General Comments

The study aimed to compare quantity of fungal bioaerosols measured under forest canopy in plot with normal and elevated atmospheric CO2 concentrations. Also, the correlations to meteorological conditions (wind speed, relative humidity, temperature) are assessed. The study attempts to answer important research question of effects climate change could have on bioaerosol emissions in forests. The manuscript is well written, and scientific results and conclusions are presented in a clear, concise, and well-structured way. However, there are several concerns regarding methodology.

The authors should make additional effort to assure that data collected by used optical particle counters represent bioaerosols. Notable quantity of inorganic particles should be present in the size range analysed and there is no evidence that bioaerosols dominate. A notable amount of sand particles or plant debris can be suspended in the atmosphere, especially in Autumn.

The hypotheses are focused on fungal spores, and it should be better addressed in results what is efficiency of used methodology for sampling expected diversity of fungal spores in studied environments. The authors clearly indicated that the size range chosen for the analysis (1-10 μm) is just the fraction of typical size range for fungal spores (1-30 μm). But in my view this creates more serious issue than just missing the effect of eCO2 on some specific fungi. It can result in complete miss the quantity.

Also, there is a concern that in high humidity conditions optical particle counter could detect small water droplets enhancing positive correlation between RH and particle counts.

Actually failure to validate that record from optical particle counter (PM 1-10) corresponds to total bioaerosols (if not total fungal spores) severely undermines the possibility to test research questions set by authors. Without such validation the authors have results for PM 1-10 and the relation to bioaerosols or fungal spores can only be discussed.

Specific Comments

More details about setup of instruments would be needed. For how long particle samples were taken every 60s and what is the volume of air sampled? At what height the sensors were positioned?

If high resolution wind measurements are available, I would encourage authors to check also the effect of turbulent kinetic energy on particle concentrations since the atmospheric instability could have more pronounced effect on spore dispersion than the wind speed alone.

Regarding the effects of eCO2 the authors should indicate what direct and what indirect effects are expected to increase fungal bioaerosol concentrations. It is not clear whether the CO2 increase is expected only in canopy layer or also at the ground layer where notable number of fungal spore sources could be growing. In my view eCO2 is expected to increase the vegetative mass of plants but such direct effect is not so straightforward for fungal spore sources since for many the growth and sporulation might have started only after the fumigation has ended. So if the most feasible is indirect effect through increase in amount leaf litter (as discussed) would not then be meaningful to have information about the quantity of leaf litter. This way the authors speculate twice: that the leaf litter is increased and that such increase relates to airborne fungal spore emission.

Finally, since the dispersion model indicated threshold wind conditions under which mixing between plots is neglectable I suggest looking into differences in particle concentrations after wind speeds above that threshold is eliminated.

I believe this manuscript addresses relevant and interesting scientific questions within the scope of BG. In general, the aim, the methods and the conclusions for the study are clearly presented. I believe the authors refer to relevant references. The paper presents a novel method for measurement of biological aerosol particles. I believe you could consider including the use of low-cost sensors in the title since this is an important concept of the paper.

The abstract is clear and straight-forward. I believe the references in the introduction are relevant. When reading the introduction I was curious if the type of sensors you are using have ever been used in the same way before? I’m also wondering if it can be made clearer why you hypothesize that the fungal bioaerosol concentration should increase with increasing CO2?

The methods section is extensive. I believe the confirmation done with the macro-fungi survey is important but I’m not sure you explain this survey thoroughly: Would it be possible to extend on this matter? I work with aerosols and bioaerosols but I am not familiar with this type of macro-fungi survey. Or put in a reference?

In the instrumentation section you make assumptions about the particle density and the refractive index,   how did you chose those? Do you have a reference for the choice?

Did you look into the literature how the presumed concentrations of bioaerosols that you measure compare to the concentrations measured with other more specific instruments (e.g. WIBS and UV-APS) in forests?

In general I believe you do not report on the statistical methods used extensively enough. For instance, the Loess curve you fit is only mentioned in figure captions but not in the text. Why did you fit a Loess curve? What relationships did you expect? In row 309 a linear relationship is mentioned but I find no account of how the linearity of the relationship was assessed. What statistical tests were used to assess if there is a difference or not in row 339?  When listing your conclusions, could you include significance? For instance for the decrease in bioaerosol at lower temperatures? How was this decrease measured? In scatter plots you show prediction intervals/confidence intervals: can you explain them in the text?

On row 256 you mentioned that RH was high throughout the measurement period, clarify what you mean by high?

Text in Figure 2-Figure 7 are too small. Legends are also missing and should be put in. Make sure you clearly explain the difference between colors in the plots. For Figure 2 a difference in color intensity is mentioned in the caption but I can’t detect this intensity difference, can you clarify this? For panel E and F in Figure 2, is the data hourly or daily? What is the resolution for the data displayed in Figure 3A?

At row 296-297, there is a sentence “Detecting events..”, is this a conclusion you’re making about the sensors? I’m not sure I follow the reasoning, can you extend on it?

When listing your conclusions, could you include significance? For instance for the decrease in bioaerosol at lower temperatures? How was this decrease measured btw?

Technical issues:

r. 34: Missing/redundant parenthesis bracket?

r. 93 (first row of method) add , UK. after “Staffordshire”.

Figure 1: can you possibly mark the array pairs in some way? (to make it obvious that they are pairs)

r. 145 Perhaps you would like to include a reference for the OPC Mie scattering since you are mentioning this

r. 177 reference missing

r. 177 maybe you can refer to Table 1 here so it’s clear that the OPC:s were moved around among the different arrays.

Table 1: To make the table easier to read, can possibly leave out the year and just have month and day for the date range?

Figure 4: Reference is missing here. Also check that the caption is correctly written.

Figure 5: Can these figures be adjusted to fit into one and the same frame. As they are now presented they take up a lot of space.

r. 343 reference missing.