Evaluation of Surrogate Aerosol Experiments to Predict Spreading and Removal of Virus-Laden Aerosols

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.  The characters in the figure 1, 2 and 13 are too small to read.

2.  CADR values of air purifiers per given fan stage should be given. Or, you can infer those values from the power consumption.

3.  There must be some kind numbers of ‘degree of mixing’ from this results.

4.  Is there any way to incorporate ceiling height into the given equations?

5. The focus of this paper is on the air purifier characteristics. Ventilation occurs during human occupancy, therefore considering concentration of carbon dioxide can be helpful to hold the paper title. How about changing the title?

Author Response

Dear Reviewer!

Thank you very much for your positive feedback on our article! We appreciate your efforts in reading the manuscript and welcome your comments and suggestions which we considered thoroughly. In the following, we would like to reply to your comments and explain our modifications in response to your feedback point by point:

1. The characters in the figure 1, 2 and 13 are too small to read.
   
   We agree completely! We used larger font size and optimized the quality by implementing vector graphics.
   
   
   
2. CADR values of air purifiers per given fan stage should be given. Or, you can infer those values from the power consumption.
   
   Unfortunately, the manufacturer only specifies the CADR for the maximum fan stage 3 with 380 m³/h. Data for the fan stages 1 and 2 are not available. Also, we did not test the air purifiers in a room specified according to a norm. We just applied our procedure to the minimum and maximum fan stage with measurements in the community hall and the seminar room and determined the respective CADR values which are listed in Table 2. We are sorry we did not measured the intermediate fan stage. However, we feel that this is not critical for the main findings and message of the manuscript.
   
   
   
3. There must be some kind numbers of ‘degree of mixing’ from this results.
   
   We agree, that in reality there is always some limited amount of mixing present. We compared our measurement with a model equation derived under the assumption of ideal We demonstrated in the paper, that the degree of mixing in the community hall is unexpectedly high with an almost uniform distribution of the aerosol indicated by the CPCs at very different measurement positions, suggesting almost ideal mixing in this case. We also give some explanations for this unexpected behaviour (e.g. ceiling heating). However, we also discussed the difficulties when doing this in the seminar room. During the experiment with an operating aerosol generator, no homogeneous mixing was obtained as can be seen by the discrepancies in concentration curves in Figure 4. However, with aerosol source and air purifiers switched off, after the short ventilation period the concentration curves approach, which enabled us to determine parameters for the loss coefficients of first and second order. We modified the text regarding the experiment in the seminar room in chapter 3.1. and hope this description makes this a little bit clearer. To deduce a degree of mixing for a transient case such as the seminar room in the atomization phase, CFD is needed to obtain a spatially resolved information. We mainly focused on the evaluation of the community hall as well-mixed case. However, a spatial resolution would be the next step in the consideration of aerosol spreading.
   
   
   
4. Is there any way to incorporate ceiling height into the given equations?
   
   We tried to describe the issue with implementation of height in chapter 3.3.4. following the determination of a theoretical aggregation coefficient. Thank you for the hint that this was not pointed out properly. We modified the text to point out that implementation of height is possible only for sedimentation loss. However, other losses linearly depending on concentration in general cannot be separated experimentally from sedimentation. Therefore, it is unfortunately not possible due scale the linear loss term linear loss term with ceiling height.
   
   
   
5. The focus of this paper is on the air purifier characteristics. Ventilation occurs during human occupancy, therefore considering concentration of carbon dioxide can be helpful to hold the paper title. How about changing the title?
   
   In the introduction we discussed different approaches to assess virus spreading and removal in indoor spaces. In this context we also discussed the assessment of carbon dioxide for this purpose which has been used in a number of studies in the past. We also tried to discuss the weaknesses of this approach and consequently followed in our paper the alternative approach of using surrogate aerosol particles. Therefore, we feel it would not be appropriate to change the title in order to include CO2 there too. We are sorry, that this was not clearly enough formulated so far. We therefore modified this section in order to make this point clearer.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, Janis Beimdiek and Hans-Joachim Schmid summarize results from surrogate experiments performed to predict spreading and removal of virus-laden aerosols. The methods used are sound, novel and well explained. The results are well presented and the findings are very relevant to better understand/model the spread of virus laden aerosols. The paper can be accepted after the following comments are addressed. 

(1) Please increase the size of text on figure 2 and 13

(2) The y axis label on figure 13 is not clear

Author Response

Dear Reviewer!

Thank you very much for your positive feedback on our article! We appreciate your efforts in reading the manuscript and are very happy that you enjoyed reading it!

Your remarks have been considered in the following way:

(1) Please increase the size of text on figure 2 and 13

We agree completely! We used larger font size and optimized the quality by implementing vector graphics.

(2) The y axis label on figure 13 is not clear

We hope the label is now easily readable and also understandable in combination with the added explanation in the figure captions (reference to corresponding equation).

Reviewer 3 Report

Comments and Suggestions for Authors

1. Original Submission

1.1. Recommendation

Accept

 Title: Evaluation of surrogate aerosol experiments to predict spreading and removal of virus-laden aerosols

  Overview and general recommendation:

The study of virus-containing aerosols is a very important field in health research on the recently spreading diseases such as Covic-19 and SARS. Recently, there has been a public health crisis due to the exposure of several viruses internationally, and many studies are being conducted on the spread of these viruses through aerosols. Therefore, research on the spread and elimination of viruses using surrogate aerosols is a very important study in public health and may attract the attention of many readers.

In this paper, the authors studied the diffusion and removal of aerosols in large community halls and seminar rooms using surrogate aerosols, and approached the model equation through this. Overall, this paper is well organized in the order of hypothesis, experiment, and model equation, and the introduction, necessity, and conclusion of the paper are clearly drawn. As a result, the research in this paper is deemed suitable for publication in atmosphere in the field of air pollution and can be published without any modifications.

Author Response

Dear Reviewer!

Thank you very much for your positive feedback on our article! We appreciate your efforts in reading the manuscript and are very happy that you enjoyed reading it!