On the understanding of tropospheric fast photochemistry: airborne observations of peroxy radicals during the EMeRGe-Europe campaign
Abstract. In this study, airborne measurements of the sum of hydroperoxyl (HO2) and organic peroxy (RO2) radicals that react with NO to produce NO2, i.e. RO2*, coupled with actinometry and other key trace gases measurements, have been used to test the current understanding of the fast photochemistry in the outflow of major population centres (MPCs). All measurements were made during the airborne campaign of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) project in Europe on-board the High Altitude Long range research aircraft (HALO). The on-board measurements of RO2* were made using the in-situ instrument Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). RO2* mixing ratios up to 120 pptv were observed in air masses of different origins and composition under different local actinometrical conditions during seven HALO research flights in July 2017 over Europe.
The range and variability of the RO2* measurements agree reasonably well with radical production rates estimated using photolysis frequencies and RO2* precursor concentrations measured on-board. RO2* is primarily produced following the photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the airmasses investigated. The suitability of photostationary steady-state (PSS) assumptions to estimate the mixing ratios and the variability of RO2* during the airborne observations is investigated. The PSS assumption is robust enough to calculate RO2* mixing rations for most conditions encountered in air masses measured. The similarities and discrepancies between measured and calculated RO2* mixing ratios are analysed stepwise. The parameters, which predominantly control the RO2* mixing ratios under different chemical and physical regimes, are identified during the analysis. The dominant removal processes of RO2* in the airmasses measured up to 2000 m are the loss of OH and RO through the reaction with NOx during the radical interconversion. Above 2000 m, HO2 – HO2 and HO2 – RO2 reactions dominate RO2* loss reactions. RO2* calculations underestimated (< 20 %) the measurements by the analytical expression inside the pollution plumes probed. The underestimation is attributed to the limitations of the PSS analysis to take into account the production of RO2* through oxidation and photolysis of the OVOCs not measured during EMeRGe.
Midhun George et al.
Status: final response (author comments only)
RC1: 'Comment on acp-2022-119', Anonymous Referee #1, 07 Mar 2022
- AC4: 'Reply on RC1', Midhun George, 23 Feb 2023
RC2: 'Comment on acp-2022-119', Anonymous Referee #2, 08 Mar 2022
- AC1: 'Reply on RC2', Midhun George, 23 Feb 2023
RC3: 'Comment on acp-2022-119', Anonymous Referee #3, 09 Mar 2022
- AC2: 'Reply on RC3', Midhun George, 23 Feb 2023
RC4: 'Comment on acp-2022-119', Anonymous Referee #4, 14 Mar 2022
- AC3: 'Reply on RC4', Midhun George, 23 Feb 2023
Midhun George et al.
Midhun George et al.
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The authors report and discuss peroxy radical measurements performed during flights with the aircraft HALO across Europe. Because there are only few flight measurements of radicals over Europe, these measurements are valuable. However, it is not very clear, what the improvement in the understanding of tropospheric fast photochemistry really is from the manuscript. The author mainly compare measurements with different approaches of steady state calculations. Results are mainly descriptive, but there is little discussion about the meaning for the understanding of photochemistry. The presentation quality needs to be improved. It is partly unclear, how equations for steady state calculations are derived and what the meaning is. This manuscript needs major improvements to be suitable for publication in ACP.
Abstract: The definition of RO2* is unclear. In the first sentence it sounds as if this is the sum of RO2+HO2, but later it looks as if also OH is included. Please clarify and be precise and accurate with definitions.
Abstract L22: How can a production rate agree with a concentration?
Abstract L23: RO2 is not directly produced from the photolysis of ozone and HONO, but OH is that then further reacts to produce RO2* species. Please be accurate how you phrase this.
Abstract L25: For an abstract the statement about the PSS is vague and not well-defined. Please expand here, which processes are considered in the PSS and what quantity is calculated.
Abstract L30: Really RO+NOx ? If RO2* is the sum of RO2+HO2+OH, it is not clear to me, why this statement is about radical interconversion, because radical interconversion reactions cancel out. Please rephrase and clarify.
L90: Reaction R25 should be mentioned as well.
L91: The first half of the sentence is not clear. What do you mean with insolation? Do you mean PSS? This would not be required to ensure rapid photochemical processes. Please rephrase and clarify.
L102: Specifically since the manuscript is about airborne measurements, the temperature and if necessary also the pressure should be given, if values for reaction rate constants are mentioned.
L103: The typical reader may not know, what exactly is meant with “weighted average rate coefficient” and why this is required. Please clarify and rephrase.
L126: It is not obvious, why the measurements of trace gases in Reactions R1 to R26 other than required in Equation 1 minimizes the number of assumptions for calculating RO2*. My expectation would have been that this would allow to perform also full model-calculations of RO2* concentrations, which could be compare PSS calculations. Please explain in more detail.
L135: Please avoid to define and use abbreviations like IOP and MPC and others that are not common. The typical reader will forget them, while reading the manuscript. It only makes it difficult to follow the line of arguments.
L143: What do you mean with “stable flight layers”?
L168: Please add also the pressure, for which you calculated the concentrations.
L172: Why do you only refer to CH3O2 as RO2? Earlier you mention “weighted average rate coefficient” implying that you not only have CH3O2.
L180: I would recommend to give a number how large the humidity effect was for measurements in this work.
L183 ff: The short description of miniDOAS data / data evaluation is hard to understand for the non-expert. Please rephrase. It is also not clear at this point, why this instrument is explain in more detail, whereas other instruments more obvious useful to determine the PSS are not explained.
L186: Please explain RT modelling.
L187: Please explain the abbreviation HAIDI.
L191: Please explain what you mean with “common and related species”.
L202: I would avoid a conclusion about the reason for high RO2 in specific regions before doing the analysis. Your arguments are plausible but there are also other plausible explanations giving the contrary conclusion.
L210: I do not understand the argument “comparable”. What is exactly compared here? Calculating RO2* from PSS can always been done as long as the time required to reach PSS is short enough that concentrations of species do not significantly change. Please explain and rephrase.
Figure 3: Wouldn’t make more sense to show percentiles instead of standard deviations to be independent from outliers?
L224: I cannot follow the argument that differences between mean and median values indicate more or less variability. Median and mean values could be exactly the same, if the distribution of values is symmetric independent on how big the range of values is. It is also not obvious, if you want to say that there is a change how similar median and mean values are. I do not see that the similarity depends on the height.
Line 235: “becomes” instead of “become”
L235: Please clarify what you mean with “low NOx conditions” and why this impacts the significance of H2O2 photolysis.
L238: Please define OVOC before using it in Eq 2
L237. This statements needs explanation. Why can you assume that photolysis of OVOCs is more important compared to reaction with OH? This is not obvious. Which were the most important OVOCs and VOCs and can you quantitatively show that your assumption is valid? Can you also show this for ozonolysis reactions? If you want to calculate the RO2* production rate you may not need to consider OH reaction, because this is a radical conversion reaction and not a primary production, which you may want to calculate. This should be clarified, if you talk about production. Please explain and extend your description.
L238: How large were the concentrations of these OVOCs? What do you mean concretely, if you take this as “surrogate”? Equation 3 only considers 4 OVOC species, which rather indicates that you neglect others.
L244: I assume that measurements allowed a calculation of the air concentration density rather than an estimate.
L245ff: Avoid explaining details of a figure that is explained in the legend and / or caption of the figure.
L291: The section header referring to PSS. From what is written earlier, one would expect calculations using Equation 1, but then you start with calculations using Eq 5. Also later in this Section Eq 5 is stated as PSS calculation instead of Equation 1 and not used at all in the end. This is confusing. Please be consistent. It is not clear, why Equation 1 is introduced earlier at all.
L297: It is a bit contradictory to state “interconversion reactions occur without losses”, because interconversion implies that the radical nature is not lost.
L298 ff: Please justify that you can calculate the loss of RO2* -RO2* reaction by an weighted average rate coefficient? What do you use as weights? Without knowing the distribution between HO2 and RO2 it is hard to imagine how this loss rate can be accurately calculated. It is not obvious how this is mathematically done, if you expand the right side of Eq. 5 using [HO2] and [RO2] concentrations. If you assumed e.g. [HO2] = [RO2] = 0.5 [RO2*], this should be clearly said and written down what this means for the equation. The assumption of [HO2]=[RO2] would be expected if the loss of [HO2] and [RO2] is dominated by reaction with NO. Please expand, if this is the case for measurements in your work. In this case, it would be also essential to show and discuss NO measurements and peroxy radical loss rates with NO. What about the loss of RO2* due to the reaction of NO2+OH? Could this have been significantly contributed to the RO2* loss? Your analysis between differences, if you divide data sets between North and South may hint that this loss process was relevant.
L318: I do not understand the statement about the validity of results. Please explain and rephrase.
L330: It would be good, if names of e.g. photolysis frequencies in Equation 5 and 6 were consistent. It should be emphasized that the point of assuming that RO2 consist only of CH3O2 is only, in order to have one RO2 species and therefore not considering differences in RO2+RO2 and RO2+HO2 reaction rate constants. In general, I would recommend to start with Equation 6 and then you easily derive Equation 5. By doing this, you also will be able to explain what you mean with average weighted reaction rate constant in Equation 5.
L338: It is rather confusing that the negative solution is mentioned at this point, but not when you discuss Eq. 5, where the form of the quadratic equation is identical.
L342: The effect that RO2* measurements can be affected by differences in the detection sensitivity of RO2 and HO2 should have been discussed for the results with delta=0.5 (Equation 5).
L344: Please make rather quantitative than qualitative statements about the level of agreement. What effect do you expect from differences in reaction rate constants among RO2, if you do not assume that all RO2 is CH3O2?
L356: It is not clear, which processes you are referring to, if you mention VOC oxidation processes. OH + VOCs would be a radical interconversion process and ozonolysis reactions and Cl chemistry may be not of importance for conditions of the campaign.
L358: Again it is confusing, if you talk about radical conversion reactions, but in fact you mean radical termination reactions. Please rephrase and be clear with the definition throughout the manuscript.
Equation 8 / 9: Similar it is confusing that you name reaction rate constants referring to radical conversion reactions and move the loss into a loss factor. What is the value of the loss factor? It would be easier, if you added more explanation, which loss reactions (products) you include. I read the first loss term as non-radical products from HO2 + NO and HO2 +O3, it is not clear to me, what for example the product of HO2+O3 would be. The factor rho associated with this term is explained as OH loss during the OH-RO2* interconversion, which does not fit, what I read from the equation. There is more explanation needed, what is meant with this term. It is also not clear to me, if the second loss term (organic nitrate formation k25 and k22) is correct and why this is connected to RO2+RO2 reactions (k16b). This needs to be explained in more detail. It would be much easier to understand, if you introduced yields of products produced from radical termination reactions.
L367 ff: It sounds as if you state that the reaction of OH+ HCHO and OH+ CH3CHO are the dominant radical precursor reactions, though so far you only discuss photolysis of them. OH reactions would also not be primary sources, but radical conversion reactions. In this context and for the same reason, it is also not clear, what you mean with RO2* production from CHOCHO and CH3OH oxidation. Please clarify and rephrase.
L373: The context of the statement about the importance of HO2+NO and HO2+O3 is not clear and seems displaced at this point.
L421: How can you exclude that there is no over-estimation of loss processes instead of an under-estimation of production processes? What is the impact in the uncertainty of the HO2/RO2 ratio in the case, when VOCs concentrations were high?
L422: Why would OH recycling processes increase the calculated RO2*, if radical regeneration terms cancel out in the calculations for the sum measurement of RO2*?
L436: It would be interesting to see a more quantitative analysis of the impact of the uncertainty in HCHO measurements on the results.
L490 ff: The calculation of OH concentrations does not really fit this manuscript and would require a much deeper description that currently done. The statement that the OH calculated from Eq 5 is higher than reported OH concentration means that OH reactivity is underestimated cannot easily be justified. I would recommend to cancel this entire paragraph. It does not add anything to the content of the manuscript and may even be rather misleading as it is now.
Section 4.4.1. / Equation 11: Again the definitions of the effective rate coefficients is not clear. Also the use of NOx makes it hard to see, what exactly is calculated. This makes it very difficult to follow any of the subsequent quantitative statements. The connection to previous Equations is also not clear. What is the difference to Equation 9, which should consider radical loss in NOx reactions? What is used for the production rate for example? The authors should make much clearer what is calculated and what the meaning of the calculation is. As it is written now, it is not clear, what the authors want to discuss in this section.