This paper provides a modification to ambient flow tube systems developed by Bertram et al 2009 and Wang et al 2018. The authors document the increased robustness due to measurements prior and after the flow tube as well as a box model to predict side reactions. While this is an interesting finding, I do not think this work is novel enough for publication in AMT. This work belongs as a technical note. I also noted that the authors need to credit Bertram et al 2009 more for use of their design. My comments are below.

MAJOR COMMENTS AND CONCERNS

1. Sections 2.1-2.3, the bulk of the flow tube design, are all pretty much the same as (T. H. . Bertram et al., 2009) yet the authors do not cite this paper in these sections for the design. A reader who has not read Bertram et al, 2009 might very well think that these aspects of the design are the authors’.

2. The residence time in the flow tube is quoted at 156s, which seems way too short for low values of surface area.

3. The authors measure very high values of gamma, much higher than observed for most ambient studies: (T. H. Bertram et al., 2009; Riedel et al., 2012b) and most flow tube work with the exception of dust particles (Mitroo et al., 2019; Tang et al., 2016), I wonder if the authors have underestimated the particle surface area by not using an APS or if the filter upstream of the CEAS is causing an artificially higher gamma than expected.

4. It is not clear that the box model presented in this work is a significant advance over the box model presented in Bertram et al 2009 and Wang 2018. The authors need to provide evidence that their work is a significant advance over previous work.

SPECIFIC COMMENTS

Abstract

1. “newly developed” on line 19. From the paper, this just seems like a slight modification instead.

Introduction

2. Line 47, also cite (Gaston and Thornton, 2016; Mitroo et al., 2019; Riedel et al., 2012a, 2013)

3. Line 59, also cite (Cosman et al., 2008; Escorcia et al., 2010; Folkers et al., 2003; Gaston et al., 2014)

4. Lines 62-65, also mention particle size as well (Gaston and Thornton, 2016). Also missing papers on organic aerosol (Escorcia et al., 2010; Gaston et al., 2014; Griffiths et al., 2009; Thornton et al., 2003)

5. Lines 71-74 reflect the findings in (Thornton et al., 2003), which should be cited here.

6. Lines 74-77 reflect the findings in (Mitroo et al., 2019; Royer et al., 2021), which should be cited here.

7. Line 105, Mitroo et al 2019 did not use an ambient flow tube.

Methods

1. Sections 2.1-2.3 are really the design of (T. H. . Bertram et al., 2009; T. H. Bertram et al., 2009), as such, the authors must use appropriate citations here.

2. Lines 215-217, wouldn’t the use of a filter upstream of the CEAS cause issues where wet, ambient particles would react with N2O5 going into the CEAS and cause a higher gamma than one would expect? That might explain the very high values of gamma observed in ambient.

3. Lines 287-288, this duty cycle is not that different from (T. H. . Bertram et al., 2009)

4. Lines 305-308, what fraction of VOCs measured had known rate constants that can be used to parameterize the reaction of NO3 with VOCs?

5. Section 3.2, it’s not clear how this box model differs from the previous studies cited.

6. Lines 437-441 is similar to the findings of (T. H. . Bertram et al., 2009)

7. Lines 489-490, 156 s for a residence time is quite short, especially for low surface areas. What is the time required for complete mixing of N2O5 in the flow tube?

8. Lines 635-639, gamma values seem really high. The authors should provide some explanation of how gamma varied as a function of air mass encountered.

REFERENCES CITED

Bertram, T.H.., Thornton, J.A.., Riedel, T.P.. (2009). An experimental technique for the direct measurement of N2O5 reactivity on ambient particles. Atmos. Meas. Tech. 2, 231–242.

Bertram, T.H., Thornton, J.A., Riedel, T.P., Middlebrook, A.M., Bahreini, R., Bates, T.S., Quinn, P.K., Coffman, D.J. (2009). Direct observations of N2O5 reactivity on ambient aerosol particles. Geophys. Res. Lett. 36, L19803, doi:10.1029/2009GL040248.

Cosman, L.M.., Knopf, D.A.., Bertram, A.K. (2008). N2O5 reactive uptake on aqueous sulfuric acid solutions coated with branched and straight-chain insoluble organic surfactants. J. Phys. Chem. A 112, 2386–2396.

Escorcia, E.N.., Sjostedt, S.J.., Abbatt, J.P.D.. (2010). Kinetics of N2O5 hydrolysis on secondary organic aerosol and mixed ammonium bisulfate-secondary organic aerosol particles. J. Phys. Chem. A 114, 13113–13121.

Folkers, M.., Mentel, T.F.., Wahner, A. (2003). Influence of an organic coating on the reactivity of aqueous aerosols probed by the heterogeneous hydrolysis of N2O5. Geophys. Res. Lett. 30, 1644, doi:10.1029/2003GL017168.

Gaston, C.J.., Thornton, J.A.. (2016). Reacto-diffusive length of N2O5 in aqueous sulfate- and chloride-containing aerosol particles. J. Phys. Chem. A 120, 1039–1045.

Gaston, C.J.., Thornton, J.A.., Ng, N.L.. (2014). Reactive uptake of N2O5 to internally mixed inorganic and organic particles: The role of organic carbon oxidation state and inferred organic phase separations . Atmos. Chem. Phys. 14, 5693–5707.

Griffiths, P.T.., Badger, C.L.., Cox, A., Folkers, M.., Henk, H.H.., Mentel, T.F.. (2009). Reactive uptake of N2O5 by aerosols containing dicarboxylic acids. Effect of particle phase, composition, and nitrate content. J. Phys. Chem. A 113, 5082–5090.

Mitroo, D.., Gill, T.E.., Haas, S.., Pratt, K.A.., Gaston, C.J.. (2019). ClNO2 production from N2O5 uptake on saline playa dusts: New insights into potential inland sources of ClNO2. Environ. Sci. Technol. 53, 7442–7452; DOI: 10.1021/acs.est.9b0112.

Riedel, T.P.., Bertram, T.H.., Crisp, T.A.., Williams, E.., Lerner, B.., Vlasenko, A.., Li, S.-M., Gilman, J., de Gouw, J., Bon, D.M.., Wagner, N.L.., Brown, S.S.., Thornton, J.A.. (2012a). Nitryl chloride and molecular chlorine in the coastal marine boundary layer. Environ. Sci. Technol. DOI: 10.1021/es204632r.

Riedel, T.P.., Bertram, T.H.., Ryder, O.S.., Liu, S.., Day, D.A.., Russell, L.M.., Gaston, C.J.., Prather, K.A.., Thornton, J.A.. (2012b). Direct N2O5 reactivity measurements at a polluted coastal site. Atmos. Chem. Phys. 12, 2959–2968.

Riedel, T.P., Wagner, N.L., Dube, W.P., Middlebrook, A.M., Young, C.J., Ozturk, F.,

Bahreini, R., VandenBoer, T.C., Wolfe, D.E., Williams, E.J., Roberts, J.M., Brown, S.S., Thornton, J.A. (2013). Chlorine activation within urban or power plant plumes: Vertically resolved ClNO2 and Cl2 measurements from a tall tower in a polluted continental setting. J. Geophys. Res. 118, 8702–8715.

Royer, H.M., Mitroo, D., Hayes, S.M., Haas, S.M., Pratt, K.A., Blackwelder, P.L., Gill, T.E., Gaston, C.J. (2021). The role of hydrates, competing chemical constituents, and surface composition on ClNO2formation. Environ. Sci. Technol. 55, 2869–2877. 

Tang, M.J., Cziczo, D.J., Grassian, V.H. (2016). Interactions of water with mineral dust aerosol: Water adsorption, hygroscopicity, cloud condensation, and ice nucleation. Chem. Rev. 116, 4205–4259.

Thornton, J.A.., Braban, C.F.., Abbatt, J.P.D.. (2003). N2O5 hydrolysis on sub-micron organic aerosols: the effect of relative humidity, particle phase, and particle size. Phys. Chem. Chem. Phys. 5, 4593–4603.

Wang et al describe an aerosol flow reactor for the measurement of the reactive uptake of N₂O₅ to ambient aerosol particles. This approach has already been reported in the literature and the approach taken by the authors is very similar to that previously reported. I suggest that the authors focus the paper on the specific aspects of the flow reactor system that are new and less on the aspects that are replication of prior work. In 2009, Bertram et al reported on the development of a flow reactor for measurement of the reactivity of ambient aerosol that is strikingly similar to this. In 2018, Wang et al reported on the use of an iterative box-model coupled to the flow reactor to improve the retrieval of the reactive uptake coefficients for N₂O₅ to ambient aerosol, which again is very similar to that used here. It is not clear what is new with this approach that would warrant a new publication. The authors need to make the case for what technological advancement has been made. It is also not clear that the uncertainty associated with the measurements have been reduced.

The authors do note that “simultaneous N₂O₅ measurement at both end of the flow tube” is a unique feature of this reactor. I find this statement to be misleading: 1) The measurement is NOT simultaneous. In this technique the top and the bottom of the flow tube are sampled sequentially within one duty cycle. 2) Sampling of the N₂O₅ concentration at the top and the bottom of the flow tube was also done in Bertram et al to retrieve daily wall loss terms (see section 3.2 of Bertram et al). The authors would need to argue that measuring the wall loss more frequently leads to a reduced uncertainty in the retrieved uptake coefficients if this is the primary technical advance of the paper.

There are a few aspects of the reported work and new directions that the authors could take this work that are (or would be) interesting:

• The residence time modeling in the flow tube was interesting, especially the conclusion that there are two flow paths. I think there is room for advancement in this technique if the distribution of reaction times was narrowed, while still preserving a long interaction time. Alternatively, it would also be interesting to try to use the RTD that is modeled within the framework of the N2O5 retrieval as it is not clear to me that an average residence time is appropriate with this type of RTD.
• While there were some nice calculations of the uncertainty in the retrieved N2O5 uptake coefficient, actual measurements are most important. I would like to see systematic evaluation of the approach in the laboratory. Some example experiments that would be extremely informative might include: i) measurement of g(N2O5) as a function of surface area for a model compound at constant RH and NO. ii) Modulation of NO (and RH above the deliquescent point) at the inlet while flowing a constant surface area concentration of a known aerosol composition. These experiments would confirm whether the modeled uncertainty holds for experimental conditions.