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Ice Nucleating Particles in the Marine Atmosphere

Periodic Reporting for period 3 - MarineIce (Ice Nucleating Particles in the Marine Atmosphere)

Reporting period: 2018-08-01 to 2020-01-31

The formation of ice in clouds is fundamentally important to life on our planet since clouds play a key role in climate and the hydrological cycle. Despite the significance of ice formation, our quantitative understanding of sources, properties, mode of action and transport of Ice-Nucleating Particles (INP) is poor. In order to improve our representation of clouds in models we need to understand the ice-nucleating ability of all major aerosol types, including those from the world’s oceans as well as those from the terrestrial environment.

Despite oceans covering over 70% of the planet and sea spray being one of the dominant aerosol types in the atmosphere, its role in the formation of ice in clouds remains poorly understood. There are strong indications that biological organic components of sea spray can nucleate ice, but there is a lack of data to quantify it. In contrast, the ice-nucleating ability of major aerosol species from terrestrial sources, such as mineral dusts or bacteria, has received significant attention over the past few decades. A similar effort now needs to be made to understand marine INP. The key limitation to accurately representing INP in models over the world’s oceans is the lack of field data, a deficiency which the MarineIce team is addressing through this project.

The specific objectives were to develop novel instrumentation for quantifying INP concentrations cover the full range of mixed phase cloud conditions. These instruments will be housed in a unique highly instrumented mobile laboratory, which will allows us to access challenging environments on land based field sites and research ships. In parallel, we are developing the representation of marine and terrestrial INP in a state-of-the-art global aerosol model and are using this to define cloud glaciation processes in climate and weather models.
In the following I set out how my team have addressed the individual ‘activities’ set out in my original proposal.
Activity 1. Construct a new-concept semi-autonomous ultra-sensitive INP counter (MicroINP)
We recently published the first paper using this new and exciting technique (Tarn et al., 2018). This paper demonstrates one of the first uses of microfluidically generated droplets in studies of heterogeneous freezing. Microfluidics has the advantage over other techniques in that it generates a very large number of droplets in a short period of time. We demonstrated that the technique works for a range of atmospherically relevant ice nucleating particle types and also some samples of aerosol sampled from the air. It has also been used to quantify ice nucleation by field collected samples, which will be included in upcoming papers. At the end of this reporting period we were developing the next generation of this instrument where we intend to count the freezing droplets in flow on the chip. This aspect of the work has been very challenging, but we are now in a position where we have an instrument to quantify ice nucleation in flowing droplets. The advantage of this technique is the huge number of droplets that can be analysed, while also working all the way down to homogeneous freezing at around -37 C..

Activity 2: Developing new capacity to make INP measurements in the remote marine atmosphere
(IcePod)

2a; commissioning the IcePod and aerosol instrumentation: The IcePod: INP concentrations in many environments are poorly constrained, in part, because the community has not had adequate means to access those environments. We have constructed the IcePod, a mobile laboratory based around a converted shipping container. The IcePod is an insulated, air conditioned, facility which allows my team to perform aerosol sampling and ice nucleation experiments in environments were ice nucleation has never been measured. The laboratory is equipped with an inlet system, aerosol instrumentation, filter sampling systems, capacity for an aerosol based INP system (the PINE chamber), the NIPI suite of instruments including the microfluidics instrument. The first campaign for the IcePod where we studied the competition between marine sources and terrestrial sources of INP in Yorkshire was recently published (O’Sullivan et al. 2018).

2b) Testing and developing of the filter based INP technique. We have made a great effort developing and using multiple filter based technique for quantifying atmospheric ice nucleation. We now routinely use a method with track etched polycarbonate filters where we wash the particles off the filter (O’Sullivan et al. 2018; Tarn et al. 2018) and a method using teflon filters where we place an array of droplets onto the filters (Price et al. 2018). This allows us to quantify low concentrations of INP relevant for mixed phase cloud conditions. A major advantage of the wash-off technique is that it allows us to treat aliquots of the samples in different ways which helps us to distinguish between biogenic (e.g. marine) and desert dust INP, for example. An addition development is the IR-NIPI instrument for quantifying low concentrations of INP using a novel IR camera technique together with multiwell plates commonly used in microbiology (Harrison et al. 2018).

2c) Construction and testing of an aerosol based INP instrument for cirrus and mixed phase cloud work. We refer to the expansion chamber as PINE (Portable expansion chamber for Ice Nucleating particle mEasurements). There are problems with CFDCs related to frost formation, which limits the detection limit, and their non-automated nature, which limits the data collection to intensive field campaign periods. I decided, in collaboration with Karlsruhe Institute of Technology (KIT), to develop an instrument using a different concept. We now have two working instruments, one owned by KIT and one funded by MarineIce. Both instruments have been put t
Key progress beyond the state-of-the-art:
1. We demonstrated that the low sea spray related INP concentrations over the Southern Ocean are key to understanding the clouds that exist there (Vergara-Temprado et al. PNAS, 2018). We demonstrate that ice nucleation in these shallow clouds has the potential to remove them and that the low concentrations that persist there allow them to exist.
2. The Development of the PINE chamber. This single instrument has the potential to transform the ice nucleation community. It is the first instrument that operates on a semiautonomous basis, producing INP concentrations over an extended period of time. We already have a company working on a commercial version.
3. We have clearly demonstrated that there is a strong terrestrial biogenic source of INP in the mid-latitudes in addition to the marine and desert sources (e.g. O’Sullivan et al. Sci Reps., 2018). This is an exciting result because it has been suggested that there is an important biological source of INP, but very few experiments have been done to test this hypothesis.
By the end of the project I anticipate the following key results beyond what we already have:
1. An unprecedented global picture of INP around the globe from our field and model work. This will highlight what we know well and where in the world we are missing sources of INP. But, overall, it will help us to underpin the description of cloud glaciation in global climate and weather models.
2. An unprecendented understanding of the impact of INP on different cloud types in marine locations.
3. An unprecedented understanding of INP in the atmosphere across the Tropical Atlantic from the comparison of our field work in Cape Verde and Barbados.
4. Evidence from multiple locations that INP of biological origin are critically important in the mid-latitude terrestrial environment.
5. A unique set of tools for quantifying INP concentrations over the full range of atmospheric concentrations and relevant for the full range of cloud types.
6. Achieved a major impact on the field with the introduction of a commercial instrument for quantifying INP concentrations.
7. The first measurements of INP from above clouds near the North Pole, which we can contrast to measurements at the surface.