the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
An oxidation flow reactor for simulating and accelerating secondary aerosol formation in aerosol liquid water and cloud droplets
Ningjin Xu
Chen Le
David R. Cocker
Abstract. Liquid water in cloud droplets and aqueous aerosols serves as an important reaction medium for the formation of secondary aerosol through aqueous-phase reactions (aqSA). Large uncertainties remain in estimates of the production and chemical evolution of aqSA in the dilute solutions found in cloud droplets and the concentrated solutions found in aerosol liquid water, which is partly due to the lack of available measurement tools and techniques. A new oxidation flow reactor (OFR), the Accelerated Production and Processing of Aerosols (APPA) reactor, was developed to measure secondary aerosol formed through gas- and aqueous-phase reactions, both for laboratory gas mixtures containing one or more precursors and for ambient air. For simulating in-cloud processes, droplets formed on monodisperse seed particles are introduced into the top of the reactor and the relative humidity (RH) inside it is controlled to 100 %. Similar measurements made with the RH in the reactor <100 % provide contrasts for aerosol formation with no liquid water and with varying amounts of aerosol liquid water.
The reactor was characterized through a series of experiments and used to form secondary aerosol from known concentrations of an organic precursor and from ambient air. The transmission efficiency of O3 and CO2 for all RH and of SO2 for low RH exceeds 90 %, while it falls to about 70 % for SO2 at 100 % RH. Particle transmission efficiency increases with increasing particle diameter from 0.67 for 0.050 μm particles to 0.98 at 0.20 μm, while that of the ~3.3 μm droplets formed on seed particles is greater than 80 %. The residence time distributions of both gases and particles are narrow relative to other OFRs and lack the tails at long residence time expected with laminar flow. Initial cloud processing experiments focused on the well-studied oxidation of dissolved SO2 by O3, with observed growth of seed particles resulting from the added sulfuric acid agreeing well with estimates based on the relevant set of aqueous phase reactions. The OH exposure (OHexp) for low RH, high RH, and in-cloud conditions was determined experimentally from the loss of SO2 and benzene, and simulated from the KinSim chemical kinetics solver with inputs of measured 254 nm UV intensity profile through the reactor and loss of O3 due to photolysis. The aerosol yield for benzene at high OHexp ranged from 18 % at low RH with dry seed particles present in the reactor to 59 % with cloud droplets present. Measurement of the composition of the secondary aerosol formed from ambient air using an aerosol mass spectrometer showed that the oxygen to carbon ratio (O : C) of the organic component increased with increasing RH (and liquid water content).
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Ningjin Xu et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2022-285', Anonymous Referee #1, 07 Dec 2022
The authors present a new flow reactor designed for investigating multiphase chemistry in aerosols and cloud droplets. This is an interesting and technically challenging idea, which could potentially be an important tool to gain insights into these understudied processes. Care was taken in the design of the reactor to avoid temperature gradients due to the UV lamps, which would interfere with multiphase partitioning, to minimize losses of soluble gases, and to strictly control RH. They use a Spot sampler to generate cloud-like droplets from seed aerosol for study in the reactor. They present preliminary results for studies of the influence of aqueous processing on SOA formation in the lab and using ambient air. I believe the manuscript is publishable in AMT after some minor issues are addressed and the manuscript is revised accordingly.
- The article is too long, with 17 figures in the main text. Surely this can be tightened up, with some material moved to SI. At the same time the most interesting figure (the second Figure S2 - there are two Figure S2s) has been relegated to the SI.
- The abstract is also too long. It's not immediately clear what "transmission efficiencies" are upon reading the abstract without first reading the manuscript.
- Why is K2SO4 used instead of other particle types which may be more representative of atmospheric aerosol or more typical of laboratory studies of aerosol and cloud chemistry? I think the question is answered several pages later on line 496 but should also be acknowledged when this is first mentioned. Also note that "neutral" isn't really representative of ambient aerosol or cloudwater pH (cf. Pye et al. 2020)
- the size dependence of the particle tranmission efficiency is mentioned, and compared with other studies, but no physical explanation for decreasing transmission for smaller particles is given. One would expect the opposite.
- The SO2 transmission efficiency at high RH is low. Presumably studying SO2 multiphase chemistry was one of the main intended purposes of this apparatus. Can the authors comment on how this issue may impact the design of future studies?
- line 444: 'described in by (Mitroo et al)'... clean that up
- residence time distribution: why is there a distribution at all if the gases or particles are introduced in the same location and sampled in the same location (if this is not the case it's not possible to tell from the text)? How are three measurements sufficient to construct the whole RTD curve and detect the absence of a long tail?
- line 490 - I would either delete or rephrase this. You are presumably introducing this apparatus for the first time in this manuscript, why are you making this statement about it not being applied very often to cloud chemistry studies when that is the most novel and unique application?
- Figure 9 and Figure S2. See my comment above about the second Figure S2. It is much more interesting than Figure 9. Figure 9 has been sufficiently described in the text lines 500-507. It could be made panel B of a figure which focuses on the data in Figure S2, moved to SI, or eliminated. How was the low SO2 transmission efficiency dealt with in these calculations?
- Is the relatively short residence time of APPA compared to a chamber experiment an issue when it comes to SOA studies?
- What is the reader supposed to take away from the SOA yield studies shown in Figure 15? Were these yields consistent with expectations?
- funding acknowledgement seems to be missing.
Citation: https://doi.org/10.5194/amt-2022-285-RC1 -
AC1: 'Reply on RC1', Don Collins, 31 Jul 2023
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-285/amt-2022-285-AC1-supplement.pdf
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AC1: 'Reply on RC1', Don Collins, 31 Jul 2023
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RC2: 'review of Xu et al.', Anonymous Referee #2, 12 Dec 2022
Xu et al. present the evaluation of their “Accelerated Production and Processing of Aerosols” (APPA) OFR that is designed to enable aqueous-phase OH-initiated oxidative aging processes. The APPA OFR combines the authors’ PFA OFR with injected K2SO4 seed particles at the inlet and controlled humidification that enables control of the particle liquid water content (the range of RH discussed in this paper is 40 - 100% RH). The authors perform characterization studies of gas and particle transmission efficiencies, residence time distributions, droplet size distributions, and radiation/oxidant profiles. To demonstrate applications of the APPA OFR, they generated sulfuric acid from SO2/O3 reactions, benzene OH-SOA in the presence of dry/aqueous seeds and “cloud droplets”, and aged ambient aerosol.
Comments
- While the APPA OFR appears to be capable of generating sulfuric acid from the aqueous-phase SO2/O3 reaction, its ability to initiate aqueous-phase OH oxidation chemistry was not conclusively demonstrated here. The authors did investigate SOA generated from gas-phase OH oxidation of benzene, followed by partitioning of OVOC/SOA into ALW/droplets. While this is a novel application that the APPA OFR seems to be well suited to, this is not a demonstration of aqSOA formation according to even the authors’ own definition: “water-soluble products of gas-phase chemistry [that] enter cloud droplets or aerosol liquid water and react in the aqueous phase with the hydroxyl radical (OH) or other oxidants” (L83-L86). In that regard, I think they should have used K2SO4 seed particles containing H2O2 and/or H2O2/FeSO4 to initiate aqSOA formation (e.g. Nguyen et al., 2013; Daumit et al., 2016), then repeated the same experiments without H2O2/FeSO4, and inferred the difference in aerosol loading and composition as aqSOA. It is not clear to me why this was not done here – this is what I would need to see to be convinced that the APPA OFR can be used to investigate aqSOA formation.
- Similarly, how did the authors conclude that aqueous phase OH oxidation was responsible for the increase in ambient OA oxidation state as RH was increased from 40%-->85%->100% (Fig. 17 and related text)? Hypothetically, couldn’t this change have been driven by the higher RH (and LWC) promoting more efficient partitioning of low-volatility gas-phase oxidation products into the aerosol? For the reasons mentioned in above comment, it is not clear to me that this evolution in OA oxidation state was in fact due to aqueous phase oxidative aging in the APPA OFR.
- Unless signification dilution flow is added downstream of the APPA OFR, its relatively low 1.5 L min-1 sample flow capacity limits its application outside of measurements that can be made with particle counter(s) and instruments such as an AMS. What design changes would need to be made to increase this flow capacity to something in the range of 5-10 L min-1 that is closer to other commonly used OFR techniques?
- L25 and L230 - Quantify “low RH”
- L70 – Clarify which “new pathways” are being referred to here
- L160 – Rather than “The APPA…is typically operated as a 254 nm-type OFR”, I suggest instead saying “the APPA reactor is typically operated in OFR254 mode”
- L376 – typo (“to a to a”)
- L446-L448 – Please indicate the relative humidity that was established in the APPA OFR when the RTD measurements were conducted, and please clarify if the lamps were on or off. Is there any humidity-dependence to the RTD?
- L450 – Why are the gas/particle RTD’s in the APPA OFR narrower than in the PFA OFR when the two reactors are nominally the same design?
- L464 – I did not notice any explicit discussion of temperature control in the APPA OFR in this section.
- L492 - The experiments describing sulfuric acid formation from SO2/O3 were not clearly described. I assume sulfuric acid was generated from SO2 + O3 --> SO3 + O2 followed by SO3 + H2O --> H2SO4, but it would be useful to clarify this. How is the concentration of “dissolved” SO2 controlled and measured? Is O3 uptake onto the K2SO4 seed particles required to initiate this reaction?
- L583 – Typo (the)
- L608- Assuming that the authors are referring to the benzene/OH system here, I disagree that the “distribution of [benzene] oxidation products and their OH reaction constant(s) are generally unknown.”See, for example, Xu et al. (2020); Priestley et al. (2021).
- L721. Please clarify the author contributions of C. Le and D. R. Cocker.
- Some of the figures should be moved to the Supplement - in my opinion, Figures 4, 8, 9, 10, 12, 13, and 16 would be a better fit there.
- The KinSim mechanism and case files that were used here should be uploaded with the Supplement.
References
T. B. Nguyen, M. M. Coggon, R. C. Flagan, and J. H. Seinfeld, Reactive Uptake and Photo-Fenton Oxidation of Glycolaldehyde in Aerosol Liquid Water. Environmental Science & Technology 2013 47 (9), 4307-4316. DOI: 10.1021/es400538j
Kelly E. Daumit, Anthony J. Carrasquillo, Rebecca A. Sugrue, and Jesse H. Kroll . Effects of Condensed-Phase Oxidants on Secondary Organic Aerosol Formation. The Journal of Physical Chemistry A 2016, 120 (9) , 1386-1394. https://doi.org/10.1021/acs.jpca.5b06160
Lu Xu, Kristian H. Møller, John D. Crounse, Henrik G. Kjaergaard, and Paul O. Wennberg, New Insights into the Radical Chemistry and Product Distribution in the OH-Initiated Oxidation of Benzene, Environmental Science & Technology 2020 54 (21), 13467-13477. DOI: 10.1021/acs.est.0c04780.
Priestley, M., Bannan, T. J., Le Breton, M., Worrall, S. D., Kang, S., Pullinen, I., Schmitt, S., Tillmann, R., Kleist, E., Zhao, D., Wildt, J., Garmash, O., Mehra, A., Bacak, A., Shallcross, D. E., Kiendler-Scharr, A., Hallquist, Å. M., Ehn, M., Coe, H., Percival, C. J., Hallquist, M., Mentel, T. F., and McFiggans, G.: Chemical characterisation of benzene oxidation products under high- and low-NOx conditions using chemical ionisation mass spectrometry, Atmos. Chem. Phys., 21, 3473–3490, https://doi.org/10.5194/acp-21-3473-2021, 2021.
Citation: https://doi.org/10.5194/amt-2022-285-RC2 -
AC2: 'Reply on RC2', Don Collins, 31 Jul 2023
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-285/amt-2022-285-AC2-supplement.pdf
Ningjin Xu et al.
Ningjin Xu et al.
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