the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Mar 2026
Evaluation of DMSO as Working Fluid in Condensation Particle Counters
Abstract. This study presents a comprehensive laboratory and field-based evaluation of dimethyl sulfoxide (DMSO) as a non-flammable working fluid for condensation particle counters (CPCs), directly compared to a butanol-operated counterpart across a wide range of pressures, temperatures, and aerosol types. Modifications to the instrument’s automatic refilling system ensured reliable operation over six months. Particle growth in the DMSO-CPC is strongly depending on the saturator temperature Tsat and the temperature difference ΔT between saturator and condenser, with optimal growth achieved at high Tsat and large ΔT values. Measurements with an optical particle counter downstream of the condenser, along with saturation and droplet size simulations, confirmed these trends and emphasized the importance of CPC internal settings for reliable particle growth. The DMSO-CPC achieved counting efficiencies and cutoff diameters comparable to the Butanol-CPC. The mean cutoff diameter was (5.8 ± 0.9) nm for the DMSO-CPC and (5.6 ± 0.5) nm for the Butanol-CPC. At the same time, the DMSO-CPC substantially reduced working fluid consumption and enabled stable long-term operation. The use of DMSO–H2O mixtures further extended the operational range and improved safety, making the CPC suitable for airborne measurements and remote monitoring. Recommendations regarding instrument modification, operational conditions, and hardware adjustments are made for operating a DMSO-CPC to gain results comparable to a Butanol-CPC. Overall, DMSO-based CPCs provide safe, efficient, and regulation-compliant operation without compromising measurement quality under challenging environmental conditions.
Competing interests: GS, CK, and LK are employed full-time by GRIMM Aerosol Technik GmbH, which may hold direct or indirect financial interests related to the work presented in this paper.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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- RC1: 'Comment on egusphere-2026-127', Anonymous Referee #1, 20 Mar 2026 reply
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The manuscript by Kirchhoff et al. describes a comprehensive study on the performance of a Grimm Sky CPC with DMSO as an alternative working fluid to butanol. DMSO has several advantages over butanol, among others, it is non-toxic, not flammable and is consumed significantly less during CPC operation. It is therefore a very promising candidate for airborne measurements. In line with this potential application, the authors investigated the use of a CPC with DMSO under various pressure levels from atmospheric pressure down to 250 hPa. They found the performance to be comparable to a butanol CPC, but also noticed that the droplet growth is clearly different with DMSO compared to butanol, resulting in a larger number of (optically) smaller droplets. Numerical simulations were conducted to explain the distribution of super-saturation levels in the saturator in order to better understand the differences.
The manuscript is well written and presents a lot of novel information, which is certainly of interest to the readers of AMT. The manuscript merits publication, but I suggest a few minor revisions before it can finally be accepted.
Introduction, general: What I miss is a brief introduction into what type of aircraft measurements the CPC is intended to be used for, i.e. at what altitudes and corresponding pressure levels.
Page 2, second paragraph: I suggest to add the information that CPCs are among the very few aerosol measurement techniques that are metrologically traceable (ISO 15900).
Page 2, line 47: Why is the higher liquid consumption of a water CPC a particular limitation for aircraft measurements? From my personal experience, water CPCs can be operated for days if not weeks before the water reservoir needs to be refilled. One main disadvantage of water CPCs is, however, not mentioned and that is its material dependence, especially of the cut-off diameter.
Page 9, section 3.2: The use of symbols is quite confusing. According to Figure 3, c0 and c1 are voltages. In the text of section 3.2, all of a sudden Th appears as a symbol for a threshold voltage. It took my quite some time to realize that c0 and c1 are actually concentrations with c1 being the concentration of particles/droplets deemed valid, based on the fact that the scattered light peak triggered a voltage signal, exceeding a defined threshold Th(c1). c0 is apparently the concentration of all droplets that cause a signal above noise level. Figure 3 and section 3.2 should be rewritten to make this clear and not leave the understanding up to the interpretation of the reader. I also wonder if Th is an appropriate symbol. Shouldn’t this rather be something like Uth to make it clearer that it is a voltage threshold?
Page 9/10, section 3.3, including Figure 4: How has this been measured? With monodisperse, DMA-classified particles? If so, for which size(s)? Or was it a polydisperse aerosol? If so, how did you know the charge level to obtain the reference number concentration from the FCE?
Page 12, Figure 6: In the caption of the figure and the x-axis, it should read surface area concentration rather than just surface area. What test aerosol was used for these measurements? How has the surface area calculated? Assuming that this result was achieved for rather compact (NaCl or AS) particles, what would happen if particles are highly agglomerated (e.g. soot), thus with a much higher surface area per particle for the same equivalent diameter?
Page 16, Figure 9: Why does the tube length start at 110 mm? Do you only show a part of the saturator or does the section from 0-110 mm belong to the condenser? How did you derive the saturation profile at the inlet of the saturator?
Page 21, Figure 14: x- and y-axis are reversed. The D50 is a function of the temperature difference and not vice versa.
Page 24, Figure 16: Earlier, you mention that the POPS measurements showed smaller droplet diameters for DMSO than for butanol and speculated that this may be due to the difference in the refractive indices. Could this also be the reason for the lower c1/c0 ratios, because the corresponding thresholds are for the voltages, which reflect the peak height of the scattered light in the optical detection of the CPC?