How clouds affect Earth’s climate
Atmospheric aerosol particles provide the condensation nuclei on which cloud droplets and ice crystals form. The aerosol particles are either directly emitted from Earth’s surface (for instance, dust from deserts) or are formed in the atmosphere by condensation of very low volatility trace gases. To understand which vapours are producing atmospheric particles, young researchers turned to the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The 3 m diameter stainless steel chamber can realistically simulate the full range of atmospheric conditions, including ionisation by galactic cosmic rays by means of a pion beam from the CERN Proton Synchrotron. Within CLOUD Initial Training Network (CLOUD TRAIN), hundreds of experiments were carried out with the CLOUD chamber. Researchers from 10 European research institutions made important discoveries. First, they found that minute concentrations of amine vapours combine with sulphuric acid to form aerosols at rates similar to those observed in the atmosphere. Second, they discovered that highly oxidised organic vapours that originate from biogenic terpenes are a major contributor to particle formation and initial growth in the atmosphere. Amines are related to ammonia and are responsible for odours emanating from the decomposition of organic matter that contains proteins. CLOUD-TRAIN experiments demonstrated that low concentrations of amines typically found in the atmosphere – only a few parts per trillion by volume – are sufficient to form particles in combination with sulphuric acid. Terpenes are organic vapours emitted by plants, especially conifers. Here it was shown by the CLOUD-TRAIN experiments that fast oxidation processes produce organic substances with extremely low volatility that rapidly form new particles – even, surprisingly, in the absence of sulphuric acid. The researchers found that ionising radiation from galactic cosmic rays has only a weak effect on the formation rate of aerosol particles from sulfuric acid with amines but, on the other hand, increases the particle formation rate from terpenes by a large factor: 10-100. Work carried out at the CLOUD facility by the CLOUD-TRAIN researchers will help to improve global climate models by reducing aerosol-cloud uncertainties. The CLOUD findings will also help identify sources of pollution (smog) particles in large cities. The CLOUD-TRAIN team have published several papers with their findings in prestigious peer-reviewed journals like Nature and Science.
Keywords
Clouds, global warming, aerosols, CLOUD-TRAIN, aerosol nucleation, ionising radiation
