the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Acoustic levitation of pollen and visualisation of hygroscopic behaviour
Sophie A. Mills
Adam Milsom
Christian Pfrang
A. Rob MacKenzie
Abstract. Pollen are hygroscopic and so have the potential to act as cloud condensation nuclei (CCN) in the atmosphere. This could have yet uncertain implications for cloud processes and climate. Previous studies have investigated the hygroscopic swelling of pollen, linked to CCN activity by the κ-Köhler theory, using methods that follow observed mass increase by electrodynamic balance (EDB) or vapour sorption analyser. This study uses an acoustic levitator to levitate pollen grains in the true aerosol phase and uses a macroscope to image the pollen to investigate hygroscopic behaviour when relative humidity (RH) is changed. Two pollen species were studied in this work: Lilium orientalis (oriental lily) and Populus deltoides (eastern cottonwood). Both species were successfully levitated, however, the smaller Populus deltoides showed greater instability throughout experiments. The quality of images taken by the macroscope, and thus calculations of pollen area and diameter ratio, varied significantly and were sensitive to lighting conditions, as well as levitated pollen grain movement and orientation. Experiments with surface-fixed pollen grains were also conducted. They showed evidence that pollen hygroscopic swelling could be observed by the macroscope. The produced results were comparable with previously reported mass increase values. Although less accurate than methods that measure mass changes, the acoustic levitator and macroscope setup offer an attractive alternative by virtue of being commercial-off-the-shelf, low-cost, and versatile. A key advantage of this method is that it is possible to visually observe particle shape dynamics under varying environmental conditions.
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Sophie A. Mills et al.
Status: open (until 27 Jun 2023)
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RC1: 'Comment on egusphere-2023-670', Anonymous Referee #1, 23 Apr 2023
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The authors presented results of pollen hygroscopic behavior obtained from an acoustic levitation technique. From the two pollen grains study, using either deposited method or levitation method, area and diameter changes were presented, and compared with results from previous studies that were mainly done with mass measurements. The authors concluded that although the direct imaging methods might suffer from stability of the levitated pollen grains, they offer direct observation on the shape dynamics of the pollen grains during their hygroscopic growth. The technique itself is quite interesting in measuring hygroscopic behaviors of large particles such as pollen grains. The manuscript is also well written. I do, however, have a major concern on how to present the observed results, and would recommend Major Revision before publication
Main:
- The authors claimed that the main advantage of the acoustic levitation technique is that it can provide direct imaging on the shape change of the pollen grain during hygroscopic growth or shrinkage. Yet, there is little information on such an advantage, i.e., either images at different RH for the same pollen grain, or a parameter to show the shape “factor”.
- Related to the point above, in addition to the “area ratio” and “diameter ratio”, would the aspect ratio be a good parameter to show that the pollen grains are becoming more and more spherical after taking up more water, as stated in the text?
- It seems from the surface-fixed results that front lit results were clearer (Table 2 and Figure 3). How come it was not used in the acoustic levitation method later on?
Minor:
- L220-230: Fig. 2 should be Fig. 3?
- Could the authors comment on how potential lateral motion during acoustic levitation would affect the determination of area and diameter ratios?
- Figures 3-6 need further modification to font size bigger.
- Is there a way to obtain some estimates of the uncertainties for the area ratios and diameter ratios, such that readers can appreciate what changes can be understood as significant?
- How was the diameter ratio defined (i.e., normalized to what)? Why is it always less than unity?
Citation: https://doi.org/10.5194/egusphere-2023-670-RC1 -
RC2: 'Comment on egusphere-2023-670', Anonymous Referee #2, 02 Jun 2023
reply
Review of “Acoustic levitation of pollen and visualisation of hygroscopic behaviour” by Sophie Mills et al., submitted to AMT
Mills et al. present an acoustic levitation technique and the results of hygroscopic water uptake experiments of two types of pollen grains, from Oriental Lily and from Cottonwood (roughly 30 to 100 microns in diameter). The commercial acoustic levitator is coupled to a humidity-controlled air flow, and the RH used ranged from dry to 95%. While the technique and its application to primary biological particles are interesting to the atmospheric science community.
However, there are some issues with the framing of the study. More precision and substantiation are needed in the introductory sketch of the state of the science. Further, the technique is not able to measure water uptake by pollen grains with enough precision and enough repeatability for the results to be conclusive. The technique does not represent an improvement to existing methods, and therefore the publication of this technique is in doubt.
Comments on the introductory text
The claim that discussion of pollen as CCN in the literature is sustained or increasing should be more carefully supported. The importance of pollen in the atmosphere is not limited to impacts on CCN, perhaps other impacts should be emphasized. (Line 8-9, Abstract (“Pollen are hygroscopic and so have the potential to act as cloud condensation nuclei (CCN) in the atmosphere. This could have yet uncertain implications for cloud processes and climate.”), line 61-62, Introduction (“While there have been increasing discussions in the literature postulating the significance pollen may have for atmospheric cloud processes and climate, …”)).
The impact of giant pollen CCN on cloud droplet number or supersaturation should be considered in more detail by the authors before being suggested. (Lines 55-59, Introduction)
The claim that acoustic levitation has progressed significantly in recent years, and that this is a good way to study aerosol, should be substantiated by citation to recent papers (Lines 114-116).
The assumption that pollen grains can restructure when humidified should be substantiated and discussed here in the context of the findings presented, as the implications of pollen restructuring are broad (Line 250)
Comments on the technique
The accuracy of the measurements is low, as noted by the authors and as evident in the spread in the area and diameter ratios displayed in Figure 6 (see, e.g., line 247-249 (“While these results show some evidence of the hygroscopic size increase we had expected with increasing RH, they also suggest apparent inaccuracy and lack of consistency which must be considered. This variability may be due to limitations of the method in terms of imaging capabilities.”), Line 271 (“yet the change may not be considered conclusive.”); line 275 (“it would be necessary to conduct more experiment instances before making conclusive assertions from these results”); line 284 (“The results for visual size changes of the Lilium orientalis pollen while levitated are somewhat inconclusive.”); line 287 (“generally also did not show a conclusive trend.”); line 293 (“These results suggest that the measurement accuracy is hindered by the fact that the grains are being levitated freely.”); Line 305 (“It should be noted that the images for this pollen grain even among the same RH increment displayed considerable variability, implying that the observed orientation was not fixed.”); lines 306-308 (“Due to the instability of the levitated pollen grain, it was difficult to capture consistent snapshots of the grain even at constant RH. This can also be observed as the data points themselves for area and diameter ratio display considerable variance indicating a large error margin.”)).
It is assumed in the calculations that the grains are always the same distance from the camera, even though migration toward and away from the camera has been noted. The uncertainty in size due to positioning should be quantified. (Line 293-295 (“The significant but unexpected changes in observed area may be largely affected by their orientation while suspended, as there is no way to guarantee we are always looking at the same angle of the pollen grain.”)).
Comments on results
Perhaps more data should be collected. (Line 299 (“it was very difficult to collect a complete set of image data across all previous humidity increments”)).
The hygroscopicity derived from the reported results falls within the wide range for pollen reported by the literature. However, hygroscopicity is an intensive property of fully dissolved molecules. The correct term to apply in pollen studies, assuming some insoluble fraction, is the “apparent hygroscopicity.” The wet pollen grain is a mixture of soluble molecules and an insoluble part.
The figure text should be roughly the same size as the caption text after the figures are resized to their final publication-ready dimensions. As submitted, the figure fonts are about 50% as large as the figure caption font, meaning that the font size should be doubled and the plot area reduced accordingly. Some guidelines also suggest that data symbols appear similar in size to the fonts.
Citation: https://doi.org/10.5194/egusphere-2023-670-RC2
Sophie A. Mills et al.
Sophie A. Mills et al.
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