section 3.1 and then into subsection 3.1.1 without a word of text, just section headings.There are many passages that make strong qualitative claims and I strongly advise to instead show the evidence, 'write it in numbers', and let the data speak for itself. Examples below.
The Abstract says that there are uncertainties regarding coupling between a-pinene and NOx in NOx loss, renoxification and oxidation chemistry, and says that the study 'provides insights into D17O transfer dynamics, nitrogen isotope fractionation, and the role of NOx-BVOC chemistry in air quality, highlighting the potential of D17O and d15N as tools for evaluating complex atmospheric processes.' Is it possible to convert some or all of these claims into quantitative statements? What specifically is the new insight into D17O transfer dynamics? Is the promise of D17O and d15N analysis realized? Has the new mechanism been able to explain something that could not otherwise be explained, or make predictions?In the Introduction line 41, reading that the 'interplay bears significant consequences' is saying, 'take my word for it'. Please rewrite to give evidence. The reader is provided with eight references that cover 'air quality, climate, global reactive nitrogen budget and secondary organic aerosols'. The authors seem to be asking us to go figure it out. Instead I would suggest taking the time to identify a significant consequence of BVOC oxidation in the presence of NOx on a few of these areas and give specific examples. This paragraph has 27 references but the reader is left uncertain regarding why exactly 'Understanding the fates of organic nitrogen and the feedback in oxidation chemistry arising from BVOC/NOx interactions is critical for accurately assessing their roles in NOx loss and recycling, O3 formation, and SOA generation.'
Line 59, 'The natural variations..may be a promising tool to enhance our insight into the intricate connections..and their implications for atmospheric composition.' A promise is made to the reader here - be sure to revisit this point in the discussion at the end. Does the present work provide the evidence?
Line 61, 'Stable isotope approaches offer novel avenues to probe and refine our understanding..unravel the dynamics..of interactions and ultimately contribute to formulating informed air quality management strategies.' Also here - this sounds nice, but would be more valuable if it is connected with specific findings. What specific, informed air quality management strategies can be made based on the work described in this paper?
How common are the conditions under which BVOC-NOx interactions will be important? Given the relatively short lifetimes of each, the overlap might be restricted to specific zones. However, the authors are claiming global impacts in addition to regional. Please walk the reader through how these larger phenomenon can arise.
There are many approximations in the model and even for experts it is difficult to disentangle the implications of the assumptions and uncertainties.
Some examples:
Line 33, 15alpha for NO2+OH is taken as 0.997. How well known is this value? What happens if the value is taken as 0.998, or 1.1?
Line 204 mass-dependent coefficient taken to be 0.52. What is the error, what are the implications of this choice?
Line 222, uncertainties were calculated to be less than 4.1, 1.4 and 0.9. Thank you, it is good, and please add just a bit to put the information in context: are these values significant?
Line 289 'This value was not measured but assumed..'
Line 297 15N/14N ratio taken as 0.003677 -- is there an error on this number?
Lines 295 to 305, a series of values are assumed. How sensitive is the result to these values?
Line 309, Wall loss is not considered in the model. How severe is this approximation, detail chamber volume, leak rate/lifetime of air in chamber, diffusion time to wall, etc to put in perspective.
Line 326, 'The δ18O of RO2/HO2 radicals has previously been suggested to be near δ18O(O2)' How near? Suggested? Are we on solid ground?
Line 327 'the derived d18O of 11.8 +/- 1.0 ‰ is near the atmospheric d18O(O2) value of 23.2 ‰' (Thank you that here, there is a space between number and unit). These two values are not within their mutual error ranges and one is twice that of the other, please help the reader understand how this is 'near'?
Please walk the reader through the inputs and outputs of the model, the assumptions and adjustable parameters. Conduct sensitivity analysis and validation. It would be helpful to provide an analysis of which parts of the model require additional research, to help guide future research/as a service to the community.Line 310 'low relative humidity conditions', but how low, there must be a measurement?
The Conclusion section is powerful, thank you. It ties the paper together. It could be revisited during the rewrite - to bring focus, brevity, specific quantified results. These could include results from the sensitivity analysis and specific research needs.
Setting 'the weighted branching ratio of α-pinene+OH+NO leading to organic nitrate versus NO2 production' to 0.222 is a very specific number and deserves further comment. Do you have any thoughts how this value would change with atmospheric conditions like humidity, temperature, pressure, NO concentration? Is it as accurate as three digits or is it merely to within e.g. 10%?
Line 699, 'Our findings strongly suggest that pNO3 in these experiments originated exclusively from organic nitrate, a conclusion supported by online AMS data. Furthermore, the Δ17O and δ15N evidence demonstrated that organic nitrate hydrolysis was not a major source of HNO3 under the studied conditions, which predominantly involved low relative humidity.' I just want to say that these are excellent results, very interesting! Thank you.
Technical Corrections
Recommended practice by professional societies such as IUPAC and SI is that there should always be a space between a number and a unit. So for example 1.7 ‰ not 1.7‰. Check throughout, current usage is inconsistent.Line 217 'corresponds to the fraction of NO3- that corresponds to the blank.' Please rewrite.
Line 260 change experiment to experimental
Figure 1 inset, y, p, r, x should be italicized. Also caption of Table 2, 'Robs' should be italicized e.g. '$R_{obs}$'. All variables should be italicized.
Table 2, what are the uncertainties on the values in the final three columns? Perhaps a blanket uncertainty could be used e.g. '+/- 5 %' or '10 % of the given value'.
Line 437 suggest rewriting this sentence for clarity, tone, grammar, break into two etc. 'The offline filter collection and extraction technique matches the trend in which more pNO3 hydrolyzed for the photochemical experiments compared to the nighttime; however, the filter technique would indicate a higher proportion of potential hydrolysable pNO3 from photochemical experiment than these previous estimates, though with a different timescale.'
Line 370 spelling 'masse'
Line 445, check word choice, consider replacing 'speciation' with 'interpretation' or 'assignment'?
Figure 3 caption, 'The observed pNO3 concentrations are faceted by the various experiments conducted'. Do I understand this correctly to mean, 'This figure shows how the variety of experiments performed impact the observed pNO3 concentrations'? I found it hard to interpret and suggest rewriting for simplicity, clarity.
Line 459, 'This comparison indicates that the developed mechanism well represents the oxidation of α-pinene and formation of oxidants under a wide range of experimental conditions. The simulations using the USC-API mechanism was a vast improvement' Please quantify; 'well represents', 'wide range' and 'vast improvement' are imprecise and will mean different things to different readers. Similar line 489, 'well-suited', line 497 'well-reproduced' and line 501, 'well-calibrated'.
Line 493 'leaded' to 'leading' (I think?) and this sentence should be rewritten to simplify, clarify: 'Oxygen isotope mass-balance indicates that the α-pinene-derived peroxy radicals + NO pathway would be the expected pathway leaded to a low Δ17 O(pNO3 ) value as only one oxygen atom in the nitro group of the generated RONO 2could derive from O3.'
526 suggest change 'enable' to 'allow'
552 'partially improve', how is this different from 'improve'? Suggest edit.
Figures 6, 7, 8, 9, 10 and 11 have largely identical captions, mit doesn't seem necessary that they all repeat the same details. They also all have the gramamtical error, 'with the black line span the collection time range'.
Table S1 and S2, suggest taking a moment to edit the superscripts and subscripts in chemical formulas (subscript numbers), term symbols (superscript numbers), etc.
Table S2, do you have the references for these rate coefficient values? Note that it is not a reaction rate but a rate coefficient. What is the difference? Consider A + B --> C with rate coefficient k. The rate of the process is r = k[A][B] = d[C]/dt. The value 'k' is sometimes called the rate constant, but it is not constant, it changes with temperature, etc.; it is the rate coefficient. It would be useful to indicate the units for these rate coefficients, 1/s, cm^3/s etc. I might guess the activation energies are given in units of Kelvin, is that correct? Add a footnote. R022 says to multiply by 'H2O', should this be '[H2O]'? R039 and R040 are not reaction rates r or rate coefficients k, they are equilibrium constants K.
Citation: https://doi.org/10.5194/egusphere-2024-3860-RC1
Model code and software
Walters-Research-Group/alpha-pinene_NOx_Chemistry_Box_Model_Simulations: v1.0 alpha-pinene NOx F0AM Box Model Simualtions Wendell Walters https://zenodo.org/records/14241585
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