the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2024
Interaction between marine and terrestrial biogenic volatile organic compounds: Non-linear effect on secondary organic aerosol formation
Abstract. Biogenic volatile organic compounds (BVOCs) are the largest source of secondary organic aerosols (SOA) globally. However, the complex interactions between marine and terrestrial BVOCs remain unclear, inhibiting our in-depth understanding of the SOA formation in the coastal areas and its environmental impacts. Here, we performed smog chamber experiments with mixed α-pinene (a typical monoterpene) and dimethyl sulfide (DMS, a typical marine emission BVOC) to investigate their possible interactions and subsequent SOA formation. It is found that DMS has a non-linear effect on SOA generation: the mass concentration and yield of SOA show an increasing and then decreasing trend with the increase of the initial concentration of DMS. The increasing trend can be attributed to OH regeneration together with acid-catalyzed heterogeneous reactions by the oxidation of DMS, while the decreasing trend is explained by the high OH reactivity that inhibits the formation of low volatility products. The results from infrared spectra and mass spectra together reveal the contribution of sulfur-containing molecules in the mixed system. Moreover, the mass spectra results indicate that acidic products generated by DMS photooxidation enhance the O:C ratio, while organosulfates are produced to contribute to the formation of mixed SOA. In addition, the trends in relative abundance of highly oxygenated organic molecules (HOMs) with C8-C10 multiple functional groups in different mixed systems agree well with the turning point of the SOA yield. The findings of this study have significant implications for understanding binary or more complex systems in the atmosphere in the coastal areas.
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RC1: 'Comment on egusphere-2023-2960', Anonymous Referee #1, 08 Apr 2024
This manuscript investigated the SOA formation from a mixture of a-pinene and DMS in laboratory chamber experiments. It is found that DMS has a non-linear effect on SOA generation: the mass concentration and yield of SOA show an increasing and then decreasing trend with the increase of the initial concentration of DMS. Potential interaction mechanisms have been proposed. Detailed offline characterization of SOA composition was conducted and utilized to investigate the SOA formation mechanism. However, the analysis has fundamental flaws. I cannot recommend publication in its current form.
Major Comments
- The SOA yield is a function of existing organic aerosol (delta_Mo)1. This fundamental concept is key in explaining the observed results, but is completely ignored throughout the discussion. Without consid
- In figure 1, there is clear difference between measured and modeled DMS, but this issue is not discussed in the manuscript. The difference is surprising given the a-pinene decay is reasonably modeled. Perhaps the DMS measurement has issues. Further, the difference challenges the reliability of modeling results (e.g., Figure 2) and any conclusions drawn based on modeling.
- The first paragraph under section 2. The effect of a-pinene + DMS interaction on SOA yield should be systematically evaluated for all experiments and illustrated graphically. It is not sufficient to compare one set of experiments only in words. Also, an alternative and more meaningful way is to compare [delta a-pinene]*SOA yield_a-pinene + [delta DMS] * SOA yield_DMS vs SOA mass formed in the mixed experiments. The SOA yields should correspond to the total SOA mass in the mixed experiments.
- The proposed explanation regarding the effects of adding DMS on OH concentration is confusing. If the initial OH increase is because of OH regeneration from DMS oxidation, how could it be possible that further adding DMS will reduce OH?
- The proposed mechanisms in Figures 8 and 10 are flawed. The proposed isomerization reactions and H-shift do not occur in the atmosphere2, 3.
Minor Comments
- The head row of Table 3 is confusing. The table seems to have two different head rows. For example, does the first column correspond to [total particles] or delta[a-pinene]?
- Describe how the volatility of each compound is estimated for Figure 6.
- Line 3. Grammar error. I assume what the authors want to express is that “OH generation before the turning point could attribute to the enhancement in SOA formation.”
References
Pankow, J. F., An absorption model of gas/particle partitioning of organic compounds in the atmosphere. Atmospheric Environment 1994, 28 (2), 185-188.
Xu, L.; Møller, K. H.; Crounse, J. D.; Otkjær, R. V.; Kjaergaard, H. G.; Wennberg, P. O., Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-Pinene and β-Pinene by Hydroxyl Radicals. The Journal of Physical Chemistry A 2019, 123 (8), 1661-1674.
Vereecken, L.; Nozière, B., H migration in peroxy radicals under atmospheric conditions. Atmos. Chem. Phys. 2020, 20 (12), 7429-7458.
Citation: https://doi.org/10.5194/egusphere-2023-2960-RC1 -
RC2: 'Comment on egusphere-2023-2960', Anonymous Referee #2, 10 Apr 2024
Summary:
This manuscript describes a system of environmental chamber experiments to examine the kinetics and product distribution both in the gas and aerosol phase from a mixture of DMS and a-pinene. The work describes the non-linear effect of DMS on the oxidation of a-pinene with respect to the mass concentration and yield of SOA. The authors attribute this observation primarily to acid catalyzed heterogeneous reactions and changing OH reactivity and concentrations. The authors present a detailed analysis of the SOA and the components that could be contributing to the observed SOA. The authors also present multiple mechanisms to help explain the observed masses. The work in its current form is confusing and contains errors and issues with the figures and supporting claims. I cannot recommend this publication in its current form. I would request major revisions before publication is reevaluated.
General comments:
The tables and figures throughout the work are confusing. Axes are hard to associate with the data and tables seem to have headers that do not match with the presented data.
The work does a good job trying to disentangle the effect of DMS and alpha pinene on SOA formation by looking at the products and yields separately as well as in a mixture. The concentrations of VOCs, NOx and H2O2 are atypical of the environment and should be discussed in more detail. I understand limitations of instrumentation and observations, but additional work (facilitated by modeling) could be used to better understand the fate of the VOC and the RO2 generated from OH oxidation within the chamber.
Overall the experimental set up and design of the chamber experiments needs to be discussed in more detail, so the reader can understand the observations better. In particular the design of the chamber is not communicated well. There is some general confusion about the [OH] concentration within the chamber and how that plays into the observations of DMS and a-pinene. The DMS observations across the chamber results seem to have a linear and unmatched decay compared to that of the model. This is a significant fraction of mass that the model is not capturing that is not addressed in the text. A further discussion on limitations or missing mechanisms within the DMS mechanism should be communicated to better understand the present results and subsequent understanding on DMS’s effect on SOA yields.
The discussion surrounding OH is confusing and deserves more explanation. In particular, more time needs to be spent describing (or citing) how OH was calculated/constrained within the chamber. Figure 2, presents OH numbers and a trend line, but descriptions of how this was derived is missing. DMS has a well known OH loss rate and is present in most of the experiments, I would recommend using that decay curve to inform your OH concentrations and compare that to your box model results. Additionally, I am confused about the connection between OH loss and aerosol acidity referenced in the work (line 190).
Overall, there seems to be a lack of citations or validation for some of the comments made throughout the work. In particular, comprehensive citations referencing mechanisms, techniques and analysis used and previous chamber work is missing.
Technical comments:
Line 46: Emerging work on DMS oxidation has found the formation of a key intermediate, hydroperoxymethyl thioformate (HPMTF). This intermediate is formed via an isomerization reaction and regenerates OH in the process. I would recommend adding context to this reaction and using it to understand the effect of DMS on the chamber observations.
Line 70: How was the chamber run? Is this a batch mode or continuous flow method? Please explain in more detail how the chamber was run and what steps were taken to account for processes like dilution.
Line 83: What are the concentrations of H2O2 used in the chamber? You use H2O2 photolysis to produce OH under high concentrations of NO. This will lead to a complex and high concentration mixture of HO2, RO2 and NO thus changing the fate of the peroxy radical formed in MT and DMS oxidation. Please devote more time to discussing this interaction.
Line 100: Wall loss for SOA and VOC can be an important driver of loss within an environmental chamber. Values are given for the wall loss terms without any validation or reasoning for the values. Could the authors please provide context and assumptions made for the values used.
Figure 1: The axis’s colors and labels do not match. I would recommend matching them to guide the readers eye.
Line 150: Can you add a more in-depth analysis of DMS oxidation? You present one DMS chamber experiment and state that your observations don’t match with Chen and Jang (2021). DMS has been studied through various oxidation methods and strategies. I would recommend further literature review to see if other work on DMS oxidation can match your observations and if not why. Just stating RH, oxidant and collection method does not explain the observed trends.
Line 157: What is SOA in the DMS photooxidation? Could you please elaborate on what the components are of SOA that are not H2SO4. Could you please elaborate on what the new particles are in this case? Is the DMS SOA pure sulfuric acid clusters that other DMS derived species build upon. Do you have any indication of NH3 or a gas-phase base to build with sulfuric acid.
Table 2: The connection between max O3, SO2, and NOx and max SOA is not a straight forward concept that should be evaluated within the text. Presenting one or two of the time traces could be an informative way to understand when the peaks are occurring and how that relates to the steady state or end of experiment concentration. The amount of O3 produced across the experiments varies by a decent amount. I would recommend addressing this variability and seeing if its formation can help understand what is happening within the chamber. Also is O3 + MT important at these concentrations?
Table 3: The header of Table 2 is added to the start of Table 3. Please fix this.
Line 205: Needs citation for the isomerization rate. Additionally, I would recommend reviewing the literature around HPMTF formation as new slower rates exist compared to Wu et al. It is also important to caution the 43% increase in OH with comments about how much isomerization is occurring in the chamber work presented here. High NO, HO2 and RO2 likely present here could arrest this channel.
Figure 6: “a” and “b” are not labeled on the Figure
Figure 7: Please define and describe the meaning of “low,middle, and High” for the mixtures within the description.
Figure 8: A hydrogen shift mechanism across 4 carbons is presented and assumed to be a reaction within the chamber. I am not aware of this being a known reaction. Could the authors please provide evidence or citations that would support this mechanism. I understand that the bicyclic nature of the molecule could bring the hydrogen and alkyl radical close, but replacing a secondary radical with a primary seems highly unlikely.
Citation: https://doi.org/10.5194/egusphere-2023-2960-RC2 -
EC1: 'Comment on egusphere-2023-2960', Drew Gentner, 26 Apr 2024
The ACP editors would like to note that, based on the reviewers' recommendations, ratings, and comments that identify major errors, we do not encourage a revised submission as “a revised manuscript is only encouraged if [the authors] have satisfactorily addressed all comments and if the revised manuscript meets the high quality standards of ACP”. However, the authors are still able to submit responses to comments if they choose to.
Citation: https://doi.org/10.5194/egusphere-2023-2960-EC1
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