Assessment of Ventilation Efficiency in School Classrooms Based on Indoor–Outdoor Particulate Matter and Carbon Dioxide Measurements

Round 1

Reviewer 1 Report

            Measuring air exchange rates (AERs) in  real-time, easy and low-cost way is very important, especially when currently attention is paid to indoor air quality. Two monitoring methods  using continuous CO2 sensors and particulate matter (PM10) as a tracer   were validated through both empirical models and test studies (10 classrooms in different schools).  AERs were determined using CO2  decay method and the exponential transit time distribution function for PM data.  In spite of limitations, these two methods provided sufficient continuous data and  demonstrated an effective way to measure AERs in various indoor environments.

            However, in my opinion, the manuscript has some  editorial  ambiguity  and the  presentation causes some confusion.   I am giving below mostly specific comments.

Table 1 - I believe that for  further  analysis, it  should be stated how many children were in  classes and what type of ventilation system was used (Buildings are typically ventilated using three mechanisms: mechanical ventilation, natural ventilation and infiltration).

l.72-77 - please clearly state, what initial concentration for the CO2 decay  curve was determined, when (after children left the rooms ?) and how long it was recorded. I understand  that for further analysis only cases with closed windows and doors were considered.  

l. 78/79 - I think, it was used simply the CO2 decay function (method).

l.80 – Fig. 1  shows that it would be better to use the method of non-linear regression instead of linear. Please explain.

l.83 - How to understand “ the theoretical CO2 mixing ratio .... - it is taken as the  background concentration (ambient air).

l. 105-106 - I am not convinced that this is a box model but rather the transit time distribution function.  Bibliography items:  [14] -  please provide pages,  [15] - does not apply to this issue.

l. 118 – “filtration factor” - please clarify the definition. For the relationship between indoor and outdoor particle concentration, three widely parameters are used: i.e. indoor/outdoor (I/O) ratio, penetration factor and infiltration factor but not filtration factor.

l. 157 – Comparing with natural ventilation, low air exchange rate indicates that infiltration mechanism becomes the primary pathway for outdoor air and particles entering the classrooms. Wind action and temperature gradient, both proved to be significant factors influencing the amount of air change through joints and cracks in buildings. Was this aspect  taken into account when discussing the high variability of AERs? If not, please pay attention to it.

l.159/160 – please clarify these” specific conditions”

Fig. 1 – please explain what could happen between  Nov. 10 and 12, and Nov. 18.

l.208-212 - this argument is not entirely clear, but it results from the lack of meaning “r”. In my opinion, in the case of the air purification system, the proposed method is not fully justified.

Author Response

Dear reviewer, thank you very much for your comments and guidance. They will certainly significantly improve the quality of our text. Most of the comments have been included in the text. Below you will find detailed answers.

Table 1 - I believe that for  further  analysis, it  should be stated how many children were in  classes and what type of ventilation system was used (Buildings are typically ventilated using three mechanisms: mechanical ventilation, natural ventilation and infiltration):

Table 1 provides information on the number of students in each classroom, and specifies the type of ventilation at all locations. As mentioned in the text, the ratio of the nominal number of students to the size of classrooms was very similar. This led to a very small variation in the AERs values required by law. Typically, for older buildings in Poland, natural (gravity) ventilation prevails in most schools. It is also worth to underline that our study is not focused on indoor bad air quality due to the particulate emission inside the building but coming from the external bad air quality conditions via ventilation systems.

l.72-77 - please clearly state, what initial concentration for the CO2 decay  curve was determined, when (after children left the rooms ?) and how long it was recorded. I understand  that for further analysis only cases with closed windows and doors were considered:

In the text, we have declared, “The developed CO2 analysers were deployed in the classrooms. The mixing ratios were evaluated using a parallel record of the indoor air humidity and temperature. Only the periods, when there was no possible CO2 production inside and the initial concentrations were high enough (approximately over 1000 ppm), were taken into account. Based on temperature and humidity indications, we analysed whether the windows in the rooms were opened (e.g. for ventilation) during the selected periods.” Certainly, these periods were excluded from the analyses.

l. 78/79 - I think, it was used simply the CO2 decay function (method):

In fact the simple box model is a numeric application of a decay function assuming that there is a perfect mixing of the air inside the classroom.

l.80 – Fig. 1  shows that it would be better to use the method of non-linear regression instead of linear. Please explain:

It was a mistake in our manuscript. A non-linear regression function was used. Certainly, this type of functional adjustment cannot be realized by the linearization of function.

l.83 - How to understand “ the theoretical CO2 mixing ratio .... - it is taken as the  background concentration (ambient air):

Because the outdoor CO2 mixing ratio was not measured in this study, the value that indoor CO2 is approaching to was calculated by fitting function and was not important parameter of the model. Additional explanation was added in the text.

l. 105-106 - I am not convinced that this is a box model but rather the transit time distribution function.  Bibliography items:  [14] -  please provide pages,  [15] - does not apply to this issue:

Typically in tracer methods, this approach using the transition and weave function is called box models. Cited literature was corrected, there has been the mistake with numeration of reference positions.

l. 118 – “filtration factor” - please clarify the definition. For the relationship between indoor and outdoor particle concentration, three widely parameters are used: i.e. indoor/outdoor (I/O) ratio, penetration factor and infiltration factor but not filtration factor:

Thank you very much for your valuable advice. In this case, the penetration factor according to definition by (Chen 2011) seems to be the most correct. The text was changed accordingly.

l. 157 – Comparing with natural ventilation, low air exchange rate indicates that infiltration mechanism becomes the primary pathway for outdoor air and particles entering the classrooms. Wind action and temperature gradient, both proved to be significant factors influencing the amount of air change through joints and cracks in buildings. Was this aspect  taken into account when discussing the high variability of AERs? If not, please pay attention to it:

Unfortunately, no additional meteorological analyses were made. We fully agree that wind and pressure can have a direct impact on natural ventilation. However, it can be assumed that during each two-week campaign, the meteorological conditions will be variable and the obtained parameters will represent average values of the ventilation parameters during the experiment. Especially in the PM-based method, which takes into account the whole range of measurements at the same time. In our opinion, the differences between locations will be significantly larger than those resulting from changes in meteorological parameters.

l.159/160 – please clarify these” specific conditions”:

It was about conditions with the initial concentration generated. Additional explanation was added in the text.

Fig. 1 – please explain what could happen between  Nov. 10 and 12, and Nov. 18:

Additional explanation was added in the text.

l.208-212 - this argument is not entirely clear, but it results from the lack of meaning “r”. In my opinion, in the case of the air purification system, the proposed method is not fully justified:

We agree that in the original text, parameter r was vaguely defined. However, in the case of the analyses of the locations, equipped with an air purification systems, the efficiency of the purification process can be easily defined. In case of Poland, we often encounter excessive concentrations of outside PM concentration. The only thing you would expect from an air purification system is to keep the PM concentration values below the limit described in the official regulations, compensating the increasing PM level during the ventilation time. This was not a case for the studied classrooms.

Reviewer 2 Report

The methods and results in this article are very well described, but the time scale of the measurements can be considered too small for this type of research and the novelty of this endeavor is unclear.

Suggestions:

• Clearly state the novelty of your work and rebuild the article so it is the centerpiece of the presented paper.
• Reason how is the timescale of the measurements sufficient.
• Introduction could be more detailed.

Author Response

Thank you for appreciation of the transparency of the methodology description and your valuable comments and guidance. They certainly significantly improved the quality of our text. The analysis of the impact of timescale on obtained results was performed in the description of AERs uncertainty by the iterative method. No significant influence of the duration of the analysis on the values of the obtained coefficients was shown for timescales longer than 9 days. A suitable comment has been added to the text.

Clearly state the novelty of your work and rebuild the article so it is the centerpiece of the presented paper:

Thank You for this comment. We stated the novelty of the method in the introduction, method presentation and conclusion sections.

Introduction could be more detailed:

We have revised the manuscript and unclear parts of the work were clarified both in the introduction and in the methodology sections according to more detailed review of the other reviewer.

Round 2

Reviewer 2 Report

After the revision is paper is suitable for publication.