CLOUD

Cosmics Leaving Outdoor Droplets (CLOUD) is an experiment being run at CERN by a group of researchers led by Jasper Kirkby to investigate the microphysics between galactic cosmic rays (GCRs) and aerosols under controlled conditions. This is a fixed-target experiment that began operation in November 2009, though it was originally proposed in 2000. The primary goal is to understand the influence of galactic cosmic rays (GCRs) on aerosols and clouds, and their implications for climate. Although its design is optimised to address the possibility of cosmic rays nucleating cloud particles, (as posed by, for example, Henrik Svensmark and colleagues) CLOUD allows as well to measure aerosol nucleation and growth under controlled laboratory conditions. Atmospheric aerosols and their effect on clouds are recognised by the IPCC as the main source of uncertainty in present radiative forcing and climate models, since an increase in cloud cover reduces global warming. Setup The core of the experiment is a stainless steel chamber of 26m³ volume filled with synthetic air made from liquid nitrogen and liquid oxygen. The chamber atmosphere and pressure is being measured and regulated by various instrumentations. The aerosol chamber can be exposed to an adjustable particle beam simulating GCRs at various altitude or latitude. UV illumination allows photolytic reaction. The chamber contains an electric field cage to control the drift of small ions and charged aerosols. The ionisation produced by cosmic rays can be removed with a strong electric field. Besides, humidity and temperature inside the chamber can be regulated, allowing for fast adiabatic expansion for artificial clouds (compare cloud chamber) or experiments on ice microphysics. According to Kirkby "the level of cleanliness and control in a laboratory experiment is at the limit of current technology, and CERN know-how has been crucial for CLOUD being the first experiment to achieve this performance."