Referee report for revision 1 of “Design of Wide Particle Size Range Aerodynamic Inlet System with New Pre-focus Structure” by J. Huang et al, 2024
The study by Huang et al. proposes a new aerosol inlet design that is essentially a miniaturized version of the Du et al., 2023 inlet, a study that has several authors in common with the current. The scientific contribution is significant, and the subject is appropriate for AMT. This revised version (#1) is greatly improved in all areas over the original version, which had many technical and presentation problems (eg, apparently all the inlet schematics in Fig 1 of the initial submission were from the wrong design). Approximately 80% of the text and most of the figures were revised for this version.
The organization is good, and the writing is clear. The major conclusions of the paper are justified by the simulations and experiments. A possible exception is that the new optical system, which is not well described, might be inadequate to actually detect ~100% of particles entering the SPAMS system, as the authors contend. In particular, it is not clear that the particle beam is similar to or smaller than the detection laser beam widths at the point of detection, as was the case for Du et al and is necessary (even in theory) to achieve anywhere near the detection efficiency reported. Additional details of the optical system and laser beams may clear this up. See specific comments below.
I recommend publication in AMT after the authors adequately address the minor comments below.
Minor comments
It is awkward that the single-particle MS used in this study (“Bio-SPAMS”) is a new instrument but is not fully described here, nor referenced. “Bio-SPAMS used in this study is not the same as Du et al.'s HP-SPAMS, and a different laser is employed.” Can you add to that basic description? Presumably this instrument is designed to detect (large) biological particles?
Fig 1. Add units to caption. Also, I think most small TSI OPC models use a fixed 2.8 lpm flow rate, ie, much higher than Bio-SPAMS – please confirm.
Fig 1/Section 2.1. Define “virtual impactor” here, as it is not currently listed in this section. It is necessary to clarify the authors’ terminology because all injection systems employing the pressure-reduction orifice plus transverse pumpout design (common in many/all the other aerosol MS systems referenced in the paper) inherently include a virtual impactor, ie, the sample air is enhanced with larger particles compared to the pumpout air. I believe the authors mean that the addition of the “separation cone” is what defines a “virtual impactor” in this study, although technically the injection system already includes virtual impaction effect without the separation cone.
Also, list approx. pressures of all inlet components.
Also, briefly describe the 2nd pumping stage downstream of the nozzle. Are one or both of the pumping lines fixed at a constant pressure, eg, using a commercial pressure controller?
Line 120. A pressure reduction (critical) orifice is missing from this list. Also add labels in Fig 1 for separation cone and acceleration nozzle.
Line 132. “Smooth” = tapered? Please clarify.
Section 2.2. Please state whether the model considers compressible flow (which is well known to occur downstream of each critical orifice and, importantly, through which all particles pass). State in the main text what symmetry was employed.
Line 142. Do the authors mean that the pressures are chosen based on numerical simulations of this injection system from Zhang’s code? Please clarify.
Line 162. A “three-way flow splitter”?
Line 178. APS = Aerodynamic Particle Sizer (TSI, Inc).
Line 179. For this new optical system, do the particle detection beams pass through focusing or shaping optics? State the approx. laser beam waists where the particles intersect the laser beams.
Line 185. Does the TE calculation also consider the flow difference?
Line 206. Again related to the “virtual impactor”, please clarify what is actually being removed for this comparison (I think it is just the separation cone and not, eg, the entire upstream pumpout region).
Fig 3. It surprising that the addition of a separation cone (aka “virtual impactor”) has no theoretical effect on small particles (blue vs orange lines). One would expect that by adding the separation cone to enhance the inherent virtual impaction effect that is already there, the impactor’s cutpoint diameter would shift to smaller sizes. But the blue line is already at 100% for the smallest particles shown. So although line 210 is technically correct (“Our team discovered that the virtual impactor used in this study is capable of transporting 100 nm particles downstream with an efficiency of over 90 %”), the same is apparently true for the injection system without the “virtual impactor”. Consider clarifying.
Para starting line 217. Similarly, given the above interpretation of the enhanced virtual impaction effect, explain why the “virtual impactor has increased its ability to focus on large particles”. If the VI cutpoint diameter was indeed reduced as I suspect, one would expect the large particle transmission to be relatively unchanged. The simulated D50 cutpoints for the two compared designs (with and without separation cone) are easy to estimate and may help illustrate.
Lines 266-274. Most of this is repetitive with the previous paragraphs.
Fig 5. The colors are inverted. Define the color bar and add units.
Line 293. Enhancement compared to what?
Line 297-299. This is incorrect. The Du et al. 2023 system has a beam waist of 300 microns at the particle beam. Were the Du optical design used on the current system, it appears that a large fraction of the particles (having radial width ~0.5-0.6 mm, Fig 3b) would not intersect the laser beams. No details are given regarding the “improved” optical design employed here (line 179), though it is likely that new scattering laser beams are focused with converging lenses, as is done in other single-particle MS systems, such that the beam waists where particles pass through them may or may not be smaller than the simulated particle beam width. Although technically this consideration does not affect the transmission efficiencies reported here (but is highly relevant to experimental detection efficiencies in Fig 7), some context regarding the laser beam versus particle beam widths for the current system is necessary. Correct and reword as it pertains to the current system.
Fig 6. The red line is actually detection efficiency, which in addition to particle transmission also includes the efficiency of particle detection by light scattering. Also, add a caption and describe error bars.
Line 316. The use of the “dual-peak signal in Bio-SPAMS” appears to contradict the statements in lines 183-184. Also, the explanation that follows is unclear. The authors appear to imply that the lower detection efficiency for 100 nm particles is simply due to a low signal-to-noise for the scattering detection…? If so, please state.
Line 321. Delete ‘over’.
Fig 7. Update the y-axis to dN/dlogD (with units) as described in line 330. Also, since the test dust is non-spherical and has high density, please clarify “diameter”. The direct measurements from each instrument are aerodynamic diameter in the continuum regime for the APS and aerodynamic diameter in the vacuum (or near-vacuum?) regime for Bio-SPAMS. They are related but shifted a bit from one another (eg, see DeCarlo et al., 2004). No conversion is necessary for the figure.
Line 350. The beam width does not actually decrease. I believe the authors mean to say that “the radial distribution of particles in the buffer chamber exhibits an inverse correlation with particle size” (line 274). Reword.
References
DeCarlo, P. F., Slowik, J. G., Worsnop, D. R., Davidovits, P., & Jimenez, J. L. (2004). Particle Morphology and Density Characterization by Combined Mobility and Aerodynamic Diameter Measurements. Part 1: Theory. Aerosol Science and Technology, 38(12), 1185–1205. https://doi.org/10.1080/027868290903907
Du, X.; Zhuo, Z.; Li, X.; Li, X.; Li, M.; Yang, J.; Zhou, Z.; Gao, W.; Huang, Z.; Li, L. Design and Simulation of Aerosol Inlet System for Particulate Matter with a Wide Size Range. Atmosphere 2023, 14, 664. https://doi.org/10.3390/atmos14040664 |
