Cosmic rays and aerosols in the terrestrial atmosphere
Authors: Danylevsky V. 
Published: July 2019

Galactic cosmic rays are considered as one of the external force influencing the Earth’s climate change. The cosmic rays are the main cause of the troposphere ionization. Ions are considered as one of the factors that participates in producing of the aerosol particles and cloud condensation nuclei, when the super saturation level of the water vapor or/and other atmosphere constituents vapor is sufficient. Aerosols are present throughout the atmosphere and affect Earth’s climate directly through backscattering of sunlight and indirectly by altering cloud properties. Both effects are known with considerable uncertainty only, and translate into even bigger uncertainties in future climate predictions. Whereas disputable, the idea is discussed by the scientists that variations in galactic cosmic rays closely correlate with variations in atmospheric cloud cover and therefore constitute a driving force behind aerosol-cloud-climate interactions. A lot of studies were performed to validate or disprove the connection between cosmic ray’s variation (e.g. the Forbush events) and changes of the aerosol content and properties in the atmosphere, cloud cover and properties and other climate parameters, but results are controversial. The enhancement of atmospheric aerosol particle formation by ions generated from cosmic rays was proposed as a physical mechanism explaining this correlation. But the main problem is to find the appropriate physical model which allows to calculate correctly the ion concentrations, nucleation and aerosol particles rate and cosmic rays intensity. Aerosol particle formation occurs in two stages: nucleation to form a critical nucleus and subsequent growth of the critical nucleus to a larger size (>2 – 3 nm) that competes with removal of the freshly nucleated nanoparticles by coagulation with pre-existing aerosols. The most used nucleation and particle growth theories are reviewed and analyzed in the article. The base of the theories is follow. Nucleation is generally defined as creation of molecular embryos or clusters prior to formation of a new phase during the transformation of vapor liquid solid. This process is characterized by a decrease in both enthalpy and entropy of the nucleating system. A free energy barrier is often involved and needs to be surmounted before transformation to the new phase becomes spontaneous. Another limitation in the nucleation and growth of atmospheric nanoparticles lies in significantly elevated equilibrium vapor pressures above small clusters and nanoparticles, also known as the Kelvin (curvature) effect, which considerably restricts growth of freshly nucleated nanoparticles. Ions are capable, under certain conditions, of suppressing or even removing the barrier to nucleation in embryonic molecular clusters of water. But results of the theories are very uncertain so far. Results of the observations of the nucleation and particles formation as well as the special CLOUD experiment results are reviewed and analyzed in the article. The molecular clusters and nuclei can not be observed by remote sensing techniques like sun-photometers, lidars or satellite instruments. The in-situ measurements of the nucleation concentration and particles growth rate are performed in the certain sites only. The observations and experiments revealed the important influence of the trace gases and organic molecules on the nucleation and particle growth rate. Sulphuric acid, ammonia, amines, and oxidised organics play a crucial role in nanoparticle formation in the atmosphere competing with ionmediated mechanism. Saturation pressure of the sulphuric acid and organics vapors at the typical atmospheric conditions is much lower than for water vapor and at typical atmospheric concentration they are capable of suppressing the nucleation barrier. Nucleation with ions started earlier and run faster but the nucleus with sizes ≥ 3 nm more than 90 % of clusters are neutral. Ion-mediated mechanism can dominate when sulphuric asid and organic molecules concentration is low. But more observations in the different atmosphere layers and locations and experiments at different conditions is required to better understanding the ion-mediated nucleation in the atmosphere. Nucleation contribution to the aerosol content and properties in the terrestrial atmosphere is also simulated by the special modules included to the regional and global models of the atmosphere and climate, e.g. GEOS-Chem and CAM5. Comparison of the simulation and observations has showed that in general the averaged model results are in good agreement with observational data at some sites but same biases were revealed at some sites too. It requires the further analysis and models developments. Also ion-mediated mechanism contribution was also estimated by the simulation not more than 10%. Analysis of the observations and models results in the article showed that cosmic rays influencing the aerosol formation also influence the microphysical and optical properties of the particles. First of all particles size distribution is influenced by nucleation mechanism and relative content of the Aitken nuclei increases. Also sulphuric acid can influence the particle refractive index increasing the single-scattering albedo of the aerosols. Modern remote sense technique such as the AERONET sun-photometers can measure the spectral AOD and sky radiance with high accuracy and the reliable size distribution, refractive index and single-scattering albedo averaged over atmosphere column can be determined from that observations, but the AERONET inversion algorithm has to be developed to obtain the particles size finer than 50 nm.