the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
Particle deliquescence in a turbulent humidity field
Abstract. The atmosphere contains aerosol particles, some of which are hygroscopic in nature. These particles have direct and indirect effects on weather and climate. Furthermore, turbulence causes fluctuations in temperature, water vapor content, and relative humidity (RH). Turbulent humidity fluctuations may influence, among others, the phase state of specific hygroscopic particles. One process of particular interest in that context is particle deliquescence which is the phase transition of solid particles to solution droplets. It occurs at a certain RH, the so-called deliquescence relative humidity (DRH), which in turn depends on e.g., the particle substance. This study investigates the deliquescence behavior of sodium chloride particles in a turbulent humidity field, in particular addressing the questions whether and how turbulent relative humidity fluctuations affect the number / number fraction of deliquesced particles. The turbulent moist air wind tunnel LACIS-T (Turbulent Leipzig Aerosol Cloud Interaction Simulator) is used for this study. The results show that the number of deliquesced particles is influenced by turbulent RH fluctuations. On the one hand, particle deliquescence can be observed although the mean RH is smaller than DRH. On the other hand, there are cases for which non-deliquesced particles are present even though the mean RH is larger than DRH. In general, the number fraction of deliquesced particles depends on a combination of mean relative humidity, strength of humidity fluctuations, and residence time of the particles in the turbulent humidity field. The study concludes that relying solely on the mean relative humidity is inadequate for determining the phase state of deliquescent particle species in the atmosphere. It is necessary to additionally consider both the humidity fluctuations and the particle history.
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RC1: 'Comment on ar-2024-41', Anonymous Referee #1, 02 Feb 2025
This is an excellent paper. I have some items the authors may wish to consider, but there are no glaring errors or omissions.
The authors cite books in several places. I’ll use Seinfeld and Pandis as an example. That book is over 700 pages, and covers a wide variety of topics. Please, at least cite a chapter in the book. For example, in line 38 when you cite Seinfeld and Pandis for deliquescence, point to the paragraph or section in the book that is relevant for this.
The section on particle generation, size selection, and pre-conditioning is comprehensive, but I think there needs to be some mention of doubly charged particles, because a DMA is used for size selection. Is the size used such that you don’t have to worry about larger particles being present in the sample? Maybe having larger particles in the sample is not so important because you are considering deliquescence, not activation? A sentence explaining how the possibility of larger, doubly charged particles affect the results of the paper would be appreciated.
Line 187: I know this is picky… “About 78.000…” That’s five significant figures. I recommend “78”.
I was particularly interested in the discussion of the time for deliquescence in the appendix. I think deliquescence is a nucleated phase transition, as noted by the authors when they cite Khvorostyanov and Curry. See also Lu et al (2008) and Cantrell et al (2002). I am not convinced that the authors are seeing evidence for nucleation in their experiments though. I would expect salt particles resulting from efflorescence of an atomized solution to be defect rich, which would lower the nucleation barrier to a value that I doubt you would detect it in these experiments.
Lu, P.D., He, T. and Zhang, Y.H., 2008. Relative humidity anneal effect on hygroscopicity of aerosol particles studied by rapid‐scan FTIR‐ATR spectroscopy. Geophysical research letters, 35(20).
Cantrell, W., McCrory, C. and Ewing, G.E., 2002. Nucleated deliquescence of salt. The Journal of chemical physics, 116(5), pp.2116-2120.
Citation: https://doi.org/10.5194/ar-2024-41-RC1 -
RC2: 'Comment on ar-2024-41', Anonymous Referee #2, 11 Feb 2025
This manuscript is well-prepared and presents the cloud simulator experiment combined with Large-eddy simulation to investigates the deliquesce behavior of NaCl particles under turbulent humidity conditions. The result highlights the importance of mean RH, the strength of RH fluctuations, and the residence time of particles on the deliquescence process. While these results are generally expected, previous studies have not provided clear experimental evidence, making this study a valuable contribution that fills a critical knowledge gap. The manuscript is well organized, and the data is presented clearly. I recommend acceptance after considering my comments below, which should further improve the quality of this manuscript.
Comments:
Although the selection of Dp 400 nm particles may minimize the impact of multiply charged particles, it depends on the number size distribution of generated particles from the atomizer. A brief explanation is needed.
In the dehumidified scheme (Case 3), the particle shape may need further correction. This could also slightly affect the size distribution in Figure 3. Please refer to Biskos et al. (2006). If it is negligible, please clarify it.
Biskos, G., et al. "Nanosize effect on the hygroscopic growth factor of aerosol particles." Geophysical Research Letters 33.7 (2006).
This study fully relies on simulated RH fluctuations. While this is reasonable given instrumental limitations, I would like to see a discussion on the accuracy of these simulated fluctuations and the possible uncertainties associated with them.
The deliquescence time scale is assumed to be 10⁻⁴ s in simulation. However, in real ambient conditions, for example, for organic/inorganic mixed particles, it can reach equilibrium on the order of seconds (Duplissy et al., 2009). While inorganic particles typically deliquesce in less than 1 s, I suspect that a value of 10⁻⁴ s may be too short for realistic atmospheric conditions. Could the authors explore how different τdel values affect their results?
Duplissy, Jonathan, et al. "Intercomparison study of six HTDMAs: results and recommendations." Atmospheric Measurement Techniques 2.2 (2009): 363-378.
How do the RH fluctuations intensities in this experiment compare to real atmospheric environments? Some discussion about it could improve the scope of this study.
Minor issue:
Line 41: Do you mean that “most” experiments were performed under laminar flow conditions?
Citation: https://doi.org/10.5194/ar-2024-41-RC2
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