the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
22 Feb 2021
22 Feb 2021
Secondary organic aerosol phase behaviour in chamber photo-oxidation of mixed precursors
- 1Centre for Atmospheric Science, Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
- 2State Key Joint Laboratory of Environmental Simulation and Pollution Control, International Joint Laboratory for Regional Pollution Control, Ministry of Education (IJRC), College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
- 3Lancaster Environment Centre, Lancaster University, LA1 4YQ, UK
- 4International Joint Laboratory for Regional Pollution Control, 52425 Jülich, Germany, and Beijing 100871, China
- 5Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science and Technology, Nanjing 210044, China
- 6National Centre for Atmospheric Science, School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
- acurrently at: Exeter Climate Systems, University of Exeter, Exeter, EX4 4QE, UK
- 1Centre for Atmospheric Science, Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
- 2State Key Joint Laboratory of Environmental Simulation and Pollution Control, International Joint Laboratory for Regional Pollution Control, Ministry of Education (IJRC), College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
- 3Lancaster Environment Centre, Lancaster University, LA1 4YQ, UK
- 4International Joint Laboratory for Regional Pollution Control, 52425 Jülich, Germany, and Beijing 100871, China
- 5Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science and Technology, Nanjing 210044, China
- 6National Centre for Atmospheric Science, School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
- acurrently at: Exeter Climate Systems, University of Exeter, Exeter, EX4 4QE, UK
Abstract. The phase behaviour of aerosol particles plays a profound role in atmospheric physicochemical processes, influencing their physical and optical properties and further impacting climate and air quality. However, understanding of aerosol phase behaviour is still incomplete, especially that of multicomponent particles which contain inorganic compounds and secondary organic aerosol (SOA) from mixed volatile organic compound (VOC) precursors. We report measurements conducted in the Manchester Aerosol Chamber (MAC) to investigate the aerosol rebounding tendency, measured as bounce fraction
, as a surrogate of particle phase behaviour during SOA formation from photo-oxidation of biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) VOCs and their binary mixtures on deliquescent ammonium sulphate seed. Aerosol phase behaviour is RH and chemical composition dependent. Liquid (bounce fraction, BF < 0.2) at RH > 80 % and non-liquid behaviour (BF > 0.8) at RH < 30 % were observed, with a liquid-to-nonliquid transition with decreasing RH between 30 %~80 %. This RH-dependent phase behaviour (RHBF = 0.2, 0.5, 0.8) increased towards a maximum with increasing organic-inorganic-mass ratio (MRorg/inorg) during SOA formation evolution in all investigated VOC systems. With the use of comparable initial ammonium sulphate seed concentration, the SOA production rate of the VOC systems determines the MRorg/inorg, and consequently the change of the phase behaviour. Although less important than RH and MRorg/inorg, the SOA composition plays a second-order role, with differences in liquid-to-nonliquid transition at moderate MRorg/inorg of ~1 observed between biogenic and anthropogenic-containing VOC systems. The real atmospheric consequences of our results are that any processes changing ambient RH or MRorg/inorg will influence their particle phase behaviour. Where abundant anthropogenic VOCs contribute to SOA, compositional changes of SOA may influence phase behaviour at moderate organic mass fraction (~50 %) compared with purely biogenic SOA. Further studies are needed on more complex and realistic atmospheric mixtures.
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Yu Wang et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2021-105', Anonymous Referee #1, 20 Mar 2021
The authors presented laboratory results of phase behaviour of secondary organic aerosol (SOA) mixed with ammonium sulphate seed particles using smog-chamber experiments and particle bouncing measurements. The key novelty about this study is the usage of “iso-reactive” (see below for a question on this) single- or mixed-precursor volatile organic compounds (VOCs) from both biogenic and anthropogenic sources, while the two hypotheses (RH rules and MR_org/inorg dominates) tested are actually well established. The results confirmed the vital role of RH and the dominating effect of the fraction of organics in the aerosol particles in the changes of phase behavior, while also suggested a second-order role of SOA composition (whether it is anthropogenic or biogenic, but not very much related to O:C ratio) when MR_org/inorg is close to unity. The experiments of this study were carefully designed and data well interpreted, in support of the conclusions made. The manuscript is also fairly well written. I have a few comments for the authors to clarify, and recommend a Minor Revision before publication.
- “iso-reactive”: I assume that this term is referring to a similar reactivity towards a certain oxidant for the VOCs or mixtures chosen (Table 1). The question is, what is the dominating oxidant in the chamber experiments? OH radicals or O3? Or a mixture of them? If it is the mixture, is it a combined reactivity that makes them “iso-reactive”? Please clarify.
- About AMS results: a) the authors used O:C and H:C ratios and specific ion signals of NO^+ and NO_2^+ in their analysis, but indicated that only V-mode data were available from AMS measurements (L188). How were those data on elemental ratios and specific ion peaks obtained? Approximated from empirical formulas or high-res fitting on V-mode data? b) L192: how is this ionization efficiency defined here? If it is the number of ammonium nitrate molecules ionized per number of ammonium nitrate molecules introduced, it should be a number much lower than 1. c) 196: please specify on what is “real-time” collection efficiency.
- Some minor comments:
- P4/L88: suggest to change “even RH up to 90%” to “even with RH of up to 90%”.
- P8/L174 and L177: “don’t” to “do not”.
- P8/L181: “100 Pa s in volatility”? Should it be viscosity?
- P9/L184: add “and” before “SOA”. Also in other places including L204, L234, L235, and L244
- P11/L255: “are” to “is”, and use “>” to denote the decreasing order?
- P12/L266: “3.5h” to “3.5 h”, and a few other places in the same paragraph.
- P12/L270: add “was” before “>0.05”.
- P14/L319: “BVOC” to “BVOC-“.
- P14/L341: “need” to “needs”.
- Figures 1-3: I assume “[-]” in the axis titles means dimensionless. How about just state “dimensionless”?
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RC2: 'Comment on acp-2021-105', Anonymous Referee #2, 04 May 2021
The manuscript by Wang et al. reports particle rebound fractions (as a measure of particle phase states) measured for SOA particles produced from the photooxidation of single and binary VOC precursors in an environmental chamber. Compared to previous studies mainly focusing on organic aerosols, this study also examined the role of ammonium sulfate seed particles by varying the organic-to-inorganic mass ratio. The authors found that particle phase states to the first order depend on the RH and organic-inorganic ratio, while the VOC precursor type plays a relatively minor role. The experiments were carefully designed and executed. The paper was well written. However, I do have a few comments that need to be addressed before I can recommend publication.
- The authors claim o-cresol as a representative anthropogenic SOA precursor. The manuscript explains that the o-cresol was chosen mainly because of its OH reactivity similar to the biogenic precursors and its modest SOA yield. However, a major source of o-cresol can be biomass burning, either from manmade or natural sources. This is not discussed in the manuscript and I think it can be misleading to simply claim that o-cresol is anthropogenic.
- The presence of inorganic species can significantly alter the rebound curve, and the composition and O:C of organic species play a relatively minor role. I wonder if it is possible to develop a simple mixing role to predict the liquid-to-nonliquid phase transition? For example, is the transition RH related to liquid water content or hygroscopic growth factor?
- Line 104-105: some studies do find that alpha-pinene SOA coating can influence the deliquescence of ammonium sulfate, although the effect is relatively smaller than isoprene SOA:
https://www.tandfonline.com/doi/full/10.1080/02786826.2010.532178
https://acp.copernicus.org/articles/12/9613/2012/acp-12-9613-2012.pdf
4. Were the rebound measurements performed for monodisperse or polydisperse aerosol particles? If polydisperse, were there particles smaller than the cutoff diameter of the impactor? This information might be provided in the literature cited in this paper, but it would be nice to briefly describe it here as well.
Yu Wang et al.
Yu Wang et al.
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