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CLOUD-MObility, Training and InnOvation Network

Periodic Reporting for period 2 - CLOUD-MOTION (CLOUD-MObility, Training and InnOvation Network)

Reporting period: 2019-09-01 to 2021-12-31

CLOUD-MOTION: CLOUD-MObility, Training and InnOvation Network

New-particle formation, or nucleation, largely determines the number concentration of ultrafine particles (<100 nm) in the atmosphere, and models predict that approximately half of global cloud condensation nuclei form in this way. Nucleation is therefore critical to aerosol-cloud interactions and climate. Due to the non-linear nature of the processes, competing mechanisms, and the complex variety of chemical compounds that influence nucleation we are still far from a satisfactory level of understanding. Ice nucleation, i.e. the formation of ice in clouds, also plays an important role governing the radiative properties of ice and mixed-phase clouds and influencing precipitation and climate, but only a tiny, poorly understood fraction of atmospheric particles act as heterogeneous ice-nucleating particles (INP).
Both of these topics - aerosol nucleation and ice nucleation - are studied with great precision at the CLOUD aerosol and cloud chamber facility at CERN. The experimental data are parametrized to improve air-quality models and climate-change predictions, two topics of highest relevance and timeliness for society.

The major research activity of CLOUD-MOTION was to perform three sets of joint experiments at the CLOUD aerosol chamber. The focus of investigations was:
a) aerosol nucleation and growth in pristine environments (tropical free troposphere and unpolluted marine environments),
b) aerosol nucleation and growth in the polluted atmosphere (urban environments),
c) the formation of ice on glassy Secondary Organic Aerosol acting as Ice Nucleating Particles (“glassy SOA as INP”).

CLOUD is the world’s first experiment to reach the technical standards required to measure nucleation and growth of aerosol particles under controlled atmospheric conditions in the laboratory. A beamline at CERN allows to simulate the role of galactic cosmic rays for aerosol formation and cloud processes.

CLOUD-MOTION was successfully finished on December 31, 2021. The scientific goals of the project were reached. Main conclusions of the project are that a) an important, so far unknown, role for the combination of nitric acid and ammonia was found to cause rapid growth of aerosol particles in urban atmospheres, b) the nitric acid-ammonia-sulfuric acid system is responsible for nucleation and growth of aerosols in the upper troposphere above the Asian monsoon, and c) the molecular steps for nucleation of iodine oxoacids were identified and the role of this process was highlighted for Arctic regions.

All early-stage researchers, employed for CLOUD-MOTION, received excellent training at their host institutions as well as within the project network.
CLOUD-MOTION was established in Sep. 2017. 15 ESRs were appointed and research plans were set up.

As the core of the research activities three experiments were conducted at the CLOUD chamber to investigate aerosol nucleation processes. Comprehensive measurements of the precursor chemistry as well as the nucleation and initial growth rates were conducted for a number of chemical systems, comprising several hundred experimental runs. The focus of investigations was on the following systems:
• tropical free troposphere: isoprene and α-pinene in combination with sulphur compounds for a range of temperatures down to -50°C (e.g. Caudillo et al., ACP, 2021).
• marine system: iodine and sulphur compounds that are representative for the atmosphere in coastal or open ocean areas (e.g. Shen et al., submitted, 2022; He et al., Science, 2021).
• urban nucleation: aromatic compounds such as toluene and cresol as well as the inorganic acid-base system of sulphuric and nitric acid with ammonia and dimethylamine (e.g. Wang et al., Nature, 2020; Martens et al., submitted, 2022).

Activation properties of secondary aerosol for cloud droplets and ice particles were also investigated. Aerosol particles were nucleated and grown in the CLOUD chamber from vapours under various conditions (chemical species, relative humidity, temperature, and ion concentrations). A wide range of secondary aerosols were investigated (inorganic, pure biogenic, multicomponent, marine, and urban). The cloud activation properties were investigated for liquid droplets (cloud condensation nuclei, CCN) and ice formation (ice nucleating particles, INP). Significant process understanding was achieved from these experiments (e.g. Bertozzi et al., 2021).

Transfer of results into global and cloud-scale models was also performed (e.g. Ranajithkumar et al., ACP, 2021; Wang et al, submitted manuscript, 2022). With these modelling studies the results of CLOUD-MOTION are directly applied to improve our understanding of important aerosol processes in the atmosphere and their influence on the radiation budget and climate.

As an exploitable result, an innovative gas chromatographic (GC) coupling design for a TOF mass spectrometer was developed, characterized and tested by the ESR at Tofwerk AG. The patenting process for this device is ongoing.

A new CCN generator was developed to produce highly charged CCN of either polarity. Expansion experiments were performed with highly-charged CCN and compared with similar experiments using uncharged CCN.

Twelve collaboration meetings and data workshops took place, three CLOUD-MOTION Summer Schools and one Winter School were held (some as online meetings due to the corona pandemic).

All ESRs received comprehensive training on the preparation, operation, troubleshooting, and data analysis procedures for their individual instruments or models. They all learned also to operate the complex CLOUD chamber for conducting the shifts during the experiments.
Progress of the ESRs was regularly reviewed. Overall, excellent progress has been achieved by the ESRs. All ESRs are co-authors of various peer-reviewed publications of the CLOUD collaboration, including publications in high impact scientific journals. Many ESRs have published first-author papers already. The process of publishing the results in scientific journals is still ongoing. The list of publications and conference presentations was submitted as Deliverable 5.2 and it can be viewed at

All Milestones were reached and all Deliverables were provided to the Participants Portal in time.
The measurements at CLOUD provided unique data sets describing the nucleation and growth properties for several chemical systems at a range of atmospheric conditions that had never been characterized before.
As a special highlight we observed exceptional growth properties for the nitric acid-ammonia system (Wang et al., Nature, 2020). Here, extremely rapid growth of nucleation mode aerosol under conditions relevant for Asian megacities in winter was observed. This system was also discovered to be of high relevance for aerosol formation in the upper tropospheric Asian monsoon outflow.
Another highlight was the detailed determination of the molecular nucleation for iodine oxoacids including the ion-induced as well as the neutral formation pathway (He et al., Science, 2021).
These high-impact publications have opened new research directions. These publications have already received considerable attention (one is highly-cited) and are contributing to our overall understanding of climate change.
CLOUD with its analysing instruments in the East Hall during the CLOUD14 cosmic ray run (image CERN)
CLOUD14 instrument layout
ESRs of CLOUD-MOTION during the Winter School, Hyytiälä Forestry Field Station in Southern Finland