the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2023
Volatile Oxidation Products and Secondary Organosiloxane Aerosol from D5 + OH at Varying OH Exposures
Hyun Gu Kang
Yanfang Chen
Jiwoo Jeong
Yoojin Park
Hwajin Kim
Abstract. Siloxanes are composed of silicon, oxygen, and alkyl groups and are emitted from consumer chemicals. Despite being entirely anthropogenic, siloxanes are being detected in remote regions and are ubiquitous in indoor and urban environments. Decamethylcyclopentasiloxane (D5) is one of the most common cyclic congeners, and smog chamber and oxidation flow reactor (OFR) experiments have found D5 + OH to form secondary organosiloxane aerosol (SOSiA). However, there is uncertainty about the reaction products, and the reported SOSiA mass yields (YSOSiA) appear inconsistent. To quantify small volatile oxidation products (VOP) and to consolidate the YSOSiA in the literature, we performed experiments using a Potential Aerosol Mass OFR while varying D5 concentration, humidity, and OH exposure (OHexp). We use a proton transfer reaction time-of-flight mass spectrometer to quantify D5, HCHO, and HCOOH, and detect other VOP, which we tentatively identify as siloxanols and siloxanyl formates. We determine molar yields of HCHO and HCOOH between 52 – 211 % and 45 – 127 %, respectively. With particle size distributions measured with a scanning mobility particle sizer, we find YSOSiA to be < 10 % at OHexp < 1.3 × 1011 s cm-3 and ~20 % at OHexp corresponding to that of the lifetime of D5 at atmospheric OH concentrations. We also find that YSOSiA is dependent on both organic aerosol mass loading and OHexp. We use a kinetic box model of SOSiA formation and aging (aging-VBS model) to reconcile the YSOSiA values found in this study and the literature. The model uses a volatility basis set (VBS) of the primary oxidation products as well as an aging rate coefficient in the gas phase, kage,gas, of 2.17 × 10-11 cm3 s-1, and an aging rate coefficient in the particle phase, kage,particle, which is ten times smaller. The combination of primary VBS and OH-dependent oxidative aging predicts SOSiA formation much better than a standard-VBS parameterization that does not consider aging (R2 = 0.970 vs. 0.847). The need for an ageing-dependent parameterization to accurately model SOSiA formation shows that concepts developed for secondary organic aerosol precursors, which are able to form low-volatile products at low OHexp, do not necessarily apply to D5 + OH. The resulting yields of HCHO and HCOOH and the parameterization of YSOSiA may be used in larger scale models to assess the implications of siloxanes on air quality.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Hyun Gu Kang et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1033', Anonymous Referee #1, 03 Jul 2023
General comments:
The authors investigated the photooxidation process of decamethylcyclopentasiloxane (D5), which is used in consumer products, using a flow reactor; the D5 oxidation products, i.e., silanols, formaldehyde, formic acid, and secondary aerosols, were measured revealing the photooxidation process in the gas and particle phase. By determining the variables of the volatility basis-set model, the measured yields of D5-derived secondary aerosols in this and previous studies were explained, and the mechanism by which the volatile silanols produced during the initial oxidation process undergo photochemical aging to form lower-volatility compounds which are partitioned into the particle phase was clarified. In particular, the model-based explanation for D5-derived secondary aerosols is considered worthy of publication in the field of atmospheric chemistry. However, I hope the authors will read the following comments and consider revisions to the draft prior to publication.
Specific comments:
- Lines 106-110. Briefly describe the methods used in previous studies and discuss the implications of using the methods of this study to investigate the mechanism of photooxidation of D5, which has been interpreted differently in previous studies.
- Lines 127. There is only one sentence at the end of the introduction explaining the aim of the study. The topic of this study would not only be to parameterize aerosol yields. The initial product analysis of D5, the HCHO production yield measurement, and the formic acid production yield measurement to be performed in this study should also be reiterated, and the overall aims of this study should be summarized. The new measurements or calculations to be performed in this study should be again highlighted.
- Line 150. By external OH reactivity, do you mean OH reactivity measured under the same conditions separately from the D5 reaction experiment? The meaning of the word "external" is ambiguous.
- Line 191. PerMaSCal ions are confusing; it would be easier to simply write diiodobenzene ions for the m/z 331 ions here.
- Lines 201-204 and section S2. Has a study been conducted to determine the effect of volatilization of particles from the filter? Also, you state that the accuracy of the balance is 0.1 mg, but if it is a semi-microbalance, wouldn't the accuracy be 0.01 mg? What was the mass of the particles collected by the filter?
- Lines 246-247. Can we explain the experimental results of photochemical aging of D5-derived secondary aerosols without considering particle phase aging? There is no guarantee that the uptake factor will be the same as for the α-pinene SOA of Zhao et al. (2015); can we assess the sensitivity of a factor of 10% to the final fitting?
- Lines 254-256. Why normalize the mass spectra before and after the reaction by the signal of m/z 371, which is the signal of D5? The signal of m/z 371 after OH exposure should be decreased by the reaction than before the exposure. How much did m/z 371 decrease by OH exposure in Experiment 12? It seems meaningless to compare before and after reaction by normalizing by m/z 371 signal without considering the decrease of m/z 371 signal intensity by reaction.
- Lines 261-266. Since the m/z ratio of protonated D5 (D5-H+) produced in the PTR is m/z 371, the formation of the ion at m/z 355 should be described as loss of methane from protonated D5 ([D5-H - CH4]+) rather than loss of methyl from D5. Similarly, the loss of OH from silanol should be expressed as loss of water molecules from protonated silanol.
- Lines 276-278. "Consistent" may be an error for "constant". In the same sentence, if the absorption of VOPi into the particle phase is discussed and then ignored, evidence should be provided that it can be ignored.
- Line 304. The abbreviation ODE is only used in one place in the text. It would be easier to understand if the abbreviation were not used and the term "ordinary differencial equations" were used again here.
- Lines 307-310. The formation of formaldehyde by subsequent oxidation is ignored in Equations 7 and 8, even though it is later considered that formaldehyde is formed by subsequent oxidation. In fact, all experimental results for OHexp=3E11-1E12 in Figure 3 are higher than the value of the fitted curve. This is probably due to the failure to account for the subsequent formation of HCHO. Fitting with an incorrect model could provide data with systematic errors for the determined γHCHO.
- Lines 344-352. This paragraph ultimately only compares the measured formic acid yield from D5 to that from isoprene. what evidence is there to conclude that D5 should be considered as a source of formic acid in the atmosphere? The current explanation is inadequate.
- Line 424. KinSim, which is not mentioned in the text, is suddenly mentioned in the conclusion. Section S5 and KinSim should be briefly explained in advance at appropriate places in the text.
- Fig. 1. The figure contains the structural formula of the analyte. However, since the figure is a mass spectrum, it may be necessary to provide an explanation of the detected ions instead of an explanation of the analyte. For example, you could simply indicate the symbols A-E in the figure and add the following explanation to the figure title; A: [D5-H]+, B: [D3T2-OH-OCHO-H - CH4]+, C: [D4T-OCHO-H]+, D: [D4T-OH-H - H2O]+, and E: [D43T2-(OH )2-H - H2O]+.
- Fig. 2. Explain in words somewhere that the color used for the functional group of the compound corresponds to the color of the plot.
- Fig. 4b. Explain in words somewhere that the light colored area represents αi for gas and the dark colored area represents αi for particles. Does this figure show the gas-particle distribution ratio for COA=10ug/m3? If so, it should be half gas and half particles in 10^1ug/m3 bins. Correct any errors in calculations or markings.
Citation: https://doi.org/10.5194/egusphere-2023-1033-RC1 -
AC1: 'Reply on RC1', Hwajin Kim, 15 Sep 2023
We appreciate the anonymous reviewers for their thoughtful reviews and comments. We have carefully considered their suggestions and revised the manuscript accordingly. In addition to changes arising from the reviewers, we have made edits to correct figure references in the text and the figures themselves. Our response file is attached.
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RC2: 'Comment on egusphere-2023-1033', Anonymous Referee #2, 17 Jul 2023
This manuscript investigated the oxidation of D5 siloxane in an oxidation flow reactor (OFR) and the formation of volatile oxidation products, such as formaldehye (HCHO) and formic acid (HCOOH), and secondary organic aerosol (SOSiA). It was found that there was substantial formation of HCHO, HCOOH and SOSiA, highlighting their environmental importance. To reconcile the discrepant SOSiA yields reported in the literature, the study employed a volatility-based multi-generation aging model (VBS) to fit the observed SOSiA in all experiments simultaneously, by tuning the volatility and aging parameters. It was found that the model was able to better capture SOSiA formation from the literature with aging accounted for. This suggested that multi-generational aging may be very important for D5 siloxane oxidation, more so than other systems like monoterpene oxidation, where SOA forms early on. This study is generally well designed, and acceptance is recommended if the comments can be addressed.
1. When simulating the OFR experiments, were the OH concentrations corrected for suppression by external reactivity?
2. Since HCHO and HCOOH are continuously formed with aging, why quantify their molar yield at zero OH exposure? The yield would change significantly with aging, is that right? Please clarify.
3. In the all the OFR experiments simulated, were seed particles added to promote SOSiA condensation? If not, how would new particle formation and kinetically limited particle growth affect the model predictions? Please include this in the discussion.
4. This may be related to 3. I think the authors should make clear in the discussion that this study is focused on reconciling the SOSiA yields by accounting for multi-generational aging only, but aging may be not the only factor affecting the yields, especially in OFR experiments where particle kinetics, phase state etc. can play important roles. Limitations should be acknowledged and if possible, sensitivities should be probed.
5. The use of “pseudo persistent” in line 68 is somewhat unclear. Please add that D5 siloxane has temporary reservoirs in the atmosphere, if that is right.
Citation: https://doi.org/10.5194/egusphere-2023-1033-RC2 -
AC2: 'Reply on RC2', Hwajin Kim, 15 Sep 2023
We appreciate the anonymous reviewers for their thoughtful reviews and comments. We have carefully considered their suggestions and revised the manuscript accordingly. In addition to changes arising from the reviewers, we have made edits to correct figure references in the text and the figures themselves. Our response file is attached.
-
AC2: 'Reply on RC2', Hwajin Kim, 15 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1033', Anonymous Referee #1, 03 Jul 2023
General comments:
The authors investigated the photooxidation process of decamethylcyclopentasiloxane (D5), which is used in consumer products, using a flow reactor; the D5 oxidation products, i.e., silanols, formaldehyde, formic acid, and secondary aerosols, were measured revealing the photooxidation process in the gas and particle phase. By determining the variables of the volatility basis-set model, the measured yields of D5-derived secondary aerosols in this and previous studies were explained, and the mechanism by which the volatile silanols produced during the initial oxidation process undergo photochemical aging to form lower-volatility compounds which are partitioned into the particle phase was clarified. In particular, the model-based explanation for D5-derived secondary aerosols is considered worthy of publication in the field of atmospheric chemistry. However, I hope the authors will read the following comments and consider revisions to the draft prior to publication.
Specific comments:
- Lines 106-110. Briefly describe the methods used in previous studies and discuss the implications of using the methods of this study to investigate the mechanism of photooxidation of D5, which has been interpreted differently in previous studies.
- Lines 127. There is only one sentence at the end of the introduction explaining the aim of the study. The topic of this study would not only be to parameterize aerosol yields. The initial product analysis of D5, the HCHO production yield measurement, and the formic acid production yield measurement to be performed in this study should also be reiterated, and the overall aims of this study should be summarized. The new measurements or calculations to be performed in this study should be again highlighted.
- Line 150. By external OH reactivity, do you mean OH reactivity measured under the same conditions separately from the D5 reaction experiment? The meaning of the word "external" is ambiguous.
- Line 191. PerMaSCal ions are confusing; it would be easier to simply write diiodobenzene ions for the m/z 331 ions here.
- Lines 201-204 and section S2. Has a study been conducted to determine the effect of volatilization of particles from the filter? Also, you state that the accuracy of the balance is 0.1 mg, but if it is a semi-microbalance, wouldn't the accuracy be 0.01 mg? What was the mass of the particles collected by the filter?
- Lines 246-247. Can we explain the experimental results of photochemical aging of D5-derived secondary aerosols without considering particle phase aging? There is no guarantee that the uptake factor will be the same as for the α-pinene SOA of Zhao et al. (2015); can we assess the sensitivity of a factor of 10% to the final fitting?
- Lines 254-256. Why normalize the mass spectra before and after the reaction by the signal of m/z 371, which is the signal of D5? The signal of m/z 371 after OH exposure should be decreased by the reaction than before the exposure. How much did m/z 371 decrease by OH exposure in Experiment 12? It seems meaningless to compare before and after reaction by normalizing by m/z 371 signal without considering the decrease of m/z 371 signal intensity by reaction.
- Lines 261-266. Since the m/z ratio of protonated D5 (D5-H+) produced in the PTR is m/z 371, the formation of the ion at m/z 355 should be described as loss of methane from protonated D5 ([D5-H - CH4]+) rather than loss of methyl from D5. Similarly, the loss of OH from silanol should be expressed as loss of water molecules from protonated silanol.
- Lines 276-278. "Consistent" may be an error for "constant". In the same sentence, if the absorption of VOPi into the particle phase is discussed and then ignored, evidence should be provided that it can be ignored.
- Line 304. The abbreviation ODE is only used in one place in the text. It would be easier to understand if the abbreviation were not used and the term "ordinary differencial equations" were used again here.
- Lines 307-310. The formation of formaldehyde by subsequent oxidation is ignored in Equations 7 and 8, even though it is later considered that formaldehyde is formed by subsequent oxidation. In fact, all experimental results for OHexp=3E11-1E12 in Figure 3 are higher than the value of the fitted curve. This is probably due to the failure to account for the subsequent formation of HCHO. Fitting with an incorrect model could provide data with systematic errors for the determined γHCHO.
- Lines 344-352. This paragraph ultimately only compares the measured formic acid yield from D5 to that from isoprene. what evidence is there to conclude that D5 should be considered as a source of formic acid in the atmosphere? The current explanation is inadequate.
- Line 424. KinSim, which is not mentioned in the text, is suddenly mentioned in the conclusion. Section S5 and KinSim should be briefly explained in advance at appropriate places in the text.
- Fig. 1. The figure contains the structural formula of the analyte. However, since the figure is a mass spectrum, it may be necessary to provide an explanation of the detected ions instead of an explanation of the analyte. For example, you could simply indicate the symbols A-E in the figure and add the following explanation to the figure title; A: [D5-H]+, B: [D3T2-OH-OCHO-H - CH4]+, C: [D4T-OCHO-H]+, D: [D4T-OH-H - H2O]+, and E: [D43T2-(OH )2-H - H2O]+.
- Fig. 2. Explain in words somewhere that the color used for the functional group of the compound corresponds to the color of the plot.
- Fig. 4b. Explain in words somewhere that the light colored area represents αi for gas and the dark colored area represents αi for particles. Does this figure show the gas-particle distribution ratio for COA=10ug/m3? If so, it should be half gas and half particles in 10^1ug/m3 bins. Correct any errors in calculations or markings.
Citation: https://doi.org/10.5194/egusphere-2023-1033-RC1 -
AC1: 'Reply on RC1', Hwajin Kim, 15 Sep 2023
We appreciate the anonymous reviewers for their thoughtful reviews and comments. We have carefully considered their suggestions and revised the manuscript accordingly. In addition to changes arising from the reviewers, we have made edits to correct figure references in the text and the figures themselves. Our response file is attached.
-
RC2: 'Comment on egusphere-2023-1033', Anonymous Referee #2, 17 Jul 2023
This manuscript investigated the oxidation of D5 siloxane in an oxidation flow reactor (OFR) and the formation of volatile oxidation products, such as formaldehye (HCHO) and formic acid (HCOOH), and secondary organic aerosol (SOSiA). It was found that there was substantial formation of HCHO, HCOOH and SOSiA, highlighting their environmental importance. To reconcile the discrepant SOSiA yields reported in the literature, the study employed a volatility-based multi-generation aging model (VBS) to fit the observed SOSiA in all experiments simultaneously, by tuning the volatility and aging parameters. It was found that the model was able to better capture SOSiA formation from the literature with aging accounted for. This suggested that multi-generational aging may be very important for D5 siloxane oxidation, more so than other systems like monoterpene oxidation, where SOA forms early on. This study is generally well designed, and acceptance is recommended if the comments can be addressed.
1. When simulating the OFR experiments, were the OH concentrations corrected for suppression by external reactivity?
2. Since HCHO and HCOOH are continuously formed with aging, why quantify their molar yield at zero OH exposure? The yield would change significantly with aging, is that right? Please clarify.
3. In the all the OFR experiments simulated, were seed particles added to promote SOSiA condensation? If not, how would new particle formation and kinetically limited particle growth affect the model predictions? Please include this in the discussion.
4. This may be related to 3. I think the authors should make clear in the discussion that this study is focused on reconciling the SOSiA yields by accounting for multi-generational aging only, but aging may be not the only factor affecting the yields, especially in OFR experiments where particle kinetics, phase state etc. can play important roles. Limitations should be acknowledged and if possible, sensitivities should be probed.
5. The use of “pseudo persistent” in line 68 is somewhat unclear. Please add that D5 siloxane has temporary reservoirs in the atmosphere, if that is right.
Citation: https://doi.org/10.5194/egusphere-2023-1033-RC2 -
AC2: 'Reply on RC2', Hwajin Kim, 15 Sep 2023
We appreciate the anonymous reviewers for their thoughtful reviews and comments. We have carefully considered their suggestions and revised the manuscript accordingly. In addition to changes arising from the reviewers, we have made edits to correct figure references in the text and the figures themselves. Our response file is attached.
-
AC2: 'Reply on RC2', Hwajin Kim, 15 Sep 2023
Peer review completion
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Journal article(s) based on this preprint
Hyun Gu Kang et al.
Hyun Gu Kang et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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- Final revised paper