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Ice Nucleating Particles in the Marine Atmosphere

Periodic Reporting for period 4 - MarineIce (Ice Nucleating Particles in the Marine Atmosphere)

Reporting period: 2020-02-01 to 2021-01-31

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 were indications that biological organic components of sea spray could nucleate ice, but there was 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 was needed to understand marine INP. The key limitation to accurately representing INP in models over the world’s oceans is the lack of field data and suitable instrumentation, a deficiency which the MarineIce team addressed through this project.

In MarineIce we constructed a new mobile laboratory (the IcePod) and built new and novel instrumentation focused on the quantification of ice nucleating particles in the atmosphere. We have used these new tools to measurem INPs in remote marine environments around the world from the Caribbean to the North Pole as well as quantifying the ability of marine organics to nucleate ice in laboratory experiments. In doing so we have provided the underpinning data required to model the global distribution of ice-nucleating particles associated with sea spray aerosol. We have shown that sea spray dominates the population of ice-nucleating particles in remote marine locations, whereas terrestrial sourced INP dominate elsewhere. This discovery shed light on why clouds over remote marine locations persist in a supercooled state.

We have gone further than this and shown that knowledge of the ice-nucleating particle population is key to reducing uncertainties in cloud-climate feedbacks. In fact, defining ice-nucleating particles is central to defining cloud feedbacks.
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
We have developed an instrument using microfluidically generated droplets in studies of heterogeneous freezing. 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. 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

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.

We have developed a new expansion chamber for counting INP, known as PINE (Portable expansion chamber for Ice Nucleating particle mEasurements), in collaboration with Karlsruhe Institute of Technology (KIT). PINE has been put through an intensive testing period and has been deployed in multiple places around the world and there is now a commercial version available.

We have used Scanning Electron Microscopy (SEM) to study the size resolved composition of aerosol samples which we do in parallel to the filter based INP studies to inform us of the composition of aerosol and help relate INP concentrations to aerosol size and composition.

Activity 3: Quantify and characterise INP in marine environments

My team have done field campaigns around the world, from Barbados to the North Pole and the Finnish forests to the Iceland. These results have shown that sea spray INP are very important in marine locations remote from land, whereas the terrestrial environment provides a much stronger source of more active INPs. These campaigns have given us the underpinning data and understanding to model the global distribution of INPS from the oceans and deserts of the world.

Activity 4: Modelling the global distribution of marine INP

As part of this activity we now have a global distribution of INP associated with marine sources (Vergara-Temprado et al. 2017). In this work we parameterised INP measurements and linked this to the production of sea spray in a global aerosol model (GLOMAP). We also represented desert dust in order that we could study the competition between desert dust and marine sources of INP. We conclude that marine INP dominate in the world’s remote oceans, such as the Southern Ocean.

We then went on to incorporate the model INP concentrations in a global weather model. We demonstrated that with our representation of marine INP (i.e. low concentrations) we could reproduce the cloud fields in this region, which helps to solve the Southern Ocean bias (Vergara-Temprado et al. 2018). This places INP of first order importance for uncertainty in cloud-climate feedbacks and climate projections (Murray et al. 2021). In addition, we have shown that INP in the tropics are of first order importance in defining the development and properties of deep convective clouds in the tropical Atlantic (Hawker et al., 2021).
1. Demonstrated that the low sea spray related INP concentrations over the Southern Ocean are key to understanding the clouds that exist there. We demonstrate that ice nucleation in these shallow clouds has the potential to dramatically reduce their albedo and that the low INP concentrations that persist there allow them to persist in a supercooled high albedo state.

2. 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.

3. Clearly demonstrated that there is a strong terrestrial biogenic source of INP in the mid-latitudes in addition to the marine and desert sources.

4. Developed a global model of INP. This is helping us to to underpin the description of cloud glaciation in global climate and weather models.

5. Shown that the temperature dependence of INP is very important for deep convective clouds over the tropical Atlantic.

6. The first measurements of INP from above clouds near the North Pole, which we can contrast to measurements at the surface.
View of an Icelandic glacial INP source from the FAAM aircraft during filter sampling
The FAAM aircraft in Cape Verde where we made INP measurements
The tower at Ragged Point, Barbados, where we made INP measurements
The MarineIce team with the IcePod at the Ragged Point observatory in Barbados
The PI, Ben Murray, on board the FAAM aircraft collecting filter samples near Cape Verde
Photograph of the whole MOCCHA team at the North Pole, the IcePod is visible on the Oden