We thank Referee 1 for their favorable review and useful suggestions. Below are our responses (in regular font) to the referee's comments (in italics) and details of how the manuscript has been revised (in quotes and highlighted in bold).

1. lines 22-23: what unit of MP, um?

The critical level (LC) and limit of detection (LOD), as defined in this study, are the minimum numbers of MPs that must be detected in an environmental sample to be >95% sure that false positives (LC) or both false positive and false negatives (LOD) are minimized. Therefore, the unit is number of MPs. For clarity, we revised the text as follows: 

"Based on analyses of blanks, the critical level and the limit of detection, number-based thresholds for minimizing false positives and false negatives, were 29 and 58 MPs per analyzed subsample, respectively."

2. line 120: why chose brass instead of stainless steel?

Stainless steel would also have been perfectly well-suited for the purpose, but since the Swagelok tube fitting (Figure 2) was made of brass, we used a cap of the same material. Our only requirements were that the connection be water-tight and not made of plastic.

3. did you take specific precautions to minimize contamination of glass dishes and other apperatus such as sonication/ combustion? [nvm - this is answered later in the manuscript.]

Yes, as described in subsection 3.5.3, we combusted/muffled glassware and metalware in an oven at 450°C for 4 hours, covering them in aluminium foil to prevent recontamination after muffling.

4. 3.2.2 Oxidative digestion: this is a good place to reiterate what is happening in this step, chemically speaking, to give the reader an idea of the chemistry behind the digestion.

This is a good idea. We have now added new text highlighted in bold: 

"After filtration, the 15 µm mesh was placed in a 250 mL glass beaker and underwent oxidative digestion using Fenton's reaction based on a protocol similar to the one described by Philipp et al. (2022). Briefly, 10 mL of hydrogen peroxide, 5 mL ultrapure water, 1 mL 2 mM protocatechuic acid and 1 mL 2 mM iron sulphate were added to the beaker. The beaker was placed in an incubator (Incubator 1000, Unimax 1010, Heidolph, Germany) and allowed to shake at 100 rpm at 40°C for up to three days. In this step, iron(II) acts as a catalyst to produce hydroxyl radicals from hydrogen peroxide, which can oxidize natural organic matter to produce gaseous carbon dioxide and water. The resulting suspension was finally filtered on the same 15 µm mesh.

5. Figure 4: please define what is true number in the figure caption as well.

We agree that this would be good to add. The figure caption has been rewritten as follows: 

"(a) Number of spherical red polyethylene (PE) particles identified during replicate FPA-μ-FTIR measurements compared to the true value of 44 particles observed on an optical microscopy image, (b) number of spherical blue polystyrene (PS) particles identified during replicate FPA-μ-FTIR measurements (true value: 22 particles), (c) number of spherical red PE particles identified during FPA-μ-FTIR measurements at different focal heights (true value: 76 particles), (d) number of spherical blue PS particles identified during FPA-μ-FTIR measurements at different focal heights (true value: 143 particles). In (c) and (d), "bottom": IR beam focused on the filter surface (red PE and blue PS spheres appeared blurry), "middle": IR beam focused ~50 µm above the filter surface (red PE spheres in focus, blue PS spheres slightly blurred), "top": IR beam focused ~100 microns above the filter surface (red PE spheres slightly blurred, blue PS spheres in focus)."

6. if authors are willing and the all parties involved (such as authors, institutions, funding sources...) allow for it, it would be nice to make the python based program available for research use.

Thank you for the suggestion. We agree and it was indeed our original goal to make it publicly available. However, it is a bit complicated because, due to how the software development process evolved, we now have two versions. We can certainly make them available upon request. The first version is capable of setting up the FTIR measurements (including generating subsampling areas) but only works with the Agilent Cary 670 and 610, operated using Agilent Resolutions Pro and running Windows 10 OS. The second version cannot set up measurements but can be used in the data post-processing of measurements done using the first version. The second version has image processing algorithms (e.g. detection of coloured surrogate standards) that we could make available as separate Python codes, as it does not depend on the FTIR instrument being used and only requires an optical image of an Anodisc filter as input. Long story short, the two software programs are not yet fully developed and documented, so we should not actively distribute them. We are happy to provide the software on request to anyone who is interested in working with it.

7. does figure 4 has error bars?

In (a) and (b), we show each replicate (n = 6) as its own bar. Therefore, the error across replicates is captured even though there are no error bars. In (c) and (d), we only did one measurement at each focal height, so no error bars are available.

We thank Referee 2 for their favorable review and useful suggestions. Below are our responses (in regular font) to the referee's comments (in italics) and details of how the manuscript has been revised (in quotes and highlighted in bold). 

General Comments

The manuscript reports on a novel sampling and analytical method for airborne microplastics, combined with an in-dept analysis of the associated measurement uncertainties. The whole chain from sampling airborne microplastics in dry and wet deposition to analytical methods for determining number, size distribution and mass of deposited material is described in detail and analysed rigorously for associated errors. The described method makes a substantial contribution to this emerging field of research and sets a benchmark for careful error analysis.

Overall, the manuscript is very well structured and fully aligns with the scope of AMT. It is clearly written and requires only few minor modifications before being acceptable for publication.

Thank you for the positive feedback.

SPECIFIC COMMENTS

1| Figure 1 is an excellent illustration of the analytical chain described in the manuscript. As mentioned in the text, the individual steps are described in the subsection of chapter 3. To help the reader, it might be worthwhile having a brief description of the process already added to the first paragraph of chapter 3, starting on line 75. This would help getting a quick overview over the entire analytical chain. Linking the individual components of the analytical chain, numbered in Fig. 1 from 1 to 6, to the subsections of chapter 3 would allow the reader to navigate more easily through the description of the analytical chain.

This is a great suggestion. We have now added a brief overview of the analytical chain to Chapter 3 and therein also linked steps 1 to 6 shown in Fig. 1 to the respective subsections as follows: 

"A schematic of the analytical chain – including sample collection, processing, and analysis, as well as QA/QC steps – developed in this study for the quantification of MPs in wet and dry atmospheric deposition is shown in Fig. 1. Briefly, prior to sample collection, a known number of red PE spheres is added to the respective sampling vessels, i.e. glass dish for dry deposition and aluminium filtration device for wet deposition (step 1, Fig. 1; Sect. 3.5.1). Samples are then collected in a passive sampler (step 2, Fig. 1; Sect. 3.1). After samples are collected and brought to the laboratory, a known number of blue PE spheres is added to the respective dry and wet sampling vessels (step 3, Fig. 1; Sect. 3.5.1). The samples undergo the following processing steps to isolate particles of interest (steps 4i-iii, Fig. 1; Sect. 3.2): size fractionation by vacuum filtration through a series of stainless steel meshes, oxidative digestion to destroy natural organic matter and optionally, density separation to remove heavier particles like mineral dust. The extracted particles are then filtered onto an aluminium oxide membrane (step 4iv, Fig. 1). The aluminium oxide membrane is analysed by optical microscopy (step 5, Fig. 1; Sect 3.3.1) and focal plane array µ-FTIR spectroscopy (FPA-µ-FTIR) to identify MPs (step 6, Fig. 1; Sect 3.3.2). Detailed descriptions of each step, including data interpretation, are provided in the following subsections." 

Note: we tried to add links to the subsections within the figure's caption but noticed that it disrupted the text of the figure caption and therefore decided against it.

MINOR ISSUES:

1| Please check the length of the abstract. It should not exceed 250 words.

We agree that the abstract is long but argue that it provides essential descriptions of the context and contents of the study. Also, to our knowledge, there is no word limit for the abstract in Atmospheric Measurement Techniques. We therefore suggest keeping the abstract as it is (except the revisions made in response to a comment from Referee 1).

2| Mathematical equations: the equations used in this manuscript are not fully consistent. In Eq. (1) on the volume calculation, multiplication is indicated by “´” while in all other Equations the symbol “×” is used. This might be harmonised.

Thank you for pointing this out. The multiplication symbols are now consistently "∙" instead of "×" in all equations.

3| In Table 1, it is not clear to which ensemble the percentage uncertainty per component refers to. Since the total sum is 160%, it is not clear if the fractions give the contributions of the individual components of uncertainty to the overall uncertainty. Please explain.

We agree that the legend of Table 1 might have been unclear. To make clear that Table 1 lists the individual components of uncertainty and the corresponding relative standard uncertainties (rather than their percent contribution to the total uncertainty), we changed the legend to the following: "Individual components of uncertainty of our analytical chain for the quantification of microplastics in wet and dry atmospheric deposition samples and their relative standard uncertainties. The individual standard uncertainties can be combined to calculate the total measurement uncertainty of the analytical chain (see Sect. 4.8). […] " We also modified the text before the table as follows: "Table 1 gives an overview of the determined individual components of uncertainty and their standard uncertainties, which are discussed below."

4| Figure 5 uses the metric “Circle-equivalent diameter” which is not introduced. This quantity should be explained in the text. Furthermore, is it more of an area-equivalent diameter? If so, please rename it.

"Circle-equivalent diameter” is synonymous to "area-equivalent diameter". We agree that it should be explained in the text and added the following text in subsection 3.4.1 "Microplastic particle size-to-mass estimation", where we believe this information would fit best (new text in bold):

"Measurements by FPA-μ-FTIR provide 2D projections of particles from which the length (L), width (W) and area of the particles are derived. These were calculated directly by Microplastics Finder. The area of the particle's 2D projection was used to calculate the circle-equivalent diameter, defined as the diameter of a circle with an area equivalent to the area of the particle's 2D projection. The circle-equivalent diameter was used as the primary metric for reporting particle size.  The values of L and W are defined as the dimensions of the smallest rectangle enclosing individual projected particles. However, there is no information on the third dimension, i.e. the height or thickness (H) of the particles, for which assumptions have to be made…"

As that section and Figure 5 are far apart, to aid the reader we again added a short text in Figure 5's legend describing what we meant by circle-equivalent diameter: "Number of microplastics by polymer type and size […] Circle-equivalent diameter refers to the diameter of a circle with an area equivalent to the measured area of the particle's 2D projection."

Note: we noticed a small error in the way the histograms were plotted and have now updated the figure with the correct bins.