A. General comments: 

  In this manuscript, BVOCs emission from Norway spruce was explored utilizing the Total Ozone Reactivity Monitor (TORM) which has been established recently based on the differential O3 monitor (Helmig et al., 2022). This study is positioned as a demonstrative research of the TORM instrument. Samples were prepared by use of the branch enclosure. Simultaneously, various BVOC species were individually monitored by the cold trap / thermal desorption / GC-MS technique which could quantify many ozone-reactive species including representative sesquiterpenes. BVOCs emission was characterized (eg. temperature dependence). Significant gap (missing reactivity) between observation by TORM and theoretical calculation based on GC-MS analysis was confirmed, suggesting experimentally that further exploration on unmeasured BVOC species could be important. Especially, missing ozone reactivity remained still significant although authors monitored representative ozone-reactive sesquiterpenes (eg. b-caryophyllene) and discussed on the possibility of the contribution by some unmeasured species like GLVs. 

 Totally, the reviewer believes that this work can be a breakthrough of O3-BVOCs chemistry in the atmosphere and has an important implication and is significant enough to be published in this journal. However, the present manuscript leaves several minor points to be clarified, in order for readers to understand descriptions and to recognize the significance and descriptions of this study more clearly. Especially, descriptions on the assumption of linear ozone decay in the reactor of TORM can be confusing for the quantitative discussion in this study. Minor revision is necessary. 

B. Specific comments: 

B1) Line 110 and around: 

  The descriptions on principles of the TORM instrument are one of the most important points in order to evaluate and understand the results of this study properly and quantitatively. Especially, the assumption of linear ozone decay in the reactor of TORM can be directly linked to invalid quantification of RO3 as suggested around Lines 220-221 and Section B2. Therefore, minimum descriptions on the principle should be added (including strict equation for exponential decay, linear assumption, and notice on fast reacting compounds). For example, … 

… Strictly, RO3 can be determined from exponential decay of ozone in the reactor:

RO3 = - {LN(O3(t) / O3(0) ) } / Dt. 

In this study, … (original Line 119) 

 RO3 ~ (original Eq.2) . 

… This assumption can be valid when fast reacting compounds are negligible and linear decay of ozone is proper. …   

B2) Line 114 and around: 

  Would you please clarify the model number and supplier of each O3 analyzer? 

B3) Lines 131 - 135 and Figure B1: 

  In the present preprint, the readers can recognize only the fact that, as a result of calibration, the TORM underestimated the standard concentrations of 4 BVOCs mixture. Why does such an underestimation occur? Why is the correction described as Eq.5? Is such a correction proper quantitatively? Would you please indicate authors’ ideas at least in the manuscript. 

  Relatedly, is the prepared standard sample (mixture of 4 BVOCs compounds, 200 ppbv each) proper as the conditions/settings of calibration during measurements in this study? Please clarify authors’ ideas in the text. Additionally, is the standard sample valid in view of the linear assumption in Eq.2? 

B4) After Line 145: 

  NO rapidly reacts with ozone and can interfere total ozone reactivity measurements. Would you add descriptions on typical concentrations of residual NO in zero air and samples? And, can such NO interference be removed or reduced in Eq.3? 

B5）Lines 219-224, Figure B2, Lines 344-363 and around: 

  In these paragraphs and figure, to evaluate the upper limit of unknown SQTs’ contribution to ozone reactivity (RO3) as ‘case2’, correction in RO3 measurement for fast reacting compounds is explained. 

1) Around line 220, such a purpose of these descriptions should be clarified by adding a little description on ‘case2’. 

2) In Appendix B2, such a purpose of these descriptions should be clarified for readers to avoid confusing between underestimations in B1 and those in B2. For example: 

    Title of B2: Correction of measured reactivity for fast reacting compounds in ‘case2’

3) In case except for ‘case2’, is such a correction not necessary? When is such a correction necessary? Would you indicate authors’ ideas at least in section B2? 

4) From the value of k (Line 350), authors may use k of b-caryophyllene as the upper limit. Would you indicate such authors’ ideas at least in section B2? 

5) Is it not difficult in principle to evaluate the strict RO3 values based on exponential decay of ozone from observed values as follows? 

   O3(t) / O3(0) = exp { - RO3 Dt } 

   O3(t) = O3(0) - DO3 

   =>=> RO3  - { LN (1 - DO3/O3(0) ) } / Dt 

 How about results in this study? Can any consideration be possible by use of strictly evaluated RO3? If it is not possible, please show us the reasons. And would you please indicate such situations in the text as possible? 

B6) Line 273 and around: 

  There is a leap in logic around ‘Therefore’. For example, please clarify as follows:

… However, these compounds were insignificantly detected by GC-MS. Therefore, … 

B7) Figure 8: 

  If the green points (or their regression line) in Fig.8b are plotted simultaneously in Fig8a, do the lower envelope of red points overlap on the green points (or their regression line) ? If so, can the new figure (simultaneous plots of red and green points) be added in the appendix of the manuscript in order to clarify the descriptions in Lines 284-304 (red points = temperature dependence of MT (green points) + spikes due to stress) ? Alternatively, would you show such an additional plot in the interactive public discussion?

C. Technical corrections: 

C1) Table 1, Figure 3, Figure 7, Figure 8: 

  Please clarify in the text where these table and figures are explained and/or referred. 

End of Comments. 

The manuscript "Undetected BVOCs from Norway spruce drive total ozone reactivity measurements" by Thomas et al. presents a set of measurements of total ozone reactivity in a branch enclosure of a spruce tree and compares these measurements with calculations using observations of BVOC by gas-chromatography. The manuscript is a nice demonstration of the potential use of ozone reactivity measurements and it is within the scope of the journal. I have a few comments that need to be addressed, but otherwise I think the manuscript is suitable for publication.

Main Comments

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The unit m3 s-2 g-1 is used for ozone reactivity throughout the manuscript. I think this is not correct because reactivity is, by definition, just the inverse of the chemical lifetime and should therefore be in s-1. The authors themselves define it as such with Equation 1. I assume that the reactivity presented in the manuscript is weighted by mass of the branch and scaled by time in order to be in m3 s-2 g-1, but this is not mentioned anywhere. I would suggest that, first of all, this information is added. And, second, that instead of "ozone reactivity" the authors use a different term, where appropriate.

In section 2.1 the authors mention that PAR measurements after August 23 were biased due to the shade of a nearby scaffolding. PAR is used in section 3.2 to discuss the emissions of some BVOCs. I would expect that only the data prior to August 23 are considered in figure 4 (and related discussion in section 3.2). Please clarify that this is the case, and if not, why unrepresentative PAR measurements have been used.

In section 2.3, the quantification of a-Farnesene and of bornyl acetate is determined assuming the same sensitivity as b-caryophyllene and nopinone, respectively. I suggest to add a comment on the potential error introduced by this assumption.

In section 3.3 (pages 11-12) the discussion focuses on the comparison between observed and measured reactivity. A correction is applied to the observations (please state explicitly) and then the data with and without correction are compared to calculations in case 1 and 2. I have questions regarding this procedure:

First, why is the correction only applied to the data compared to case 2? Sesquiterpenes are present also in case 1, the only difference between the cases is that the unidentified SQT1 is given a slow rate coefficient, but the issue described in figure B2 should be valid for all sesquiterpenes not just SQT1.

Second, if the correction is calculated with a box model (as described in appendix B2) to account for the differentials in BVOC ozone reactivities, I would expect it to be dependent on the particular mixture of BVOC, which means both the species in the mixture and their concentrations. I would therefore expect the result of the model calculation to be different for each data point of the campaign. Instead, from equation B3 it looks like a constant correction factor was applied. The authors should explain this point better, because it hugely impacts both the interpretation of figure 6 and discussion on missing reactivity, and therefore the overall conclusions of the paper.

In appendix B1, two calibrations are shown. I would argue that the 20.10 calibration cannot be considered reliable. It has only two points, so the slope is kind of meaningless. I suggest that only the 19.10 calibration should be used in the manuscript and all the numbers updated accordingly.

Minor Comments

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line 3 (and elsewhere in the text): should it be "low volatility"?

line 26: what do you mean with "other emissions"?

line 36: delete "on".

line 234: you should state explicitly that R(O3,corr) was corrected for the underestimation observed during the calibration, so that the procedure is more clear. Also please add the detection limit and uncertainty of all instruments somewhere in this section.

line 189: was SQT1 correlated to any other identified BVOC and/or temperature, PAR or other observed parameters? Were there other compounds that were present but not identified?

line 212: add some references for previous enclosure studies.

line 249: change to "very low".

lines 249-250: what does it mean "partly"? By higher rate you mean the faster or the slower rate coefficient? Please be more quantitative and precise.

figure 2: relative humidity is clearly high inside the branch enclosure, often around 100%. Can the authors comment on whether the high humidity can affect the determination of ozone by the Differential Monitor?

figures 3 and 5: I suggest you indicate the rainy and sunny periods to help with the interpretation of the figure, and related text.

figure 6: I suggest to increase the height of this figure and add the detection limit of TORM.