Based on years of measurements in the CLOUD chamber, researchers have for the first time been able to build a computer model simulation of particle formation throughout the atmosphere based entirely on data from laboratory measurements.
All global climate models use parameterisations for aerosol production that are based on classical theories or uncontrolled in-situ observations, rather than experimental data. Now, data collected by CLOUD have been used to build a model of secondary aerosol production based solely on laboratory measurements. This simulation was published in Science magazine on 27 October.
Since 2009, the CLOUD collaboration has been making innovative measurements of nucleation rates with an unprecedented level of control, cleanliness and accuracy. "To understand what these cutting-edge measurements mean for the climate system, we needed to construct a parameterisation from the data which could be used in a climate model. In this paper, for the first time, simulations are presented which use this parameterisation to estimate the effect of different factors on nucleation. This marks a big step forward in our understanding of how aerosol formation affects the climate", says researcher Eimear Dunne from the Finnish Meteorological Institute.
The inorganic parameterisation presented in the paper accounts separately for four nucleation pathways: binary (sulphuric acid only) and ternary (additional stabilisation from ammonia), further subdivided into neutral and ion-induced. The model also included a parameterisation of organic nucleation based on previous CLOUD measurements. The complex interactions of different components of the nucleation system have never been so completely explored.
Based on these simulations, nucleation is found to be the source of 43% of the aerosol particles which are activated in clouds. Ternary nucleation is the source of around 85% of new particles formed in the troposphere, meaning that simulations which only include binary nucleation cannot describe the complete system. In particular, while ion-induced pathways account for an estimated 28% of nucleation, the system as a whole is not sensitive to changes in cosmic ray induced ionisation.
The Earth's climate is a complex and changing system. Computer models are used to predict its future behaviour, and are especially important in light of persistently increasing atmospheric concentrations of anthropogenic greenhouse gases like CO2. The largest cause of climate uncertainty has consistently been aerosols, particularly their interactions with clouds. These small solid and liquid particles can scatter and absorb light directly, and change the optical properties of clouds. Primary particles enter the atmosphere in solid or liquid form, like sea spray or dust. But the vast majority of aerosol are secondary particles, which form from gases in a process called nucleation.
Sulphuric acid gas can condense into aerosol at a very low vapour pressure, making it an important component of atmospheric nucleation. But theory showed that nucleation rates observed in the atmosphere were too high to be cause by sulphuric acid alone. Other chemicals like ammonia, amines, and organic compounds were believed to assist in forming new particles. Ions can also stabilise newly formed particles, and increase nucleation rates. But quantifying the contributions of these different species to the aerosol population was extremely difficult.
The European Union's Marie Curie FP7 CLOUD-ITN project was composed of an international team of Europe's top aerosol scientists, working in collaboration with particle physicists at CERN. The CLOUD chamber was built at CERN to extremely high technical standards, making it possible to reproduce the extremely clean and cold conditions of the free troposphere for the first time in a laboratory situation. Since Finnish scientists made the first ever observations of atmospheric nucleation in the 1990s, and have been heavily involved in nucleation research ever since, they were of course an important part of the CLOUD project.
Researcher Eimear Dunne, tel. +358 50 436 5545, email@example.com
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