Next-generation ice nucleating particle sampling on aircraft: Characterization of the High-volume flow aERosol particle filter sAmpler (HERA)
Abstract. Atmospheric ice nucleating particle (INP) concentration data from the free troposphere are sparse, but urgently needed to understand vertical transport processes of INPs and their influence on cloud formation and properties. Here, we introduce the new High-volume flow aERosol particle filter sAmpler (HERA) which was specially developed for installation on research aircraft and subsequent offline INP analysis. HERA is a modular system constisting of a sampling unit and a powerful pump unit and has several features which were integrated specifically for INP sampling. Firstly, the pump unit enables sampling at flow rates exceeding 100 L min−1, which is well above typical flow rates of aircraft INP sampling systems described in the literature (~10 L min−1). Consequently, required sampling times to capture rare, high-temperature INPs (≥-15 °C) are reduced in comparison to other systems and potential source regions of INPs can be confined more precisely. Secondly, the sampling unit is designed as a seven-way valve, enabling switching between six filter holders and a bypass with one filter being sampled at a time. In contrast to other aircraft INP sampling systems, the valve position is controlled remotely via software so that manual filter changes in-flight are eliminated and the potential for sample contamination is decreased. This design is compatible with a high degree of automation, i.e., triggering filter changes depending on parameters like flight altitude, geographical location, temperature, or time. In addition to the design and principle of operation of HERA, this paper presents laboratory characterization experiments with size-selected test substances, i.e., SNOMAX® and Arizona Test Dust. The particles were sampled on filters with HERA, varying either particle diameter (300 nm to 800 nm) or flow rate (10 L min−1 to 100 L min−1) between experiments. The subsequent offline INP analysis showed good agreement with literature data and comparable sampling efficiencies for all investigated particle sizes and flow rates. Furthermore, the deposition efficiency of atmospheric INPs in HERA was compared to a straightforward filter sampler and good agreement was found. Finally, results from the first campaign of HERA on the High Altitude and LOng range research aircraft (HALO) demonstrate the functionality of the new system in the context of aircraft application.
Sarah Grawe et al.
Status: open (until 14 Jun 2023)
- RC1: 'Comment on amt-2023-88', Gabor Vali, 27 May 2023 reply
Sarah Grawe et al.
Sarah Grawe et al.
Viewed (geographical distribution)
The paper presents a newly designed sampling device for atmospheric ice nucleating particles (INPs). It is designed for use in research aircraft. The design is described in detail and test results are presented to show the sampling efficiency and reliability of the device. The significance of the results is the prospect that it will aid in obtaining research results on atmospheric INPs at higher spatial and temporal resolution than has been possible with other systems. This can be specially useful when coupled with cloud and precipitation studies. The paper addresses the relevant questions thoroughly, although with some limitations and with some superfluous material. The topic of the paper fits well within the scope of AMT.
The manuscript presents an evaluation of the HERA sampling unit specifically for INPs. To some extent this is justified as the stated goal for HERA is to collect samples for INP studies. On the other hand, it makes the paper somewhat convoluted and focus is lost on the fact that the novelty is the HERA sampling, not the filter processing. The latter issue has been extensively treated in previous publications. Some new data are included here on pore-size dependence but really that should be a topic examined on its own.
Except for rather special situations, INPs can be expected to be dynamically just like other aerosol particles. Thus, transmission and collection efficiencies for INPs of given sizes, densities and shapes can be expected to be the same as for similar particles of other kinds. In view of that, it would be better if the paper separated the HERA sampling efficiency from INP processing issues. In fact, it is curious why the authors focus only on INPs and do not leave open the use of HERA for aerosol sampling for other types of aerosol analyses.
The paper also gets complicated by not clearly defining that installation-dependent other factors, specially inlet characteristics, are not subjects of this paper. Only in the discussion of the flight result (Section 4) do these issues have a place, as a specific instance of information needed to interpret the results.
The HERA unit is aimed to facilitate the collection of filter samples for INP analyses by eliminating in-flight handling of the filters. This is a worthwhile goal, although controls used in many previous cases showed that manual exchanges of filters didn't produce noticeable contamination. The potential of HERA will be more fully evident once the operation is fully automated and the need for a technician attending to the collector is eliminated. Then, the remaining limitation will be the number of filters per flight to six. Selection of six truly meaningful sampling intervals will require difficult decisions by the flight directors. The paper could be more realistic about these issues. Also, it should be clarified if filter holders could be exchanged during flights or not. Perhaps this is already in the paper and was missed in reading it.
The design of HERA is described in great detail. Yet, one misses information about overall size, weight and power requirements. Those are important factors for aircraft deployment.
Sampling efficiency is presented in terms of theoretical calculations. It would have been good to have support for the computed efficiencies from actual tests. The tests with Snowmax are in that direction but for smaller sizes than the calculations (Fig. 2).
A design criteria of 0.5 μm lower limit for particle sizes is given and transmission efficiencies are calculated for size above that limit. Small particles are less subject to losses. This is fine, but the justification given deserves some comments. Two references are cited in support of the decision for INPs. Both references show correlations of INP concentrations with particle sizes >0.5 μm but also show temperature dependence and variations with ambient conditions. None of the correlations are strong at higher nucleation temperatures and, in any case, correlations can arise from source or transport similarities without the INPs actually being >0.5 μm. Both studies were made with samples at ground level and there are other limitation as well in the two references. It is out of place for this manuscript to review the literature about INP sizes. It should however not give the appearance of a firm justification for the choice. Results obtained with the use of the HERA will have to be evaluated with the size question in mind, specially since the impact of INP sizes is also an issue for INP extraction and detection with the filter method.
With the foregoing in mind, it is recommended that the Introduction focus on the sampling issues from aircraft, that Section 3.1 be re-written to focus on showing the lack of sensitivity to flow-rate for small particles (as expected) for one filter setup. Sections 3.2, 3.3 and 4 are the main results to present. The use of 'deposition efficiency' should be avoided and it is easily confused with deposition nucleation tests. The discussion about cell fragments versus protein aggregates isn't effective because of the differences in physical dimensions; the sources of differences are more likely to be a question of sample aging and other treatment differences. The observed drop-off in detection at the higher temperatures is concerning but not a point to be treated in this paper.
In Section 3.3. the experimental setup would be of interest and the reason for the wind-speed dependence more understandable. This is more to the central point of the paper (sampling efficiency).
In summary, this is paper about an important new instrument and its characterization for INP sampling. Much good material is presented. Improvements can be gained by simplification of the paper.