Particle-phase processing of α-pinene NO3 secondary organic aerosol in the dark
- 1Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, 5232 Villigen, Switzerland
- 2Department of Environmental Science, Stockholm University, Sweden
- anow at: ETH Zurich, Department of Chemistry and Applied Biosciences, 8093 Zurich, Switzerland
- 1Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, 5232 Villigen, Switzerland
- 2Department of Environmental Science, Stockholm University, Sweden
- anow at: ETH Zurich, Department of Chemistry and Applied Biosciences, 8093 Zurich, Switzerland
Abstract. The NO3 radical represents a significant night-time oxidant present in or downstream of polluted environments. There are studies that investigated the formation of secondary organic aerosol (SOA) from NO3 radicals focusing on yields, general composition, and hydrolysis of organonitrates. However, there is limited knowledge about how the composition of NO3-derived SOA evolves as a result of particle phase reactions. Here, SOA was formed from the reaction of α-pinene with NO3 radicals generated from N2O5, and the resulting SOA aged in the absence of external stimuli. The initial composition of NO3-derived α-pinene SOA was slightly dependent upon the concentration of N2O5 injected (excess of NO3 or excess of α-pinene), but was largely dominated by dimer dinitrates (C20H32N2O8-13). Oxidation reactions (e.g. C20H32N2O8 C20H32N2O9
C20H32N2O10 etc...) accounted for 60–70 % of the particle phase reactions observed. Fragmentation reactions and dimer degradation pathways made up the remainder of the particle-phase processes occurring. The exact oxidant is not known, though suggestions are offered (e.g. N2O5, organic peroxides, or peroxy-nitrates). Hydrolysis of −ONO2 functional groups was not an important loss term during dark aging under the relative humidity conditions of our experiments (58–62 %), and changes in the bulk organonitrate composition were likely driven by evaporation of highly nitrogenated molecules. Overall, 25–30 % of the particle-phase composition changes as a function of particle-phase reactions during dark aging representing an important atmospheric aging pathway.
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David M. Bell et al.
Status: closed
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RC1: 'Comment on acp-2021-379', Anonymous Referee #1, 05 Jun 2021
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AC1: 'Reply on RC1', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC1-supplement.pdf
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AC1: 'Reply on RC1', David Bell, 01 Oct 2021
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RC2: 'Comment on acp-2021-379', Anonymous Referee #2, 12 Jun 2021
General Comments
This manuscript (which is a companion paper to one submitted by Wu et al.) describes results of a laboratory study of the effect of aging in the dark on the mass and composition of SOA formed from the reaction of NO3 radicals with a-pinene for a few different concentrations of a-pinene and N2O5, which was the source of NO3 radicals. Experiments were conducted in a Teflon chamber, SOA mass and size were monitored with an SMPS, and gas and particle composition were monitored with a FIGAERO-CIMS and EESI-TOF. The observations are thoroughly discussed, and various possible explanations, such as evaporation, oxidation, and monomer-dimer reactions are proposed. In general, however, given the complexity of the system, the lack of information on the molecular structures of the SOA components (only elemental formulas are available), and the non-quantitative MS analyses, it was not possible to draw convincing conclusions about the physical or chemical processes that might have altered the SOA in the dark. Nonetheless, the data set is interesting, and future studies may provide more detailed data that can help to explain the results. I think the manuscript can be published after the following comments are addressed.
Specific Comments
- Line 205: The reaction RO2 + NO3 forms RO + NO2 + O2, not peroxynitrates (ROONO2). I assume you meant RO2 + NO2 –> ROONO2.
- How do you propose that peroxynitrates are converted to nitrates? The only ROONO2 reactions I am aware of are reversible formation of RO2 + NO2 and decomposition to R(O) + HNO3. It seems more likely that the additional nitrates observed in the excess N2O5 experiments 1 and 3 are formed by reactions of alcohols with N2O5: ROH + N2O5 –> RONO2 + HNO3, which is a well-known reaction that is used to synthesize organic nitrates from the corresponding alcohols.
- Line 255: Because the RO2 + RO2 and RO2 + NO3 reactions both lead to the same alkoxy radicals, and these can go on to form monomers that then form dimers in particles, an alternative explanation for the similarity in SOA dimer composition in the two radical regimes is that most of the dimers are formed in the particles and that gas-phase dimers are minor. Since these MS methods are not quantitative, it is not possible to draw conclusions on the importance of gas-phase dimers.
- Line 329: Since the EESI is not calibrated, how can you measure a mass flux?
- Since neither the EESI-TOF or the FIGAERO-CIMS signals have been calibrated, the authors cannot assume that all compounds have the same sensitivity. This makes it difficult to draw conclusions from the changes observed in MS signals over time. For example, if reversible (non-oxidative) monomer exchange reactions were occurring in the particles to form dimers with different structures and detection sensitivities, then this could appear as oxidation when it is not. One can imagine a variety of such scenarios that confuse an interpretation of the MS observations.
Technical Comments
- Please define ag s–1.
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AC2: 'Reply on RC2', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC2-supplement.pdf
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EC1: 'Comment on acp-2021-379', Nga Lee Ng, 07 Jul 2021
I have a few quick comments for the authors to consider:
- It appears that Experiments #1 and #3 were conducted under conditions that were almost identical (Table 1), why is the maximum SOA different by a factor of ~3? Same question applies to the data shown in the figures, how shall the results from Experiments #1 and #3 be explained and interpreted? Shall one expect the results to be comparable or different?
- Line 42-45: For the sentence “When experiments were conducted in the dark….”, it might be better to separate this sentence into two to avoid confusion, as Takeuchi and Ng did not use the changes in elemental ratios (N:C and O:C) to evaluate organic nitrate hydrolysis (NO3,org/Org is used as a proxy to infer hydrolysis in Takeuchi and Ng).
- Line 208-211: As the experimental conditions in the study by Takeuchi and Ng and the study by Claflin and Ziemann are different (e.g., RO2+NO3 dominant, low OA loading at ~60 ug/m3 and RO2+RO2 dominant, high OA loading on the order of hundreds of ug/m3, respectively), it is hard to directly use these studies to note that similar differences were observed between ESI and FIGAERO-CIMS in this work.
- Line 352-353: As seed aerosols are not used in this work, perhaps the relative small amount of particle water could be the reason for negligible hydrolysis observed for dimer dinitrates? Also, it would be useful to note in Section 2.1 that seed aerosols are not used.
-
AC3: 'Reply on EC1', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC3-supplement.pdf
Status: closed
-
RC1: 'Comment on acp-2021-379', Anonymous Referee #1, 05 Jun 2021
-
AC1: 'Reply on RC1', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC1-supplement.pdf
-
AC1: 'Reply on RC1', David Bell, 01 Oct 2021
-
RC2: 'Comment on acp-2021-379', Anonymous Referee #2, 12 Jun 2021
General Comments
This manuscript (which is a companion paper to one submitted by Wu et al.) describes results of a laboratory study of the effect of aging in the dark on the mass and composition of SOA formed from the reaction of NO3 radicals with a-pinene for a few different concentrations of a-pinene and N2O5, which was the source of NO3 radicals. Experiments were conducted in a Teflon chamber, SOA mass and size were monitored with an SMPS, and gas and particle composition were monitored with a FIGAERO-CIMS and EESI-TOF. The observations are thoroughly discussed, and various possible explanations, such as evaporation, oxidation, and monomer-dimer reactions are proposed. In general, however, given the complexity of the system, the lack of information on the molecular structures of the SOA components (only elemental formulas are available), and the non-quantitative MS analyses, it was not possible to draw convincing conclusions about the physical or chemical processes that might have altered the SOA in the dark. Nonetheless, the data set is interesting, and future studies may provide more detailed data that can help to explain the results. I think the manuscript can be published after the following comments are addressed.
Specific Comments
- Line 205: The reaction RO2 + NO3 forms RO + NO2 + O2, not peroxynitrates (ROONO2). I assume you meant RO2 + NO2 –> ROONO2.
- How do you propose that peroxynitrates are converted to nitrates? The only ROONO2 reactions I am aware of are reversible formation of RO2 + NO2 and decomposition to R(O) + HNO3. It seems more likely that the additional nitrates observed in the excess N2O5 experiments 1 and 3 are formed by reactions of alcohols with N2O5: ROH + N2O5 –> RONO2 + HNO3, which is a well-known reaction that is used to synthesize organic nitrates from the corresponding alcohols.
- Line 255: Because the RO2 + RO2 and RO2 + NO3 reactions both lead to the same alkoxy radicals, and these can go on to form monomers that then form dimers in particles, an alternative explanation for the similarity in SOA dimer composition in the two radical regimes is that most of the dimers are formed in the particles and that gas-phase dimers are minor. Since these MS methods are not quantitative, it is not possible to draw conclusions on the importance of gas-phase dimers.
- Line 329: Since the EESI is not calibrated, how can you measure a mass flux?
- Since neither the EESI-TOF or the FIGAERO-CIMS signals have been calibrated, the authors cannot assume that all compounds have the same sensitivity. This makes it difficult to draw conclusions from the changes observed in MS signals over time. For example, if reversible (non-oxidative) monomer exchange reactions were occurring in the particles to form dimers with different structures and detection sensitivities, then this could appear as oxidation when it is not. One can imagine a variety of such scenarios that confuse an interpretation of the MS observations.
Technical Comments
- Please define ag s–1.
-
AC2: 'Reply on RC2', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC2-supplement.pdf
-
EC1: 'Comment on acp-2021-379', Nga Lee Ng, 07 Jul 2021
I have a few quick comments for the authors to consider:
- It appears that Experiments #1 and #3 were conducted under conditions that were almost identical (Table 1), why is the maximum SOA different by a factor of ~3? Same question applies to the data shown in the figures, how shall the results from Experiments #1 and #3 be explained and interpreted? Shall one expect the results to be comparable or different?
- Line 42-45: For the sentence “When experiments were conducted in the dark….”, it might be better to separate this sentence into two to avoid confusion, as Takeuchi and Ng did not use the changes in elemental ratios (N:C and O:C) to evaluate organic nitrate hydrolysis (NO3,org/Org is used as a proxy to infer hydrolysis in Takeuchi and Ng).
- Line 208-211: As the experimental conditions in the study by Takeuchi and Ng and the study by Claflin and Ziemann are different (e.g., RO2+NO3 dominant, low OA loading at ~60 ug/m3 and RO2+RO2 dominant, high OA loading on the order of hundreds of ug/m3, respectively), it is hard to directly use these studies to note that similar differences were observed between ESI and FIGAERO-CIMS in this work.
- Line 352-353: As seed aerosols are not used in this work, perhaps the relative small amount of particle water could be the reason for negligible hydrolysis observed for dimer dinitrates? Also, it would be useful to note in Section 2.1 that seed aerosols are not used.
-
AC3: 'Reply on EC1', David Bell, 01 Oct 2021
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-379/acp-2021-379-AC3-supplement.pdf
David M. Bell et al.
David M. Bell et al.
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Cited
2 citations as recorded by crossref.
- Secondary organic aerosol and organic nitrogen yields from the nitrate radical (NO<sub>3</sub>) oxidation of alpha-pinene from various RO<sub>2</sub> fates K. Bates et al. 10.5194/acp-22-1467-2022
- Photolytically induced changes in composition and volatility of biogenic secondary organic aerosol from nitrate radical oxidation during night-to-day transition C. Wu et al. 10.5194/acp-21-14907-2021