the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Oct 2024
Design and performance of the Cluster Ion Counter (CIC)
Abstract. A dilute plasma is continuously maintained in the troposphere by ionising particle radiation from galactic cosmic rays and radon decay. Small ions in the 1–2 nm size range play an important role in atmospheric processes such as ion-induced nucleation of aerosol particles. Consequently there is a need for precise and robust instruments to measure small ions both for atmospheric observations and for laboratory experiments that simulate the atmosphere. Here we describe the design and performance of the Cluster Ion Counter (CIC, Airel OÜ), which simultaneously measures the number concentrations of positively- and negatively-charged ions and particles below 5 nm mobility diameter, with low noise and fast time response. The detection efficiency is above 80 % for ions and charged particles between 1.2 and 2.0 nm, and above 90 % between 2.0 and 3.0 nm. The ion concentrations measured by the CIC agree well with reference instruments. The noise level (1 σ of background measurements) is typically between 20 and 30 ions cm-3 at 1 Hz sampling rate and an air flow rate of 7 l min-1 per analyzer. The noise level improves when higher flow rates and longer sampling periods are used. The CIC responds rapidly with 1 s time resolution to pulses of ionisation produced in the CLOUD chamber by a CERN particle beam.
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CC1: 'Atmospheric ion measurements: air conductivity versus ion counting', Karen Aplin, 27 Nov 2024
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The paper esign and performance of the cluster ion counter (CIC) by Mirme et al describes the latest instrument in over fifty years of development of Estonian atmospheric ion spectrometers, begun by the late Prof Hannes Tammet. It seems another excellent instrument, which is well-characterised both theoretically and experimentally. This paper is a carefully and clearly written description that I hope could be further improved with some additions to the introductory material.
The Ebert ion counter and the Gerdien condenser are aspirated condensers, developed at around the same time at the start of the twentieth century (Flagan 1998). In the CIC paper’s introduction, the different types of aspirated coaxial cylindrical condenser are listed all together, implying they are essentially identical. There are however some meaningful differences between them. An ion counter, such as that designed by Ebert, operates at a sufficiently high voltage for the electric field in the condenser to collect all the ions passing through the device. In contrast, a Gerdien-type instrument operates in a lower electric field regime, such that only a portion of the ions are collected, which measures atmospheric conductivity rather than counting ions directly (Chalmers 1967). The ion concentration can be estimated from the atmospheric conductivity if a suitable ion mobility can be assumed or separately determined. Understanding the distinctions between these types of instrument is important in interpreting their data.
The paper states that “one limitation of many devices” is their inability to measure bipolar ions, which the CIC avoids by simply having two sampling tubes biased at opposite polarities. The Gerdien condenser can also be operated, as the name suggests, as a capacitor, with a rate of voltage decay that is inversely proportional to the air conductivity. This “voltage decay mode” (Aplin and Harrison 2000) was commonly used in the first half of the twentieth century, and in many radiosonde ascents (Nicoll 2012), because measuring a voltage was simpler than measuring a small current. The voltage decay approach is less frequently used in modern devices but has been exploited in combination with the current measurement approach for self-calibration (Aplin and Harrison 2001). As the operating principle extends to other geometries, this type of instrument is also used in planetary atmospheric electricity, in which context it is known as a “relaxation probe” (Aplin 2013). In the voltage decay mode, a bias voltage is temporarily applied to charge the condenser. It is then released and the capacitor allowed to decay, with a time constant related to the air conductivity. Both positive and negative ions are involved in this process. The form of the decay also provides information on the ion mobility spectrum (Aplin 2005).
Finally, it is worth noting that the total ionisation rate near the surface, combining both cosmic rays and natural radioactivity is 10 cm-3s-1, so C.T.R. Wilson was indeed within a factor of two of the modern average.
References
Aplin, K. L. 2005. “Aspirated Capacitor Measurements of Air Conductivity and Ion Mobility Spectra.” Review of Scientific Instruments 76(10).
Aplin, K. L., and R. G. Harrison. 2000. “A Computer-Controlled Gerdien Atmospheric Ion Counter.” Review of Scientific Instruments 71(8).
Aplin, K. L., and R. G. Harrison. 2001. “A Self-Calibrating Programable Mobility Spectrometer for Atmospheric Ion Measurements.” Review of Scientific Instruments 72(8).
Aplin, Karen L. 2013. Electrifying Atmospheres: Charging, Ionisation and Lightning in the Solar System and Beyond. Dordrecht: Springer Netherlands.
Chalmers, John Alan. 1967. Atmospheric Electricity. Second edition. Oxford: Pergamon Press.
Flagan, Richard C. 1998. “History of Electrical Aerosol Measurements.” Aerosol Science and Technology 28(4):301–80.
Nicoll, K. A. 2012. “Measurements of Atmospheric Electricity Aloft.” Surveys in Geophysics 33(5):991–1057.
Citation: https://doi.org/10.5194/amt-2024-138-CC1 -
RC1: 'Comment on amt-2024-138', Anonymous Referee #2, 17 Dec 2024
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Air ions promote new particle formation through ion-induced nucleation, so measuring air ions, especially small ones, is crucial. This manuscript describes a newly designed instrument, Cluster Ion Counter (CIC), which measures the number concentrations of air ions below 5 nm. Such a device could complement the family of instruments for studying the new particle formation. This manuscript is well written. I believe this manuscript could be published in AMT after addressing the following comments.
Major comments:
- Neutral cluster and Air ion Spectrometer (NAIS), which was designed by same research group of this study, is widely used for the observation of air ions. I’m curious to know what differences or advantages the CIC has over NAIS. This study appears to have done a parallel comparison experiment of CIC and NAIS (Figure 5), but the results are not mentioned in the manuscript.
- CIC looks quite small compared to the NAIS. What is the weight of the CIC? Can mobile observation be an advantage for CIC?
- The authors state that CIC is capable of making precise and robust long-term measurements. Did the CIC operate under ambient conditions before? How does CIC perform in field measurements? I think it is important to make it clear that CIC can be used not only for chamber experiments but also for long-term field measurements.
Minor comments:
Abstract: I suggest adding a description of CIC application prospects to the abstract.
Line 146: The full name of NAIS needs to be given here.
Citation: https://doi.org/10.5194/amt-2024-138-RC1
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