In this paper, the authors investigated the homogeneous freezing properties of lubrication oil droplets using a PINE expansion chamber. These laboratory experiments conclude that oil droplets freeze at temperatures below 235K and suggest that reducing the number concentration of these droplets could mitigate the climate impact of aircraft contrail cirrus clouds. Detailed understanding of contrail ice clouds is currently poorly understood and also poorly represented in the model. This study aims to fill this gap; however, it lacks originality. The main concerns are as below.

Page 93-107: There are no direct measurements that support that lubrication oil droplets are found in the exhaust plumes. In fact, as per Karcher et al. (2016) lubricant-derived aerosol particles are too few and do not influence contrail formation. Measurement conditions (ground level) are significantly different from high-altitude aircraft cruising altitudes. The atmospheric relevance of lubrication oil (section 3.1) used as a surrogate for the actual oil (if any) is not clear. Is the oil used (section 3.1) found in the actual exhaust plume? Chemical composition of oil droplets sampled downstream of the exhaust plume and surrogate oil droplets used in this study should be compared. Such experiments are missing in this paper.

Section 3.1, 3.2: The experimental setup to generate the oil droplets is not similar as the actual aircraft engine generating the oil droplets. Does the aerosol generation mechanism change the chemical properties of oil droplets? The physical and chemical properties of generated oil droplets within this paper are similar to actual exhaust plume? Note that the aircraft engine is operated at different thermal and turbulent conditions. The study will be unique if actual aircraft engine exhaust plume is sampled.

Section 4.1: The setup shown in Figure 2 describes that diffusion dryers are used upstream of the PINE chamber. If the air is dried, then what is the source of humidity in the PINE chamber? If correct, there must be some source of water vapor to activate oil into water droplets. What is the RH of the air upstream (after dryers) and within the PINE chamber? If the air is not completely dried (maybe RH = 5% then the dewpoint temperature is -21 degC of 20 degC room temperature air that is entering the chamber), then as PINE is cooled most of the incoming water vapor will condense on the interior walls of the PINE chamber as soon dew point is reached instead on the oil droplets. This will result in very few droplets activation (Figure 3). What is the droplet activation fraction at saturated supercooled temperature conditions? Such thermodynamic analysis (trajectory analysis) of oil droplets (particles) that are sampled is missing. As there are no RH measurements within the PINE, it is difficult to understand the activation behavior of droplets. Also, this poor understanding makes other ice nucleation groups to reproduce these results.

Section 4.2: The freezing behavior of oil droplets has been widely studied in the past. See Tabazadeh et al. (2002) and many papers that cite this work. It is very well known the T and RH conditions where these oil droplets freeze homogeneously. The results shown in Figure 4 are well described in the literature. It is not clear the uniqueness of the lubricant oil that is used in this study. As mentioned above, the atmospheric relevance is missing.

References:

Karcher, B., The importance of contrail ice formation for mitigating the climate impact of aviation. Journal of Geophysical Research-Atmospheres, 2016. 121(7): p. 3497-3505.

Tabazadeh, A., Y.S. Djikaev, and H. Reiss, Surface crystallization of supercooled water in clouds. Proceedings of the National Academy of Sciences of the United States of America, 2002. 99(25): p. 15873-15878.

The manuscript titled “Jet aircraft lubrication oil droplets as contrail ice-forming particles” by Ponsonby et al., investigates water and ice nucleation affected by lubrication oil droplets relevant to aircraft engines. Basic thermodynamics of supersaturation conditions are first presented followed by nucleation results. The authors claim cloud condensation nuclei activation with a hygroscopic parameter close to 0, and claim that ice nucleation occurred homogeneously. The contrail mixing line is cleverly plotted together with their results to highlight the importance of T and RH ranges at which ice or liquid could nucleate when exhaust plumes mix with colder dryer ambient air.

The study is performed well, and the conclusions are sound. The negative result that particles nucleate with no hygroscopicity and homogeneously is important to be published. It will guide future work on used oil and help to aid in interpreting ice nucleation from aircraft emitted particles. Certainly, it also shows the suitability of the PINE instrument to measure nucleation in general. The paper is written and presented well, and suitable for publication. I only have a few minor comments to be addressed.

1. Figure 1: It is not so clear what is water partial pressure and saturation vapor pressure. Would the authors please identify this? pw,x is the saturation vapor pressures where x is i for ice or w for water. The lines are predicted water vapor partial pressure. It would be appreciated for this to be claimed in the caption.
2. Figure 1 and p5 l126-127: G is the slope of the colored lines in Fig. 1A. Although claimed that G=1.64 is a typical value, it would be beneficial to show or state a range. I would expect that the different types, manufactures and sizes of engines, G may have different values.
3. Figure 1 and p8 l196: Would it be worth to state or show where homogeneous liquid nucleation (kelvin equation) would occur? Supersaturation with respect to water is predicted to be very high, and it would be interesting to show where nucleation on 100nm particles that are completely hygroscopic would occur.