From the journal Environmental Science: Atmospheres Peer review history

16-Oct-2023

Dear Dr Crowley:

Manuscript ID: EA-ART-09-2023-000137
TITLE: NO₃ reactivity measurements in an indoor environment: A pilot study

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Reviewer 1

Great measurement and interesting application! I included comments and edits in the attached document.

Reviewer 2

This paper presents the first indoor measurements of NO3 radical reactivity using a flow tube instrument coupled with a cavity ring-down spectrometer for detection of NO3. In addition to the reactivity measurements, the authors also measured mixing ratios of NO, NO2, O3, and N2O5 simultaneously. Together with the reactivity measurements, this allowed the authors to estimate the mixing ratio of NO3 in this indoor environment. The measurements were conducted in an unoccupied laboratory with an air exchange rate of approximately 4 hr-1. This relatively high exchange rate resulted in mixing ratios of ozone and NO2 similar to that outdoors. However, measured mixing ratios of indoor NO3 were below the detection limit of the instrument, apparently due to loss of NO3 on the inlet tubing and filter. But based on the measured NO3 reactivity and the measurements of NO, NO2, and O3, the authors were able to estimate expected steady-state mixing ratios of NO3.

The calculated NO3 mixing ratios lead to calculated N2O5 mixing ratios that are consistent with the measurements, suggesting that the measured NO3 reactivity and the calculated NO3 mixing ratios are consistent with the measured N2O5. The calculated NO3 mixing ratios are similar to some previous measurements in other indoor environments but are approximately a factor of 100 greater than previous predictions of NO3 in residential environments. The results suggest that indoor mixing ratios of NO3 may be greater than previously believed. The authors conclude that NO3 mixing ratios may be significant indoors under conditions where indoor O3 and NO2 are elevated due to high air exchange rates, and that measurements of NO3 reactivity may be used in conjunction with measurements of NO2 and ozone to estimate NO3 concentrations.

The paper is well written, and the pilot study presents new information about the potential importance of NO3 radicals in indoor environments. I recommend publication after the authors have addressed the following comments:

1) The authors state that the fact that their measurements of NO3 were undetectable due to loss of NO3 on the inlet tube and filter. This is in contrast to the description of NO3 loss in their instrument as described in Sobanski et al. (2016) which stated that the transmission of NO3 through the filter was approximately 70%. While the picture of the filter in the supplement clearly shows that the filter was contaminated in this study, it isn’t clear why the authors did not change the filter more frequently given that it appears that the system is equipped with an automatic filter changer according to the description in Sobanski et al.

2) The authors state that their measurements of NO3 reactivity are corrected for the impact of NOx (page 8), so the additional loss of NO3 by reaction with NO is added in Equation 2. I assume that this is related to the numerical simulations required to extract the NO3 reactivity from the measurements. This should be clarified, perhaps in the supplement, as the discussion in Liebmann et al. (2017) seems to suggest that a bias in measurements is due to the reaction of NO3 with NO2 under high NO2 conditions.

Minor comments:

It would be useful to provide a schematic of the experimental set up in the supplement, illustrating the inlet lengths and location for both the reactivity instrument and the CRDS instrument.

Page 3, lines 90-91: The authors state that the measurements were conducted over four days in October, but state that the only extended sunny day was July 16th…

Reviewer 3

This paper presents an interesting pilot investigation of the indoor NO3-N2O5 chemistry by measuring many related parameters mainly including NO3 reactivity, NOx, O3, and NO3, N2O5. NO3 reactivity measurement showed a comparable level with previous observation in an outdoor rural environment, and highlights that NO3 chemistry may be important to indoor air pollution. They showed NO and VOC are the dominate NO3 loss pathways with comparable contribution, and N2O5 uptake on the inner wall and aerosol surface contributes minor. Overall, the indoor oxidation by NO3 is less concerned compared with ozone and OH radicals, this study is helpful in promoting the understanding of the indoor NO3 chemistry when the indoor air is well exchanged with the outdoor air. This topic is certainly within the scope of ESA. This paper is well written, and should be published subject to the following minor comments.

1. Line 75-80, we know that the exchange rate should be smaller than 4.3 h-1, are there a more detailed sensitivity analysis of the indoor oxidants or wall loss estimation for giving the result of 4.0 h-1?
2. Line 160-165, In Figure 1, the plot of Kno3 and Lno3 is a little bit confused. I can understand that the upper limit of kno3 is caused by the high NO, and the VOC-induced NO3 reactivity cannot be derived, while in the figure, the plot of a flat line of kno3 may misunderstanding since the kno3 is not quantified rather than equal to 1.7 s-1. In addition, I believe the plot of Lno3 during the period just only considers the contribution of NO. All these detailed points should be clarified.
3. I suggest the author separate the measurement and calculation of NO3 and N2O5 into two panels, it’s too busy to see the performance during some period, by the way, the case in Figure 4 is clear although used the same plot style.
4. Line 232, the LNO3 can be replaced by another abbreviation such as kNO3-total (it is not the same with the loss rate in Line 14), since the LNO3 is often regard as the NO3 loss rate rather than the overall NO3 loss rate coefficient.
5. Line 270-273, the usage of fresh-air via filters indicates that a lower N2O5 uptake coefficients, while this statement seems to contradict the previous one that NO3 detection is largely affected by the indoor aerosols (Fig. S4), are there possible that the filter of in the NO3 instrument is not changed with a relative high frequency? The author should clarify it.

Reply to the Referee Comments
In the following, the referee’s comments are reproduced (black) along with our replies (blue) and changes made to the text (red) in the revised manuscript. Line numbers refer to those in the initial submission.

1) Reply to Referee 1

Summary
Finally, some attempts at direct measurements of NO3 indoors without injecting O3! Great job. This is also the first report of an NO3 reactivity measurement in an environment relevant for indoor air research. I enjoyed the manuscript. The authors highlight how high ventilation rates in a commercial building can bring in higher levels of O3 than are typically measured in residential buildings, which, as they show, are high enough to promote formation of N2O5 and NO3. One drawback of this study is that a contaminated filter likely was responsible for preventing detection of NO3. The authors use steady-state analysis to determine NO3 mixing ratios and reconcile measured N2O5 with the thermal equilibrium calculation. This calculation is fairly convincing that, despite the high air change and no measurements of N2O5 from the outdoor air coincident with measurements from indoor air, NO2 + O3 was happening fast enough to generate NO3 at ppt levels when O3 levels were relatively high. Highlighting the connection between ventilation rates, outdoor air pollution, and indoor oxidant production is of high significance.
I recommend the manuscript for publication, but there are some edits I would like the authors to consider and address.
We thank the reviewer for this positive assessment of our manuscript and for providing helpful comments.
Major Comments
(Line 8; pg. 1) Please replace “air exchange” with “air change” when discussing this rate. Exchange implies a plug flow movement of fluids when in reality indoor spaces resemble more of a completely mixed reactor.
Correction made, we replaced “air exchange” with “air change” throughout the whole manuscript.
(Line 30; pg. 2) Could the authors provide the relevant wavelength ranges for the photolysis of NO3 outdoors?
Done, we now write in L30/31:
NO3 + hv (lambda ≈ 400 – 640 nm)  NO2 + O (R5a)
NO3 + hv (lambda ≈ 585 – 640 nm)  NO + O2 (R5b)

(Line 32; pg. 2) I recommend moving this statement to the next paragraph or removing it entirely.
We notice the redundancy with L39-41 and merge the statement in L32 with that in L40/41. We now write in L40:
In some environments (e.g. forested regions), reactions with unsaturated hydrocarbons not only become the dominant NO3 removal process at night, but even compete with R4, R5a and R5b during the day (Liebmann et al., 2018a; Liebmann et al., 2018b).
(Line 34; pg. 2) There are two studies that may be more appropriate to cite here than the Weschler and Moravek studies.
Zhou, S.; Kahan, T. F. Spatiotemporal characterization of irradiance and photolysis rate constants of indoor gas‐phase species in the UTest house during HOMEChem. Indoor air 2022, 32 (1), e12964.
Gandolfo, A.; Gligorovski, V.; Bartolomei, V.; Tlili, S.; Alvarez, E. G.; Wortham, H.; Kleffmann, J.; Gligorovski, S. Spectrally resolved actinic flux and photolysis frequencies of key species within an indoor environment. Building and environment 2016, 109, 50-57.
We agree and replaced in L34 the studies by Weschler et al. (1992) and Moravek et al. (2022) with those mentioned above.
(Line 35; pg. 2) This statement “Indoor ozone (and NOx) are found in buildings equipped with ventilation systems or when windows are open, so that indoor NO3 production rates become significant.” is misleading. All buildings have O3. Outdoor air gets indoors through several process including infiltration (outdoor air change via leakage points in a building), mechanical ventilation, or natural ventilation (opening a window). I recommend restating this as: “Production rates of NO3 indoors have been calculated to be high enough to form N2O5 and to potentially influence VOC oxidation (Moravek et al., 2022).”
We agree and modified the statement in L35:
Elevated indoor ozone (and NOx) levels are particularly common in urban buildings with ventilation systems or when windows are open, so that indoor NO3 production rates become significant.
(Line 45; pg. 2) I recommend removing this line. Alkyl nitrates are a broad class of molecules that may partition to aerosol depending on which alkyl nitrates are being considered. There are also no citations for the “…indoor particle levels are generally relatively low…” or “…alkyl nitrates and HNO3 are expected to…”. Dry deposition is sure to be an important sink of HNO3 indoors, but alkyl nitrates range in volatility such that some may deposition efficiently and some may be lost from indoor air principally through ventilation.
Due to the lack of both VOC measurements necessary to assess the volatility of the corresponding organic nitrates in our experiment and studies on the indoor fate of organic nitrates, we have removed the sentence in L45 following the referee's comment.
(Line 71-79; pg. 3) The authors approach to measuring the air change rate is unorthodox (see reference, though the ASTM method recommends NO as a tracer gas which I think is inappropriate because of reaction with O3) (ASTM, 2017). What the authors are actually measuring is a decay rate of the two chemicals. They attempt to reconcile the two different decay rates from the chemicals by suggesting the reactivity of limonene is greater than DMB. I recommend some edits:
Unless the authors can provide an analysis to understand how much reactive loss was contributing to the decay of the two molecules to reframe their measurement as one that measured the upper bound of the true air change rate.
We now emphasize this point by adding in L78:
Due to the bias caused by deposition and chemical loss processes, the values derived by our approach can only serve as an upper limit of the true air change rate.
Edit Line 78 to say “An exchange rate constant The air change rate of our laboratory during the measurement period was less than about 4 h-1 is thus appropriate for our laboratory.
Done, we modified this sentence in L78 accordingly.
Include a statement saying that an alternative method of measuring the air change rate that future studies could use is to purchase a low-cost CO2 sensor, generate CO2 through respiration, then measure the decay that way.
We added in L80:
Note that a more volatile and less reactive tracer such as carbon dioxide (CO2), which is readily available through respiration and can be detected by inexpensive sensors, would have been a more appropriate choice.
In the SI the authors fit the NO3 decays to exponentials. In tracer gas methods generally the natural log of the mixing ratio is analyzed because in natural log space deviations from linearity indicate interference from non-linear or coupled linear processes. For instance, mixing in indoor spaces generally can be measured as a fast first-order log-linear process and then air change is often slower. In the authors case, because of the reactive nature of limonene, chemistry is happening on some timescale that is competitive with air change that is producing a high decay constant compared to DMB. I recommend the authors determine decay constants by taking the natural log of the NO3 concentration.
Thanks for raising this point and drawing our attention to the ASTM method. As proposed in this standard, we now fit our data with an exponential decay function without any offset accordingly, which results in an only slightly higher decay constant of 5.76 h-1 for limonene (the value for DMB remains unchanged). However, we prefer to work with the original values instead of taking its natural logarithm for the following reasons:
Deviations from linearity also become evident when plotting the original values on a logarithmic scale.
When fitting the logarithm of the data with a linear function, points at large x-values (where [NO3]0-[NO3] approaches 0 pptv), are overemphasized, which results in a bias of the slope (Motulsky and Ransnas, 1987).
As shown in the plots below, both approaches lead to very similar decay constants, as long as large x-values are not considered in the regression (fitting interval marked in orange).

We modified the plots in Fig. S1b and Fig. S1c according to those above on the left and updated the value of the limonene decay constant.
(Line 93; pg. 4) Is kNO3 defined in this manuscript? This is not an intuitive concept for many studying indoor air quality. I recommend presenting an equation for kNO3 and briefly describing what kNO3 tells us about indoor air composition and what measuring it implies for indoor air quality.
The benefit of the NO3 reactivity measurement is already indicated in the paragraph beginning in L55. To underline our point, we add in L62:
Quantifying VOC-induced NO3 reactivity together with the above-mentioned set of measurements enables identification of the dominant NO3 loss processes which may allow drawing qualitative conclusions about the formation of organic nitrates indoors.
In addition, we elaborate in L94:
After accounting for the impact of NOx (see below), this instrument quantifies the total gas-phase NO3 reactivity (i.e. the inverse of the NO3 lifetime) towards VOCs, so that k^(NO_3 ) is equal to the summed first-order loss rate Σki[VOC]i with the concentration of a VOC [VOC]i and the corresponding rate coefficient ki for its reaction with NO3 (R7).
(Line 180; pg. 6) One of the major drawbacks of this study is that outdoor air composition, through a direct outdoor measurement or an indirect measurement in a supply vent, was measured only for about 10 minutes. Indeed with an ACR of ≈ 4 h-1 the indoor air mixing ratios are quite strongly affected by outdoor air composition and it’s not clear that indoor N2O5 levels were not affected by outdoor air composition. Can the authors provide some comment on the possible influence of N2O5 from outdoor air?
An air change rate of 4 h-1 would indeed be sufficient to transport outdoor N2O5 into the laboratory. However, outdoor air during periods B, D, E and (partially) A is affected by sunlight which drastically reduces the lifetime of NO3 (typical photolysis rate of 0.17 s-1 at noon, Johnston et al. (1996)) and thus N2O5, which is why neither is typically observed during the day (Brown and Stutz, 2012). To assess the potential impact of outdoor N2O5 on our measurement, we calculate the in-situ indoor N2O5 production rate Pindoor(N2O5), which is, according to R2 and Eq.4, equal to k2[NO2][NO3] ≈ k1k2[NO2]2[O3]/LNO3. Our measurements of NO2, O3, NO and kNO3 thus allow us to calculate Pindoor(N2O5). As depicted below, Pindoor(N2O5) varies from 0.1 to 2.3 pptv s-1 during periods A-E. In Schuster et al. (2009), outdoor N2O5 at the institute of up to 80 pptv were reported in October 2007 at night. With an ACR of 4 h-1, even 100 pptv would result in a production rate caused by infiltration of outdoor N2O5 Poutdoor(N2O5) of only 0.012 s-1 (orange line) which is by far not sufficient to compete with Pindoor(N2O5). An outdoor N2O5 mixing ratio of 1 ppbv (Poutdoor(N2O5) = 0.12 s-1, blue line) would be required to influence the indoor N2O5 mixing ratios throughout the whole measurement period. Therefore, we conclude that contribution from outdoor N2O5 to our indoor N2O5 budget cannot be excluded, but appears to be unlikely unless several hundreds of pptv of N2O5 were constantly abundant outside, which would be unusual for this urban environment near ground level.

We now write in L177-179:
Outdoors, NO3 and N2O5 are usually only observed during nighttime due to NO3 photolysis rates of typically up to 0.17 s-1 at noon […]
However, an air change rate of 4 h-1 is sufficient to entrain N2O5 originating from outside into the laboratory especially at night. To assess the potential impact of outdoor N2O5 on our measurement, we compare the calculated in-situ indoor N2O5 production rate Pindoor(N2O5) with the production rate Poutdoor(N2O5) caused by infiltration of N2O5 originating from outside. According to R2 and Eq.4, Pindoor(N2O5) is equal to k2[NO2][NO3] ≈ k1k2[NO2]2[O3]/LNO3. Calculation of Pindoor(N2O5) from our measurements of NO2, O3, NO and kNO3 yields values of 0.1-2.1 pptv s-1 during periods A-E. Schuster et al. (2009) reported nighttime N2O5 mixing ratios outside the institute in October 2007 of up to 80 pptv which would result in an production rate Poutdoor(N2O5) = [N2O5]outdoor x kchange of ≈ 0.01 pptv s-1. Unless several hundreds of pptv of N2O5 were constantly abundant outside, Poutdoor(N2O5) is very unlikely to affect our N2O5 levels measured inside.
(Line 180-182; pg. 6) I recommend removing these lines because this information is repeated in lines 184-186.
Correction made, we removed L180-182:
As mentioned in Section 2, the laboratory is a ventilated indoor environment, so that the indoor measurements as shown in Figure 1 should be affected by the ambient air composition. In order to verify this, we determined the effective air exchange rate and compared O3 and NOX levels inside the laboratory and directly outside.
(Line 184; pg. 6) The true ACR is, at most, 4 h-1. The authors are reporting the ACR from the decay of a reactive chemical and they’ve demonstrated through the decay of limonene that chemistry is playing a role in determining decay and inflating to some degree the true ACR.
Correction made, we now write:
With an air change rate of up to 4 h-1, the composition of the air in the laboratory is strongly influenced by that outside.
(Line 210; pg. 7) It is not clear how the authors arrived at the conclusion that reaction of NO + O3 was the most important loss process determining I/O(O3). On line 203 the authors describe what they would expect the I/O(O3) to be under steady-state conditions and using a kexchange of 4 h- 1 and ksurface of 2.5 h-1 come to the conclusion that they explain I/O(O3) as being predicted just by air change and surface uptake. Where is the kgas-phase reaction term?
The referee is right, the impact of NO + O3 was not considered in our discussion of I/O(O3). Since O3 levels well above our LOD only occurred when NO was close or below ca. 150 pptv, kgas-phase reaction is expected to be low: Assuming an NO level of 150 pptv and using the IUPAC-recommended rate coefficient for NO + O3 results in an NO-induced ozone loss rate of 0.25 h-1. Gas-phase reactions therefore do not affect the I/O ratio during these periods. We removed the sentence in L210.
Furthermore, our measurement of I/O(O3) suggests that the reaction between NO and O3 is the most important factor of most observations:
(Lines 195-215; pg. 7) In Figure 3 it looks like the authors took one sequential 20-minute measurement of indoor and outdoor O3 and NO2. It’s not clear why this much explanation of the attempt to reconcile indoor and outdoor levels of O3 and NO2 is provided with so few measurements. As the authors point out by referencing the (Nazaroff and Weschler, 2022) paper it is well known that O3 levels are lower indoors compared to outdoors through a variety of reasons. There are many examples of studies that try and characterize O3 losses through HVAC systems, on indoor surfaces, and through reactions in indoor air so non-rigorous speculation. It seems sufficient to just state the I/O(O3) ratio and suggest that although the O3 indoor loss rate was not quantified the (Nazaroff and Weschler, 2022) study that reports a surface loss rate of 2.5 h-1 for a laboratory. I think if the authors want to attempt to budget the I/O(O3) they need to include gas-phase losses into their steady-state assessment.
Our goal was to underline that the surface removal rate is not necessarily similar for two different laboratories, hence the lengthy discussion. We have shortened this paragraph and eliminated the speculative nature in our statements for the sake of clarity. We removed the statement in L207 and now write in L200:
Note that windows are shut at all times in the laboratory, and that outdoor air passes through a compressor, metal piping and filters before entering the labs so O3 is partially lost, which explains the lower than unity indoor / outdoor ratio of concentrations as reflected in an average I/O of 0.25 recently reported in a review summarizing measurements in ca. 2000 indoor environments.
Our observed value for I/O(O3) is much higher than the above-mentioned average of 0.25, which can be linked to low losses in the ventilation system, the lack of human emissions inside the laboratory during the measurements and low NO mixing ratios during the transport of O3 to our laboratory.
As explained in the previous comment, gas-phase losses are insignificant compared to surface losses and air change, so we added in L209:
Since indoor O3 only reached elevated levels when NO ≤ 150 pptv resulting in a maximum O3 loss rate of 0.25 h-1, the contribution of R6 was neglected.
(Line 220-223; pg. 8) I don’t see any presentation of particle measurements. This seems to be an important point because the measurement period was four days and my understanding is that over the course of four days the Teflon filter collected what looks to be (Figure S4) many particles. This much particle loading on the filter over four days would indicate that there was a lot of particles in the room. Further, this is the explanation the authors provide why they did not measure NO3. Can the authors provide data showing particle measurements? Am I misunderstanding something that the authors can clarify?
Particle number densities were only measured on a few occasions (but not during our pilot study) and showed that indoor particle numbers are a factor of ca. 0.1 lower compared to outdoor levels. Losses to particles are not a crucial point for our observations though. Even in outdoor environments NO3 loss via particles is generally insignificant compared to gas-phase losses (Liebmann et al., 2018a; Liebmann et al., 2018b). Furthermore, the visible particles that accumulated on the filter only serve as an indication of likely reactivity as shown by Tang et al. (2010).The number of particles on the filter is not representative for those being available in ambient air. NO3 is very sensitive to filter losses (also when a fresh filter is used), which is why even changing the filter after several hours results in an increase in the signal (Crowley et al., 2010). We add in L149, L220 and L223:
Total (i.e. non-size-segregated) particle number densities were sporadically measured after the pilot study using a condensation particle counter (CPC, TSI, model 3025a).
A few checks (after our pilot study) showed that the typical number density indoor-to-outdoor ratio, I/O(Npart), was close to 0.1.
As shown below, the low particle number density results in low aerosol surface areas, hence leading to insignificant losses of e.g. N2O5 or NO3 to particles in ambient air compared to other surfaces and/or reactants. Nevertheless, accumulation of ambient particles on our inlet filter results in quantitative NO3 removal prior to entering our cavity (Tang et al., 2010), which is why membrane filters are usually changed on an hourly basis during measurements to avoid this issue (Crowley et al., 2010; Sobanski et al., 2016).
(Line 248; pg. 8) On line 169 the authors said NO3 was undetected, but here on this line they say “…under which indoor N2O5 (and NO3) are observable…” Please clarify.
With that we wanted to imply that, when N2O5 is observed, NO3 is expected to be present due to its thermal equilibrium with N2O5. To avoid confusion, we now removed the expression in the brackets.
In order to analyze the conditions under which indoor N2O5 (and NO3) are is observable […].
(Line 257-259; pg. 9) Thanks for putting these levels of oxidants into context of VOC oxidation. Can the authors add one more comparison to this and use the estimated ACR of 4 h-1 to say how important limonene oxidation is as a sink compared to ventilation? From the numbers the authors provided this suggests that in the study, during times of NO3 production, NO3 oxidation would account for about 70% of the reactive sink for limonene, but only about 30% (oxidation + ventilation) of the total sink.
That is a good point, we now mention in L257-259:
Note that the reaction with NO3 would only contribute ca. 30% to the total VOC loss rate when the maximum air change rate of 4 h-1 is taken into account.
(Line 285; pg. 10) The authors make a good point about the detection limit of NO being an important factor in influencing how interpretable the production of NO3, and subsequent impacts on VOC oxidation, can be determined. The instrument used in this study has a much better detection limit than many commercial NOx analyzers and so without an instrument like the one used by the authors other studies may have challenges constraining the elusively NO3 chemistry indoors.
Thanks for pointing this out.
(Comparison to Other Studies) In the previous section the authors presented some comparison of loss constants for NO3, OH, and O3. The study of Price et al. (2019) put expected levels and ventilation rates into the context of VOC loss for indoor spaces by doing an analysis of VOC reactivity flux. For the indoor environment they focused on, Figure 5 of that paper demonstrates that oxidation chemistry is a small sink for VOCs. Can the authors put the observations of NO3 reactivity from their high ventilation laboratory into context by comparing their results to the Price study?
We have added this study in Table 1 (and L49) and now consider the results of this study in our discussion in section 4. In L329 we write:
In a study by Price et al. (2019), oxidation via NO3 contributes ~10% to the total VOC loss in a museum that is ventilated with an air change rate of only 0.8 h-1. The dominant loss process in this museum is ventilation with the residual contribution of 90%. Despite the fact that our air change rate is higher by a factor of 5, we estimated a higher NO3 contribution of ~30 % (see above). This discrepancy to our study is explained by different indoor NO3 levels: According to steady-state and model calculations, only 0.04 pptv and up to NO3 were present in the museum which is two orders of magnitudes lower compared to our values.
(Comparison to Other Studies) The authors could consider citing our study that was accepted several days ago to ES&T. If it is published by the time the authors address these comments, it would be a nice additional point of comparison as the air change rate measured in our residential building (ACR ≈ 0.2 h-1) is the lowest of any study to date that considers the role of NO3 radical chemistry in influencing indoor air quality. However, this is not critical so feel free to ignore this comment.
M.F. Link, J. Li, J.C. Ditto, H. Huynh, J. Yu, S.M. Zimmerman, K.L. Rediger, A. Shore, J.P.D. Abbatt, L.A. Garofalo, D.K. Farmer, D. Poppendieck. Ventilation in a Residential Building Brings Outdoor NOx Indoors with Limited Implications for VOC Oxidation by NO3 Radicals, Environmental Science and Technology, (Accepted October 3rd , 2023).
We have added this study in Table 1 (and L49) and now consider the results of this study in our discussion in section 4. In L330 we write:
Link et al. (2023) identified ventilation to be the dominant VOC sink (88%) in a residential building featuring an even lower air change rate and modelled NO3 level of 0.2 h-1 and 0.02 pptv, respectively. When 70 ppbv of O3 were added artificially, oxidative processes competed with ventilation. In this case, ozonolysis (and OH production) drastically reduced the fractional contribution of R7 to ~10%. Again, our higher fractional contribution of R7 to the overall VOC loss is consequently reflected in our higher indoor NO3 levels compared to those in Link et al. (2023).
(Conclusions) Can the authors provide any recommendations for themselves or others attempting to make a measurement like this related to the dirty filter?
We recommend changing the inlet filter on an hourly basis and added in L344:
Furthermore, our measuring attempt of NO3 emphasized the necessity of frequent inlet filter changes as common in field measurements.
Minor Comments
(Figure 1) Remove the (a), (b), (c), ect. on the left side of the figure for each panel.
We prefer to keep the letters because they help to guide the reader through the discussion of Fig.1 in section 3.
(Significance Statement) Recommended edit: “Our measurements suggest that NO3 can be the dominant indoor oxidant of e.g. limonene, which is often released indoors owing to its presence in cleaning agents.”
Correction made.
(Line 7; pg. 1) “Indoor mixing ratios of O3 ranged from < 2-28 ppbv…” Less than 2 ppb is because the detection limit right? Indoor O3 follows outdoor O3. Can this be restated as “Daytime O3 mixing ratios typically were 15-20 ppbv…”
During many time periods of our measurement, O3 was actually below the LOD due to high NO mixing ratios. We thus prefer to keep the statement as is.
(Line 22; pg. 1) Recommended edit: “NO3 is produced by the oxidation of nitrogen dioxide (NO2) by ozone (O3), both of which are usually abundant present in ventilated indoor air environments owing to transport from outside from ventilation of outdoor air:”
It is more typical for residential buildings to have O3 mixing ratios that are about 25% of the O3 mixing ratio outside (Nazaroff and Weschler, 2022). There are many examples of residences where O3 levels are less than 5 ppb typically (Zhou et al., 2018) with a “…central tendency between 4-6 ppb..” (Nazaroff and Weschler, 2022). The indoor space in this study indeed has high levels of O3 as a result of the mechanical ventilation system of the commercial building.

Correction made.
(Line 33; pg. 2) Recommended edit: “NO3 photolysis rates indoors are sufficiently diminished compared to outside so that R5a and R5b can be neglected and the nitrate radical NO3 gains in relative importance, relative to O3 and OH, as an oxidizing agent.
Correction made.
(Line 41; pg. 2) Recommended edit: “NO3 reacts rapidly with many unsaturated VOCs and, if formed in sufficiently high levels, it can represent the dominant oxidizing agent for unsaturated hydrocarbons.”
This sentence has already been modified after addressing a previous comment.
(Line 55; pg. 2) Recommended edit: “This limited number of studies implies that, in non- or poorly ventilated rooms, high…” Most indoor spaces are ventilated at least through infiltration.
Correction made.
(Line 58; pg. 2) Recommended edit: “Rapid ventilation of “fresh” outside air results…”
Correction made.
(Line 61; pg. 2) Recommended edit: “…together with O3 from a well-ventilated laboratory and inside a laboratory serving as a ventilated test-indoor-environment over a weekend period…”
Correction made.
(Line 70, 71, 76, 78, 79; pg. 3) Replace “exchange rate” with “air change rate”
Done, we replaced “exchange rate” with “air change rate” throughout the whole manuscript.
(Line 219; pg. 7) Recommended edit: “…2) that R6, similarly to outdoors, explains the distinct anti-correlation between O3 and NOx measured indoors as observed in Figure 1.”
Correction made.

2) Reply to Referee 2
This paper presents the first indoor measurements of NO3 radical reactivity using a flow tube instrument coupled with a cavity ring-down spectrometer for detection of NO3. In addition to the reactivity measurements, the authors also measured mixing ratios of NO, NO2, O3, and N2O5 simultaneously. Together with the reactivity measurements, this allowed the authors to estimate the mixing ratio of NO3 in this indoor environment. The measurements were conducted in an unoccupied laboratory with an air exchange rate of approximately 4 hr-1. This relatively high exchange rate resulted in mixing ratios of ozone and NO2 similar to that outdoors. However, measured mixing ratios of indoor NO3 were below the detection limit of the instrument, apparently due to loss of NO3 on the inlet tubing and filter. But based on the measured NO3 reactivity and the measurements of NO, NO2, and O3, the authors were able to estimate expected steady-state mixing ratios of NO3.

The calculated NO3 mixing ratios lead to calculated N2O5 mixing ratios that are consistent with the measurements, suggesting that the measured NO3 reactivity and the calculated NO3 mixing ratios are consistent with the measured N2O5. The calculated NO3 mixing ratios are similar to some previous measurements in other indoor environments but are approximately a factor of 100 greater than previous predictions of NO3 in residential environments. The results suggest that indoor mixing ratios of NO3 may be greater than previously believed. The authors conclude that NO3 mixing ratios may be significant indoors under conditions where indoor O3 and NO2 are elevated due to high air exchange rates, and that measurements of NO3 reactivity may be used in conjunction with measurements of NO2 and ozone to estimate NO3 concentrations.

The paper is well written, and the pilot study presents new information about the potential importance of NO3 radicals in indoor environments. I recommend publication after the authors have addressed the following comments:
We thank the referee for the positive evaluation of our manuscript.

1) The authors state that the fact that their measurements of NO3 were undetectable due to loss of NO3 on the inlet tube and filter. This is in contrast to the description of NO3 loss in their instrument as described in Sobanski et al. (2016) which stated that the transmission of NO3 through the filter was approximately 70%. While the picture of the filter in the supplement clearly shows that the filter was contaminated in this study, it isn’t clear why the authors did not change the filter more frequently given that it appears that the system is equipped with an automatic filter changer according to the description in Sobanski et al. (2016).
The referee is right, it would have been necessary to change the inlet filter more frequently, which is why the instrument is usually equipped with an automatic filter changer. Unfortunately, the filter changer was not available during the measurement period. We clarify this in L127 in our manuscript:
To reduce NO3 loss through the inlet, an automatic filter changer normally replaces the inlet filter every hour. Unfortunately, the filter changer was not available during the measurement period.

2) The authors state that their measurements of NO3 reactivity are corrected for the impact of NOx (page 8), so the additional loss of NO3 by reaction with NO is added in Equation 2. I assume that this is related to the numerical simulations required to extract the NO3 reactivity from the measurements. This should be clarified, perhaps in the supplement, as the discussion in Liebmann et al. (2017) seems to suggest that a bias in measurements is due to the reaction of NO3 with NO2 under high NO2 conditions.
This correction is indeed related to the reactions that are taking place in the flowtube. Note that the bias of the measurement is generally very sensitive to NO2. During the residence in the flowtube a significant fraction (ca. 20%) of NO is converted to additional NO2 due to the presence of ca. 40 ppbv. However, ambient NO was extremely low compared to NO2 during periods A-E rendering the correction for NO unnecessary. We prefer to keep this correction for the sake of consistency though, since this instrument provided purely VOC-induced reactivities in previous publications. We add in L116:
Note that conversion of NO to additional NO2 via R6 in the flowtube affects NO2/k^(NO_3 ) which is why accounting for the impact of NO becomes significant in polluted conditions not only because of R4.

Minor comments:

It would be useful to provide a schematic of the experimental set up in the supplement, illustrating the inlet lengths and location for both the reactivity instrument and the CRDS instrument.
Such a schematic does not really provide useful insight as the air is well-mixed within the laboratory (see Fig. S2).

Page 3, lines 90-91: The authors state that the measurements were conducted over four days in October, but state that the only extended sunny day was July 16th…
Correction made, it was October 16.
3) Reply to Referee 3
This paper presents an interesting pilot investigation of the indoor NO3-N2O5 chemistry by measuring many related parameters mainly including NO3 reactivity, NOx, O3, and NO3, N2O5. NO3 reactivity measurement showed a comparable level with previous observation in an outdoor rural environment, and highlights that NO3 chemistry may be important to indoor air pollution. They showed NO and VOC are the dominate NO3 loss pathways with comparable contribution, and N2O5 uptake on the inner wall and aerosol surface contributes minor. Overall, the indoor oxidation by NO3 is less concerned compared with ozone and OH radicals, this study is helpful in promoting the understanding of the indoor NO3 chemistry when the indoor air is well exchanged with the outdoor air. This topic is certainly within the scope of ESA. This paper is well written, and should be published subject to the following minor comments.
We thank the referee for the positive assessment of our manuscript.

1. Line 75-80, we know that the exchange rate should be smaller than 4.3 h-1, are there a more detailed sensitivity analysis of the indoor oxidants or wall loss estimation for giving the result of 4.0 h-1?
The value of 4 h-1 is derived after accounting for oxidation losses. We now write in L78:
The faster decay term for limonene is likely related to its indoor oxidation and wall loss, which are both expected to be more rapid than for 2,3-dimethyl-2-butene. Due to the bias caused by deposition and chemical loss processes, the values derived by our approach can only serve as an upper limit of the true air change rate. In the case of limonene, gas-phase losses contribute ≈ 30 % to the overall decay rate. The air change rate of our laboratory during the measurement period was less than 4 h-1.

2. Line 160-165, In Figure 1, the plot of k^(NO_3 ) and L_(NO_3 ) is a little bit confused. I can understand that the upper limit of k^(NO_3 ) is caused by the high NO, and the VOC-induced NO3 reactivity cannot be derived, while in the figure, the plot of a flat line of k^(NO_3 ) may misunderstanding since the k^(NO_3 ) is not quantified rather than equal to 1.7 s-1. In addition, I believe the plot of L_(NO_3 ) during the period just only considers the contribution of NO. All these detailed points should be clarified.
L_(NO_3 )contains the contribution of both NO and k^(NO_3 ). We prefer to keep the upper limit data points, because missing data rather imply that the instrument did not run. To reduce misunderstandings, we extended the caption of Fig. 1 addressing the above-mentioned points and added Eq.1 and Eq. 2 at the axis labels.
Figure 1: Overview of directly measured and derived quantities during a weekend period inside a ventilated laboratory: (a) Relative humidity RH (blue, left axis) and temperature T (orange, right axis); (b) NO2 (blue, left axis) and NO (orange, right axis) (c) O3 (blue, left axis) and NO3 production rate P_(NO_3 )from Eq. 1 (blue, right axis); (d) Total NO3 loss rate L_(NO_3 ) from Eq. 2 (blue, left axis) and NO3 reactivity k^(NO_3 ) (LOD of 1.7 s-1, orange with same-coloured shaded area to mark its contribution to L_(NO_3 )); (e) N2O5 (blue circles, left axis), calculated N2O5 from Eq. 4 (blue line, left axis), NO3 (orange circles, right axis) and calculated NO3 from Eq. 3 (orange line, right axis).

3. I suggest the author separate the measurement and calculation of NO3 and N2O5 into two panels, it’s too busy to see the performance during some period, by the way, the case in Figure 4 is clear although used the same plot style.
Adding an additional panel would shrink the other panels too much. As pointed out by the referee, the figure concept was appropriate in Fig. 4. To increase readability of Fig. 1, we reduced the size of the data points and made the lines partially transparent. The point of Fig. 1 (panel e) is to convey that the calculated N2O5 is generally in agreement with the measured N2O5 (which is why it is difficult to distinguish between the two quantities in the plot), and that [NO3]ss is always higher than the measured NO3. Both can be seen in Fig. 1. Details in the performance are shown in Fig. 4 and Fig. 5.

4. Line 232, the L_(NO_3 ) can be replaced by another abbreviation such as k^(NO_3-total) (it is not the same with the loss rate in Line 14), since the L_(NO_3 ) is often regard as the NO3 loss rate rather than the overall NO3 loss rate coefficient.
L_(NO_3 )is a pseudo first-order loss rate in units of s-1 and is supposed to be the same as in L14. We modified L232 and Eq. 2 to clarify that our L_(NO_3 ) is an approximation:
Thus, in order to derive the overall NO3 loss rate L_(NO_3 ) (assuming only the overall gas-phase loss rate kgas is relevant compared to heterogeneous loss rate khet), the pseudo-first-order loss rate constant for reaction with NO has to be added:
L_(NO_3 )=k_gas+k_het≈k^(NO_3 )+k_4 [NO] (Eq.2)

5. Line 270-273, the usage of fresh-air via filters indicates that a lower N2O5 uptake coefficients, while this statement seems to contradict the previous one that NO3 detection is largely affected by the indoor aerosols (Fig. S4), are there possible that the filter of in the NO3 instrument is not changed with a relative high frequency? The author should clarify it.
Even in outdoor environments NO3 loss via particles is generally insignificant compared to gas-phase losses (Liebmann et al., 2018a; Liebmann et al., 2018b). Furthermore, the visible particles that accumulated on the filter only serve as an indication of likely reactivity as shown by Tang et al. (2010).The number of particles on the filter is not representative for those being available in ambient air. NO3 is very sensitive to filter losses (also when a fresh filter is used), which is why even changing the filter after several hours results in an increase in the signal (Crowley et al., 2010). We add in L223:
As shown below, the low particle number density results in low aerosol surface areas, hence leading to insignificant losses of e.g. N2O5 or NO3 to particles in ambient air compared to other surfaces and/or reactants. Nevertheless, accumulation of ambient particles on our inlet filter results in quantitative NO3 removal prior to entering our cavity (Tang et al., 2010), which is why membrane filters are usually changed on an hourly basis during measurements to avoid this issue (Crowley et al., 2010; Sobanski et al., 2016).
In fact, the inlet filter was not changed during the entire measurement period. This information is now added in L149 (see replies to Referee 2).

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03-Nov-2023

Dear Dr Crowley:

Manuscript ID: EA-ART-09-2023-000137.R1
TITLE: NO₃ reactivity measurements in an indoor environment: A pilot study

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Reviewer 1

The authors did an outstanding job addressing my comments! I look forward to seeing this work published as well as future measurements of indoor chemistry from this group.