the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Aug 2024
A Novel Methodology for Assessing the Hygroscopicity of Aerosol Filter Samples
Abstract. Due to US regulations, concentrations of hygroscopic inorganic sulfate and nitrate have declined in recent years, leading to an increased importance in the hygroscopic nature of organic matter (OM). The hygroscopicity of OM is poorly characterized because only a fraction of the multitude of organic compounds in the atmosphere are readily measured and there is limited information on their hygroscopic behaviours. Hygroscopicity of aerosol is traditionally measured using Humidified Tandem Differential Mobility Analyzer (HTDMA) or Electrodynamic Balance (EDB). EDB measures water uptake by a single particle. For ambient and chamber studies, HTDMA measurements provide water uptake and particle size information but not chemical composition. To fill in this information gap, we have developed a novel methodology to assess the water uptake of particle collected on Teflon filters, thereby providing an opportunity to link the measured hygroscopicity with ambient particle composition. To test the method, hygroscopic measurements were conducted in the laboratory for ammonium sulfate, sodium chloride, glucose, and malonic acid, which were collected on 25 mm Teflon filters using an aerosol generator and sampler. Constant humidity solutions (CHS) were employed to maintain the relative humidity (RH) at approximately 84 %, 90 %, and 97 % in small chambers. Hygroscopic parameters, including the water-to-solute (W/S) ratio, molality, mass fraction solute (mfs), and growth factors (GF), were calculated from the measurements. The results obtained are consistent with those reported by the E-AIM model and previous studies utilizing HTDMA and EDB for these compounds, highlighting the accuracy of this new methodology. This new approach enables the hygroscopicity and chemical composition of individual filter samples to be assessed so that in complex mixtures such as chamber and ambient samples, the total water uptake can be parsed between the inorganic and organic components of the aerosol.
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Status: open (until 14 Sep 2024)
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RC1: 'Comment on egusphere-2024-2482', Anonymous Referee #1, 14 Aug 2024
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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.
Citation: https://doi.org/10.5194/egusphere-2024-2482-RC1
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