Review of “A Novel Methodology for Assessing the Hygroscopicity of Aerosol Filter Samples” by Raparthi et al.

This study introduces a new way of quantifying hygroscopicity of aerosol particles collected on Teflon substrates. The paper is interestingly written, specifically Section 2, in such a way to take the readers on a ride to see what worked and didn’t work on their way to the final desired method that seems robust based on their analysis. The paper is important in that hygroscopicity is a critical aerosol parameter to quantify and new methods are helpful especially if they can be easily related to composition. They clearly have done a lot of tests and show that their method is effective based on comparisons with literature.

I found the paper a bit confusing at times, especially Section 2. If someone scans the abstract and conclusions it isn’t even clear what the method really is other than knowing there are Teflon filters involved. I suggest the authors try to be a bit more clear in different key parts of the paper how their method works (abstract and conclusions in particular). Section 2 is full of good details but can benefit from a bit more of a clearer picture of the method using potentially a figure (see a comment below about this). I am supportive of publication but hope the authors can address my comments below first.

Comments:

Line 34-35: for where it says “For ambient and chamber studies, HTDMA measurements provide water uptake and particle size information but not chemical composition.”, note that the next sentence claims there is a gap being filled in this study but I would disagree since this new method still just aims to quantify hygroscopicity (just like a HTDMA) and cannot additionally measure composition. I would fix this disconnect in line 34-35 and the next sentence claiming a gap is being filled. Or rather, revise the next sentence to better articulate what is the novelty of this work.

Line 36: do the authors mean “water uptake on particles…”?

Highlights: It is always dangerous to claim things like this is the “first” study to ever show something. In this regard, I am aware of other published works that quantified hygroscopicity of aerosol using Teflon substrate samples. Please revise this highlight and similar claims in the paper and also provide more credit in the literature review towards studies that used Teflon substrates to examine hygroscopicity.

Line 123: define “mfs” here instead of in line 131.

Line 133: Although not as common as the HTDMA, the authors can acknowledge the DASH-SP instrument that can go above 90% and measure sometimes near 95% (doi:10.1002/2015JD024498).

Line 136-138: at this point the reader is confused as to whether you are claiming your technique measures just water uptake properties or water uptake AND composition. The hints in the text claim it will do both and so if the method is just for water uptake properties, it would be good to be more explicit and drop the hints earlier in the text. My sense from reading the paper is that the paper focuses on just the water uptake measurement and that composition can separately be done with other techniques, which is the advantage of filters.

Line 179: should be “…Lide (2004) provided…”

Line 219: potential typo around “leaving it was….”?

Line 249: Doesn’t read well to see “from 97.5% RH”. Do the authors mean “at 97.5% RH”?

Line 311: should be “represent” and not “represents”

Line 327: “weigh” should be “weight”

Section 2 was a bit of a tour into the authors’ process to arrive at their final method and I wonder if they can benefit from some visual schematic of what their method boils down to in order to make it more visual for readers. Sometimes new instrument papers show a flow diagram and I am wondering if their method can be shown as such. I worry that Section 2 becomes a bit tedious and if the authors can make it easier to understand what their method is. The conclusion section for instance doesn’t even summarize the method leading readers in suspense – consider explaining the method briefly.

Line 464-465: please be consistent in having spaces between numbers and units. This issue occurs throughout the paper.

I wonder if the comparisons in Section 3.2 would be better presented in the form of a table.

Figure 6: This can be improved as it is hard to see the markers and there is a ton of wasted space in each panel. Consider using the space better to zoom in more on the action.

Line 541: double period

Regarding sodium chloride: does a shape factor need to be accounted for in the authors’ study?

A key aspect of the paper that is missing is the application potential of this method. I would hope the authors can share a paragraph at least in a revision about how they envision this new method can be applied on a larger scale to help the research community. For instance, can this method meant to be employed with the IMPROVE network collection of measurements in the future? If not or even if so, how else can this be applied in future research by other groups. Some examples would be helpful to provide more impact to this paper.

The manuscript was clear and relatively rigorous in the experimental setup. I like that the method does not seem costly, potentially allowing it to be applied more easily. One general comment: the authors mentioned a few times throughout the manuscript that this new method allows aerosol hygroscopicity to be related to chemical composition. It would be helpful to have a few short sentences of the chemical speciation methods that could benefit from these types of filter samples.

See below for specific comments/questions.

Line 131: you already defined "mfs" in the abstract and used it on line 123, so no need to define it again here.

Line 207: add a reference for the IMPROVE aerosol sampler (e.g., UC Davis or a previous paper that described this sampler in detail).

Line 217: what is considered "sufficient mass to produce measurable water uptake"? I would suggest the author to put something more quantitative here (e.g., at least 100 mg of each compound was collected on a Teflon filter...).

Line 219: change "was" to "to be"

Line 223-225: I suggest changing "the collected samples" to "the collected dry samples" for clarity. It was clear when I read the next section but a little confusing when I just read this sentence on line 223-225.

Line 254: is there any concern of water interacting with the dry particle-loaded filter in the chamber differently when it is in an open pouch vs. when it is not inside a pouch? Why not put the filter in the pouch after 24hrs of being inside the chamber?

Line 328-329: along the line of the previous question, could the theoretical increase in the mass of air higher than the measured values due to the RH inside the pouch being different from the RH inside the chamber? i.e., there was not an efficient amount of water entering the pouch over the 24 hours of it being in the chamber.

Line 370 Figure 5: I suggest putting error bar on the bar graphs to reflect the uncertainties in measured water mass. Also, were the measurements made at roughly the same time every day over the 5-day period? 

Line 374: "mfs" and "GF" are defined in the abstract so no need to define them here.

Line 378: mass fraction "of" solute.

Line 381-382: I think you are missing Eq. (6). There was Eq. (5) on line 317 then Eq. (7) here.

Line 411: edit MW unit to be "mol/mg" from "mol/gm"

Line 419-424: from Eq. (17) to Eq. (22), when you took partial derivatives of the (W/S) ratio for each input variable, why did you eliminate the term (MW_s/MW_w)? For example, Eq. (17) should be abs(1/ (DS-DB)) * (MW_s/MW_w).

Line 523 Figure 6: It may be nice to put the species label on the panel itself (e.g., Ammonium sulfate on panel a). I would make the markers larger and add error bars for both %RH and GF uncertainties.

Line 541: extra "."

Line 545-549: perhaps add this information to the SI and reference it at the beginning of the paper like on line 217 (see my comment above). In addition, it would be helpful if the authors have repeated the sodium chloride experiments with higher dry sample masses to confirm the hypothesis/statement on line 541: "The lower mass increases uncertainty due to the limitations in the precision of the balance).