the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2024
Surface equilibrium vapor pressure of organic nanoparticles measured from the Dynamic-aerosol-size Electrical Mobility Spectrometer
Abstract. Aerosol particles undergo continuous changes in their chemical composition and physical properties throughout their lifecycles, leading to diverse climate and health impacts. In particular, organic nanoparticle’s surface equilibrium vapor pressure stands as a critical factor for gas-particle partitioning and is pivotal for understanding the evolution of aerosol properties. Herein, we present measurements of evaporation kinetics and surface equilibrium vapor pressures of a wide array of laboratory-generated organic nanoparticles, employing the Dynamic-aerosol-size Electrical Mobility Spectrometer (DEMS) methodology, a recent advancement in aerosol process characterization. The DEMS methodology is founded on the principle that the local velocity of a size-changing nanoparticle within a flow field has a one-to-one correspondence with its local size. Consequently, this approach can facilitate the in situ probing of rapid aerosol size-changing processes, by analyzing the trajectories of size-changing nanoparticles within the classification region of a differential mobility analyzer (DMA). We employ DEMS with a tandem DMA setup, where a heated sheath flow in the second DMA initiates particle evaporation in its classification region. Through analysis of the DEMS response and the underlying mechanism governing the evaporation process, we reconstruct temporal radius profiles of evaporating nanoparticles and derive their surface equilibrium vapor pressures across various temperatures. Our results demonstrate a good agreement between the vapor pressures deduced from DEMS measurements and those documented in literature. We discuss the measurable vapor pressure range achievable with DEMS and elucidate associated uncertainties. Furthermore, we outline prospective directions for refining this methodology and anticipate its potential to contribute to the characterization of aerosol-related kinetic processes with currently unknown mechanisms.
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RC1: 'Comment on egusphere-2024-610', Anonymous Referee #1, 06 Apr 2024
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This paper develops a modified tandem DMA method to examine particle vapor pressures. Somewhat uniquely, size changes within a DMA are used to infer evaporation rates, and hence vapor pressures. Though the idea to use a DMA as a “reaction cell” of controlled residence time is not new, I think this is a novel contribution to the literature and I only have several minor suggestions and questions for the authors.
Comments:
1. The authors are essentially exploiting the fact the DMA is a controlled residence time instrument, with the residence time determined by the ratio of the sheath velocity to DMA length, and the resolution on the residence time equivalent to the DMA resolution. I think it is fine if the authors want to name the use of a DMA to monitor reaction kinetics through controlled residence time the “Dynamic-aerosol-size Electrical Mobility Spectrometer.” However, I think they should note that there are other studies that have used this technique previously. I believe the most correct work from the Hogan group the authors should reference is:
Li, Chenxi, and Christopher J. Hogan Jr. "Direct observation of C 60− nano-ion gas phase ozonation via ion mobility-mass spectrometry." Physical Chemistry Chemical Physics 21.20 (2019): 10470-10476
This work uses the DMA to monitor reaction kinetics (the cited references are all focused on equilibrium vapor uptake. Fernandez de la Mora also utilized a nearly identical approach to the authors to look at evaporation of clusters:
Fernandez de la Mora et al. "Measuring the kinetics of neutral pair evaporation from cluster ions of ionic liquid in the drift region of a differential mobility analyzer." The Journal of Physical Chemistry A 124.12 (2020): 2483-2496.
Not using a DMA but a drift cell, the Clowers group has used mobility controlled residence times to look at hydrogen-deutrium exchange reactions in the gas phase:
Schramm, Haley M., et al. "Evaluation of Hydrogen–Deuterium Exchange during Transient Vapor Binding of MeOD with Model Peptide Systems Angiotensin II and Bradykinin." The Journal of Physical Chemistry A 127.42 (2023): 8849-8861
Schramm, Haley M., et al. "Ion-neutral clustering alters gas-phase hydrogen–deuterium exchange rates." Physical Chemistry Chemical Physics 25.6 (2023): 4959-4968.
2. Equation (7). There are other published equations for the transition regime condensation/evaporation equation provided in equation 7. How sensitive are results to this equation, in comparison to the equation of Dahneke or Gopalakrishnan & Hogan? Also, how is the vapor mean free path explicitly defined for this equation? Different references commonly give different definitions of this parameter (it needs to be proportional to the vapor diffusion coefficient divided by the vapor mean thermal speed, but the proportionality coefficient depends on the theory).
3. Table 2. Could the authors comment further on the disparity between the vapor pressures determined for glycerol and the literature values? Unlike the PEGs, the glycerol vapor pressure appears to be much lower by DEMS than reported elsewhere. The PEG4 DEMS results are also lower in vapor pressure than Kreiger et al.
Citation: https://doi.org/10.5194/egusphere-2024-610-RC1 -
RC2: 'Comment on egusphere-2024-610', Anonymous Referee #2, 19 Apr 2024
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The authors present an application of the recently developed DEMS methodology in the aerosol evaporation kinetics measurement. Evaporation of laboratory-generated single-component organic particles, including Glycerol, PEG4, PEG5, PEG6, and Dibutyl sebacate were measured and investigated across temperatures ranging from 295 to 343 K. The vapor pressures deduced from DEMS measurements are generally in good agreement with literature values, demonstrating that the DEMS is capable of characterizing aerosol evaporation kinetics. This manuscript is well-written and fits the scope of Atmospheric Measurement Techniques. The reviewer recommends accepting this manuscript after addressing the following minor comments.
- “DEMS is designed to conduct in-situ measurements of rapidly evolving aerosol systems” is slightly ambiguous here. The DEMS is indeed an advanced technique for probing the aerosol kinetic processes (e.g., evaporation and/or condensation), and this process should be within the classification region. However, it is not designed to handle rapidly evolving aerosols in the atmosphere.
- To better understand the dominating factor of the uncertainties of DEMS, I recommend the authors provide the uncertainty analysis in a more quantitative way, e.g., from the simulation perspective, such that the readers can directly compare uncertainties of the vapor pressure raised from various factors. Currently, only the uncertainty of diffusion coefficient is taken into account, and it would be better to demonstrate the uncertainties in numbers (e.g., put in a table). Other potential factors, including the assumption of constant Knudsen number, Kelvin effect, and particle residence time may also be considered for this estimation. Note as particles continue to shrink to a fairly small size, Brownian diffusion may also raise uncertainties, and this should be considered for future applications to nano-sized particles.
- Line #299: “values we found from” – values were found from?
- Line #323: “Fig. S4, Fig. S5: - Fig. S5, Fig. S6?
Citation: https://doi.org/10.5194/egusphere-2024-610-RC2
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