the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Chemical composition of secondary organic aerosol particles formed from mixtures of anthropogenic and biogenic precursors
- 1School of Earth and Environmental Science, University of Manchester, Manchester, M13, 9PL, UK
- 2National Centre for Atmospheric Science
- 3Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO105DD, UK
- anow at: Department of Life and Environmental Sciences, Bournemouth University, Dorset, BH12 5BB, UK
- bnow at: Environment & Sustainability Center, Qatar Environment & Energy Research Institute, Doha, Qatar
- 1School of Earth and Environmental Science, University of Manchester, Manchester, M13, 9PL, UK
- 2National Centre for Atmospheric Science
- 3Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO105DD, UK
- anow at: Department of Life and Environmental Sciences, Bournemouth University, Dorset, BH12 5BB, UK
- bnow at: Environment & Sustainability Center, Qatar Environment & Energy Research Institute, Doha, Qatar
Abstract. A series of experiments were designed and conducted in the Manchester Aerosol Chamber (MAC) to study the photooxidation of single and mixed biogenic (isoprene and α-pinene) and anthropogenic (o-cresol) precursors in the presence of NOx and ammonium sulphate seed particles. Several online techniques (HR-TOF-AMS, Semi-Continuous GC-MS, NOx and O3 analyser) were coupled to the MAC to monitor the gas and particle mass concentrations. Secondary Organic Aerosol (SOA) particles were collected onto a quartz fibre filter at the end of each experiment and analysed using liquid chromatography ultra-high resolution mass spectrometry (LC-Orbitrap MS). The SOA particle chemical composition in single and mixed precursor systems was investigated using non-targeted accurate mass analysis of measurements in both negative and positive ionization modes, significantly reducing data complexity and analysis time, providing an more complete assessment of the chemical composition. This non-targeted analysis is not widely used in environmental science and never previously in atmospheric simulation chamber studies. Products from α-pinene were found to dominate the binary mixed α-pinene / isoprene system in terms of signal contributed and the number of particle components detected. Isoprene photooxidation was found to generate negligible SOA particle mass under the investigated experimental conditions and isoprene-derived products made a negligible contribution to particle composition in the α-pinene / isoprene system. No compounds uniquely found in this system contributed sufficiently to be reliably considered as a tracer compound for the mixture. Methyl-nitrocatechol isomers (C7H7NO4) and methyl-nitrophenol (C7H7NO3) from o-cresol oxidation made dominant contributions to the SOA particle composition in both the o-cresol / isoprene and o-cresol / α-pinene binary systems in negative ionization mode. In contrast, interactions in the oxidation mechanisms led to the formation of compounds uniquely found in the mixed o-cresol containing binary systems in positive ionization mode. C9H11NO and C8H8O10 made large signal contributions in the o-cresol / isoprene binary system. The SOA molecular composition in the o-cresol / α-pinene system in positive ionization mode is mainly driven by the large molecular weight compounds (e.g. C20H31NO4, and C20H30O3) uniquely found in the mixture. The SOA particle chemical composition formed in the ternary system is more complex. The molecular composition and signal abundance are both markedly similar to those in the single α-pinene system in positive ionization mode, with major contributions from o-cresol products in negative ionization mode.
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Yunqi Shao et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-127', Anonymous Referee #1, 14 Mar 2022
It is interesting to determine the chemical composition and interactions during SOA formation in mixed VOC systems (photooxidation of α-pinene, isoprene, o-cresol and their binary and ternary mixtures in the presence of NOx and ammonium sulphate seed particles) by using non-targeted LC-Orbitrap MS. The method is innovative. But more detailed information about the methods can be provided.
Introduction:
- What are the pros and cons of using non-targeted LC-Orbitrap MS analysis for data interpretation can be addressed?
Method:
- There are lots of anthropogenic VOC precursors, why o-cresol was chosen as an anthropogenic precursor in this study?
- Humidity and temperature are important factors for SOA formation, they are controlled by the humidifier and by controlling the air conditioning during the experiment. These parameters should be added in the manuscript.
- Why was the mass concentration of seed particle doubled in single isoprene experiment?
- How many repeated experiments performed in each experiment type?
- Before filter sampling, any denuder was used to remove VOCs, NOx and oxidants?
Results and discussion:
- Online data from gas chromatography mass spectrometer (GCMS), condensation particles counter, differential mobility particle sizer (DMPS) and aerosol mass spectrometer (AMS) are very useful for data interpretation. But the results were not reported in this study.
- Lots of data were presented in this study, (e.g. number of detected SOA compounds, molecular composition, compositional analysis). The novel part of this study is about the unique-to-mixture products due to the interactions between VOC products. This section can be extended and provide more mechanistic understanding of their formation.
- AC1: 'Reply on RC1', Yunqi Shao, 27 May 2022
- AC3: 'Reply on RC1', Yunqi Shao, 27 May 2022
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RC2: 'Comment on acp-2022-127', Anonymous Referee #2, 01 Apr 2022
General Comments:
This work by Shao et al. is a follow up to the work by McFiggins et al. in Nature 2019 on the impacts of mixed VOC systems on SOA formation. They performed a series of batch mode chamber experiments with single and mixed precursors of biogenic and anthropogenic origin in the presence of NOx and aerosol seed. Offline analysis of the SOA composition was performed primarily with LC-MS to elucidate which species dominate and dictate the SOA formation in mixtures and identify any cross products. This work is novel and of value to the community, although I find it to be overly verbose and rambling and suggest editing to make it more concise and flow better if possible. This is appropriate for ACP after addressing the other suggestions below.
Specific Comments:
Line 134: I think this is well established and suggest re-wording “might be the reason” to something more definitive
Line 207: What is the residence time in the chamber?
Section 2.4.1: Is it possible for chemical transformations to occur during the 2 hr ambient temperature rest, sonication, or drying? Would this be observable? Can you comment on how this may impact results?
Line 304: Where would the sodium and potassium come from?
Figure 2: Are these common molecular structures or molecular composition?
Figure 4: Why does essentially all the signal contain nitrogen for cresol and any mixtures with cresol? This is discussed ~line 520 but not the reasoning for why N-containing species are highly dominant.
Line 467: Suggest using HOM definition from Bianchi et al (https://pubs.acs.org/doi/10.1021/acs.chemrev.8b00395): highly oxygenated organic molecules
Lines 466-472: This section on HOM is not well fleshed out and doesn’t seem to flow with the discussion. Suggest removing or re-writing. Please add a reference for this sentence, or remove: “Autoxidation may therefore contribute to CHO products with carbon numbers 16 – 20 in α-pinene oxidation”
Line 477: Please state how much SOA was formed. It is confusing that this line (and above) states ~0 µg/m3 was formed but the section goes on to discuss the compounds measured in the particle phase
Lines 493-494: This doesn’t reflect the current state of knowledge and is an insufficient explanation/discussion. Several recent studies have shown that small particles are detected in SOA as a result of decomposition, typically via thermal processes, during analysis. While this work doesn’t utilize heating techniques, it does involve substantial sample prep (see comment on section 2.4.1).
Lines 496-498: I’m confused why the experiment would be designed in a way that is well documented to not make SOA when the stated point of this work is to make SOA and measure the particle phase composition? Please explain the reasoning for this experimental design and how this advances our understanding of multi-component SOA formation.
Line 501: Can you be sure these species are created from isoprene + OH and not impurities in your isoprene source or chamber contamination?
Line 679: Here you mention the possibility of fragmentation of larger species resulting in the smaller species measured in the particle phase. Please include references (e.g. https://pubs.acs.org/doi/abs/10.1021/acs.est.5b04769).
Line 765: It isn’t clear to me that accretion reactions have occurred during SOA formation rather than alterations during sample prep and analysis. Additionally, if they did occur during the experiment, can you be sure that accretion products would still form under atmospherically relevant precursor and SOA concentrations?
Technical:
Throughout manuscript: NOx should have a subscript “x” and be NOx
Throughout manuscript: change instances of “ml” to “mL”
Throughout manuscript: change instances of “ug” to “µg”
Line 222: particles counter à particle counter (plural to singular)
- AC2: 'Reply on RC2', Yunqi Shao, 27 May 2022
- AC4: 'Reply on RC2', Yunqi Shao, 27 May 2022
Yunqi Shao et al.
Yunqi Shao et al.
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