Review of „Determining the ultraviolet radiation dose experienced by

aerosols using ultraviolet-sensitive dyes“ by Fu et al. 2023

General Remarks

The paper deals with the possibility of determining the UV-radiation dose on airborne materials using dye. During the study, an impactor was used to collect coloured DEHS aerosol. The use of a non-evaporating liquid allows a continious probe sampling and a uniform colour distribution for soluble dyes. The aerosols produced were analysed before and after UV-light illumination. A variety measuring instruments were used during this study, such as a SMPS, TEOM and Impactor. A spectrophotometer was used to determine dye degradation. Whilst claiming to have a feasible method for quantification, this paper offers no theory/calculation to determine quantitative results and compares inadequately with previous studies.  A more comprehensible description on the method of determining the degradation of the dye would be helpful.

This manuscript needs additional details, clarification and further explanation to be suitable for publication in AR

Specific Questions and Comments

1. Normalisation would be useful to improve comparability. This would use the power of the light source, the irradiated volume and averaged over the aerosol size. The CMD (count median diameter) and GSD (global standard deviation) of the total aerosol population or that collected by the impactor should be used. The distance between LED and central aerosol flow would also be helpful as well.

2. The study mentions that DEHS is used as UV-VIS spectrum baseline. Please also show this as a graph in the study.

3.DEHS eliminates the evaporating complexity. But water solved dye could be dried, leaving just dye behind, without chemical interactions. Elsewise; high humidity levels reduces the UV-induced inactivation capacity (Peccia et al. 2001). Please discuss this.

4. Instrumentation: Please also provide the exact specifications for the measuring devices used (Cut-Off Diameters for Impactor; Flow Rates for DMA just to name a few). The SMPS; Model 3938 includes the possibility to determine the size distribution and number concentrations by CPC (condensation particle counter) or OPC (Optical Particle Counter), are both used?.

5. The use of an impactor allows the collection of larger aerosol particles. With sufficiently large droplets, shielding would be possible, preventing the dissolved dye inside (core) the aerosol from interacting with the radiation.

6. In Introduction; add more information on possible tracer methods (fluorescence or radiation (Talaat et al. 2021))

7. Please add the chemical structures of your final chromophores (#4 and #7). Are those used in prior studies (Putt et al.)?

8. Missing data from UV-Vis spectra of aerosol droplets exposed to UV-light. Please show a graph with DEHS without UV exposure and DEHS with UV; together with dye-loaded DEHS with and without UV exposure, to show the amount of degradation. (One example, one concentration, one dye, is sufficient.

9. Missing approaches/estimations (qualitative and quantitative) between UV-lethality doses on viruses and UV-radiation to which the aerosols are exposed to. Provide quantitative information on your methods. Compare with prior studies (e.g. Putt et al. 2012).

Major Comments

Line 33-39; references to other methods that deviate from the topic are too one-sidedly negative

Line 44; Citation missing for the statement “UV radiation presents a more environmentally friendly and energy-efficient alternative to liquid disinfectants and heat disinfection for sterilizing liquids, air, and surfaces “

Line 54; Power of radiation device is given for an area. For comparability, it would also be important to know at what distance from the source to the object this applies.

Line 65; Citation needed for the statement:” experiments involving pathogenic microorganisms due to the stringent requirement of biosafety laboratory” - Statutory text or the Ordinance on Industrial Safety and Health would be appropriate here

Line 71; Citation needed for the statement:” …as been used to measure the radiation doses of UV light with a wavelength of 254 nm, serving as chemical indicators for UV sterilization processes”

Line 114; sedimentation is mentioned as an exclusion criterion. The uniform distribution within the aerosol is important here.

Line 117; Please elaborate further on the similarities between the colour and DNA/RNA

Table 1; Please add peak wavelength details

Line 140f; Please add more details of the LPI (Tube lengths; Flow rates, Distance between nozzle and impact plate; cut-off diameter)

Line 163: Please add picture of the 3d-printed object or refer to figure 2

Line 184; Please add length of the quartz tube or refer to table 2; Please add estimations of sedimentation losses

Line 190; add brief function description of the cyclone

Line 194-198: Add more details for the instrumentations (flow rates, cut-off sizes)

Line 198: Add citation for the statement : “This dilution technique did not significantly alter the particle size distribution”

Figure 4; flow rate consistency inside your figure

Line 206: Please add citation and another point of view, like: experiments with pathogenic microorganisms for aerosol studies are often simulated with smoke aerosols (Chen et al. 2021)

Line 207: with “stable aerosol droplets” is meant; with a low vapour pressure /nearly non evaporating aerosol (Liqiao et al. 2020)

Line 208; Do you mean Suspension?

Line 210; Dye concentration of 100 ug/mL; Why? Virus load comparable, comparability for later UV-VIS spectra; low noise to signal ratio?

Line 215; Table 3: Add Type of dye and colour/ absorption wavelength peak

Line 221: Baseline with DEHS: Please add a figure of the pure DEHS spectra; and error estimations for the subtraction method

Line 223; “undergo chemical reactions” observed by? Caused by (chromophore frame alterations)

Line 216; Line 225-229; Desired Absorption Range of around 260 nm; Please refer to Fig 6; AND Where is the Data for 260 nm Wavelength? UV-VIS Spectra goes hardly below 270 nm wavelength.

Line 240: change quantify to estimate; or elaborate the process with uncertainties.

Figure 6; Desired Wavelength (260nm) is not represented here. At least one picture with higher resolution at desired wavelengths.; Add a statement for the baseline subtraction in the picture description, if this is not raw data

Figure 7; Absorbance higher 1? Absorbance is only relatable between 0.2 and 0.8. ; Colors are hardly distinguishable

Line 248; “smaller particle sized aerosols” Please refer to your aimed CMD of your aerosol size distribution

Line 252: Please add information on what the mobility or stokes diameter is based on

Line 255: Please refer to fig 8 for the statement “size distribution remained relatively unchanged”

Line 269: “slight fluctuation in the number concentration is likely due to the instability of the aerosol generator” – If you refer to the “jumps” between 200 and 300 nm it is most likely, this happens due to the change of the measurement system (with internal corrections applied). It looks like the SMPS change its number concentration measurement from an OPC (from  1000-300 nm; straight line”) to a CPC(more precisely) . Please state the operation ranges and flow rates of the DMA (if those changes as well)

Line 274: Please add a citation for statement “… size can change due to evaporation during the sampling process…”

Line 279; “collected … for one hour”; Are there fluctuation in the aerosol generation? Is the Complete Aerosol size distribution used, or a fraction (DMA selected?)? If so, please describe.

Line 281; “… mass output of the aerosol generator to the collected liquid…” How was the mass output determined? Please add a statement of the 10% missing aerosol mass is from lower droplet sizes, that are not “collected” by the impactor.

Figure 10; red points does not follow the curve! Discuss; With standard curve, you mean: calculated by lambert-beer law and extinction coefficient? – If so, please add this statement in the figure description.

Line 285; “… dependence of the dye concentration in the droplet…” The deviation is most likely be caused by the presence of stray radiation within the sample, resulting in a negative deviation from Beer´2 Law. There is a text by Thomas Wenzel at libretexts.org describing the Beer´s law in detail.

Line 285; “theoretically calculated” – Please add and refer to an equation

Line 287; Can you add an estimated error for the consumption rate?

Line 295-298; Missing citation

Line 304; …”prior study” please refer or use a citation

Line 308-310; Please elaborate further and describe similarities between those studies and what conclusions can be derived from it.

Figure 11: Are this data from sampled aerosol droplets or from the dye solution within a quartz cuvette exposed to UV-light. Please add this information in the figure description. When the Power is multiplied by the amount of residue time, then the unit would be without the unit s (seconds)

Line 315; Please add information on the effective volume, or normalization on the aerosol

Line 318; How was the 10% Determined, please at Data, method and Figures for this procedure. Could this degradation be caused by dilution, shown in Figure 7? Please show a graph with DEHS without UV exposure and DEHS with UV; together with dye loaded DEHS with and without UV exposure, to show the degradation quantity.

Line 323; If Sedimentation was an issue, is there a chance to alter the experimental set-up? Longer tube, vertical stand?

Line 325; Please make it clear, in table 2, conclusion and experimental set-up description, what flow rates were actually used for later examination. State the sedimentation problem earlier in the text.

Figure 12; Please change “survival fraction” to dye integrity or similar. Make a statement about the reason of non-linear degradation

Line 327: Use citations. E.g. this Review paper: DOI: 10.1016/j.jhin.2021.05.005

Line 342; please add the size of the tube and flow rate as well.

Line 344; “feasibility of quantitively” change to “approach for qualitatively determining”

Minor Remarks

Line 41; the radiation is also capable of breaking chemical bonds. Resulting in the destruction of the genome.

Line 55; Use of non-breaking space for ²

Line 189; delete “overall”

Line 190; Briefly

Line 259; “2.0 bar”, Can you add information for the respective slm (liter per minute) information?

Figure 8: Number concentrations at the Y-Axis please in log scale.

Figure 9; Number concentrations at the Y-Axis please in log scale

Line 276; Please refer the “lab build impactor” to a figure or “as described in …”; Please refer to the Cut-Off size of the Impactor

Line 279; Dye solutions #4 and #7 with….

Line 313; Please refer to table 2

Line 320; please change survival to degradation loss or similar.

Line 341; please change survival rate to dye degradation or similar.

Abstract

“UV light-based air disinfection methods”

References

Chiappa F, Frascella B, Vigezzi GP, Moro M, Diamanti L, Gentile L, Lago P, Clementi N, Signorelli C, Mancini N, Odone A. The efficacy of ultraviolet light-emitting technology against coronaviruses: a systematic review. J Hosp Infect. 2021 Aug;114:63-78. doi: 10.1016/j.jhin.2021.05.005.

Chen B, Jia P, Han J. Role of indoor aerosols for COVID-19 viral transmission: a review. Environ Chem Lett. 2021;19(3):1953-1970. doi: 10.1007/s10311-020-01174-8. Epub 2021 Jan 13. PMID: 33462543; PMCID: PMC7805572.

Liqiao Li, Eon S. Lee, Charlene Nguyen & Yifang Zhu (2020): Effects of propylene glycol, vegetable glycerin, and nicotine on emissions and dynamics of electronic cigarette aerosols, Aerosol Science and Technology, DOI: 10.1080/02786826.2020.1771270

Jordan Peccia, Holly M. Werth, Shelly Miller & Mark Hernandez (2001) Effects of Relative Humidity on the Ultraviolet Induced Inactivation of Airborne Bacteria, Aerosol Science & Technology, 35:3, 728-740, DOI: 10.1080/02786820152546770

Putt, K. S., Kernick, E. R., Lohse, B. K., Lomboy, J., O’Brien, T., and Pugh, R. B.: The use of chromophore and fluorophore degradation to quantitate UV dose: FD&C dyes as chemical identicators for UV sterilization, Journal of Microbiological Methods, 91, 215–221, https://doi.org/10.1016/j.mimet.2012.08.015, 2012.

Talaat, K., Xi, J., Baldez, P. et al. Radiation Dosimetry of Inhaled Radioactive Aerosols: CFPD and MCNP Transport Simulations of Radionuclides in the Lung. Sci Rep 9, 17450 (2019). https://doi.org/10.1038/s41598-019-54040-1

In this manuscript, a method using UV-sensitive dye for UV dose determination is developed. Although the amount of work is much appreciated, there is very limited data analysis and discussion. The experimental design is questionable and many methods used are not well described and need clarification.

The major concern I have is the applicability and feasibility of using the UV-sensitive dye for UV dose measurement as described in the manuscript. The main challenge for measuring the UV dose exposed to pathogens in aerosols in air disinfection studies is that it is very difficult to account for physical shielding provided by the aerosol chemical constituents, which normally from saliva or nasal fluids that contains proteins and absorb UV. The study proposed using DEHS as the dye carrier but it is unclear whether DEHS can simulate the physical shielding and other potential interactions in real pathogen containing aerosols. In addition, the authors failed to demonstrate an actionable procedure using the proposed method for UV dose determination in other experimental settings.

Specific comments:

Line 42: Please consider using other references here. UV inactivation mechanisms were not investigated in these two studies.

Line 93: You mean “no observable sedimentation” ?

Line 105: Need more background on DEHS and why it has been selected for droplet generation. Any previous studies? Are there any other factors that should be considered for the carrier liquid selection, such as density, viscosity, absorbance, and potential chemical reaction with the dye?

Line 112: “solutions”.

Line 117: I don’t understand why having a similar “UV susceptibility” to nucleic acids is necessary here. You may have different relative sensitivity to DNA/RNA across wavelengths and simply having a sensitivity spectrum documented. Also, how did you determine the “UV susceptibility”? Both absorbance the quantum yield are needed to estimate the susceptibility but it seems you only have the absorbance documented.

Line 119: I don’t understand how the solubility is calculated here. From absorbance and concentration, you may calculate the molar absorptivity according to Beer’s law, no sure they are related to the solubility. Also, it is not clear why solubility should be considered for the dye selection here.

Line 141: Droplet evaporation likely occurs in the real-world. Does it affect the accuracy of UV fluence measurements using the dye?

Line 160: Is 275 nm the peak emission wavelength? Need to show the UV LED emission spectrum, especially considering LED tends to have a wide emission peak, covering up to 20 nm.

Line 164: What is the dimension of the cuvette. I’m assuming there is no mixing for the dye solution in the cuvette so the UV irradiance decreases across the solution depth, which could be problematic.

Line 165: Need more details on the UV light meter. Is it calibrated specifically for this UV LED? UV light meter normally has different sensitivity across wavelengths and often needs to be calibrated for specific UV device, especially for UV LED considering the wide emission peak. Also, what is the size the meter sensor aperture and is this the same as the size of the cuvette?

Line 181: The measurement taken here is likely not the average irradiance across the tube. Light irradiance is proportional to the inverse square of the traveling distance (inverse square law), not a linear decrease.

Line 210: Please explain why 100 ug/mL is tested here. A dye could still be considered as a good selection as long as it can provide sensitivity and measurable responses to UV irradiation at a lower concentration, even the solubility less than 100 ug/mL.

Line 218: It is not true. The 260 nm peak absorbance only applies to > 240 nm. Higher absorbance can be observed in the far UVC range (<230 nm).

Line 224: Please explain why board peaks indicate chemical reactions or polarization?

Figure 6: Recommend just showing the germicidal UV range (200 – 320 nm). The data in the visible range is not useful in this study.

Figure 7. I don’t understand why the relationship between concentration and absorbance is needed here. It is well known that the absorbance is proportional to concentration (Beer’s law).

Line 245: Please include references.

Line 295: Move to introduction.

Figure 11. Why is 2000 to 6000 mJ/cm2 selected here? This range is way too high for disinfection application, and the sensitivity at reasonable UV fluence range is unclear from this study. Also, a mathematical relationship between the UV fluence and absorbance needs to be developed here.

Figure 12: How was the “dye survival” determined, absorbance? Again, a mathematical relationship between the UV fluence and “dye survival” needs to be developed here. This relationship needs to be compared with the results in Figure 11.