Review of “A Dual-Droplet Approach for Measuring the Hygroscopicity of Aqueous Aerosol” by Choczynski et al.

The authors presented a linear quadrupole electrodynamic balance (LQ-EDB) method for levitation of dual droplets that can be used for accurate measurement of hygroscopicity of aerosol particles. By using NaCl and LiCl as probe droplets, the range of accurate relative humidity (RH) determination was extended compared to the previous version. The improved technique was applied to study particles containing viscous species and volatile ones, demonstrating its abilities to investigate particles of kinetic limitations and evaporative loss. The experimental setup is well designed and implemented, with great details provided. The experimental results of a few atmospherically relevant species are also clearly presented, backed by literature data and/or thermodynamic model results. The manuscript is generally well written, but I found some places difficult to follow (some examples in technical comments). I therefore recommend publication of this manuscript after Minor Revision. My comments, as shown below, are rather minor too.

Minor comments:

1. Section 3.2. It would be helpful to have a schematic or table to show how these three methods to obtain dry size differ, aiding the description here.

1. L142: Why a lower temperature in the chamber than the ambient temperature can avoid condensation in the tubing? Should it be easier to have condensation for a hotter air stream going to a colder region?

1. Is a standalone Section 5 of Conclusions necessary? Can it be merged into Section 4 as “Conclusions and Implications”?

Technical comments:

1. P3/L72: is it “sample droplet” of “probe droplet” that was used to infer water activity?

1. P5/L166: “using” to “used”?

1. P6/L180: what are “these” referring to? Same in L246.

1. P6/L186: please delete “Cotterell et al.,” in the citation. Same in L210.

1. P6/L195: ammonium sulfate is indeed one of the most important (and abundant) inorganic components. But it would be inaccurate to say that it is the most abundant salt in the atmosphere, since global estimate suggested that sea salt, which contain mostly sodium chloride, would have a higher burden (a factor of 3 – 4) than that of ammonium sulfate.

1. P7/L223: “come” to “comes”?

1. P8/L241: add “is” before “challenging”.

1. P9/L276: larger than what?

1. P9/L295-296: this sentence is a bit difficult to follow. Maybe better to present it in two sentences.

1. Figures 2 – 7: it would be good to put a “(%)” after the “RH” in the x-axis title, and use rGF as defined in the text as the y-axis title.

The authors demonstrate a dual-droplet approach for hygroscopicity measurements of micron size aerosol droplets in a linear quadrupole electrodynamic balance. One of the droplets serves a relative humidity sensor; the other is the particle under study. The authors use a spectroscopic method (Mie resonance spectroscopy) for measuring size and refractive index of the particles. They need to switch the two particles alternatively into the measuring volume by adjusting the DC voltage compensating the gravitational force. They illustrate the method by showing several examples of hygroscopicity for different aqueous model systems and show that the accuracy in RH reaches 0.5% at 50% RH and better than 0.1 % at 90% RH. This is significantly better than what can be achieved with traditional RH sensors in electrodynamic balances.

The paper is well written, the topic timely and of interest for the readers of AMT and I recommend publishing but ask the authors to take the following comments and suggestions into account.

The main weakness of the presented approach is in my opinion that the technique allows measuring the size growth factor but not directly the mass growth factor. The main advantage of the electrodynamic balance technique is that the inverse of the mass growth factor is the concentration (mass fraction of solute) of the aqueous droplet as long as only water partitions between gas and condensed phase in the experiment. As the authors explain, going from size growth to concentration requires knowledge of the density, which is typically not available for unknown samples. This makes the method less attractive for practical applications where no independent density data exist.

Even more restrictive may be any application to multicomponent particles in which one of the compounds has low solubility and hence the droplet spherical symmetry is no longer conserved. This will make sizing with Mie resonance spectroscopy difficult if not impossible. In their paper describing the setup in detail (Davies, 2019) they make use of the electrostatic force between two particles in the balance to calculate the characteristic constant of the EDB and measure density of a particle by a combination of the optical measurement and the DC voltage to compensate gravitational force. Could this be applied to the two-droplet method here? Please add a discussion.

Minor comments:

In Section 3.1. a more extended discussion on why there is an upper limit in RH measurements because of sizing problems for large particles would be helpful.

Lines 265-274: Lienhard et al. (2012) provide experimental density data and refractive index data for citric acid. May be you could compare with those? Fig. 4 would provide an opportunity.

Line 339: How well is the temperature dependence of water activity really known for the reference systems you have in mind? For NaCl there are data available, but the situation is probably not as good for LiCl, correct? For example, Lienhard et al. (2015) reported water diffusivities at temperatures as low as 210 K.

Technical comments:

Line 322: Typo TEG

Fig. 2 and Fig.3: May be you add a residual panel (E-AIM and the continuous NaCl probe data) to illustrate the agreement.

Fig. 5: I recommend using a solid line for the simulated spectra instead of the dashed one; it makes it easier to read. Typo in figure caption: use (a) instead of (A) etc.

References:

Davies, J. F., Aerosol Science and Technology, 53, 309–320, (2019).

Lienhard, D. M. et al., J. Phys. Chem. A 116, 9954-9968 (2012).

Lienhard, D. M. et al., Atmos. Chem. Phys. 15, 13599-13613 (2015).

Upon looking again on Fig. 5 of the manuscript (see stext line 285), I would like to ask the authors to comemnt on the significant differences in scatteretd intensity between measured and simulated spectra, e.g. at 670 nm in panel (a). While I agree that resonance positions seem to agree reasonable well this is definetly not true for scattered intensity. 

Please explain the clack and Blue arrow in (b).