Title: Galactic cosmic rays, particle formation, natural variability of global cloud properties, and climate implications
Author: Fangqun Yu
Affil: Atmospheric Sciences Research Center, State University of New York at Albany
Email: yfq@asrc.cestm.albany.edu
Abstract: A distinct correlation between solar activity, which modulates galactic cosmic ray (GCR) fluxes, and the average properties of low-level clouds has recently been reported. At the same time, questions have been raised, including the absence of a physical mechanism to explain the correlation, and the lack of an obvious correlation between solar activity and high cloudiness (where, it is argued, the strongest impact should be expected in the upper troposphere, closer to the peak in the GCR ionization rate).
We propose that ion-mediated nucleation may offer a physically-based link between galactic cosmic ray fluxes and global low-level cloud properties. Due to enhanced growth and stability of the charged clusters as a consequence of electrostatic interactions, air ions from GCR ionization may play an important role in the production of new particles under typical tropospheric conditions. We find that the dependence of particle nucleation rate on ionization rate is highly non-linear and is a complex function of ionization rate itself as well as other parameters (e.g., sulfuric acid vapor concentration [H2SO4], temperature, relative humidity, and the surface area of preexisting particles, etc). Thus, the effects of changes in GCR fluxes on cloud properties (if any) are expected to be different at different altitudes and latitudes. The implications of such spatially dependent influence of GCR variations on condensation nuclei (CN) production to the proposed GCR-cloud connection will be discussed.
While our primary study indicates that GCR variations may have a significant effect on particle formation rate, we currently do not know if the natural GCR variations will have an obvious effect on the abundance of cloud condensation nuclei (CCN) and cloud properties. Much more work is needed to clearly understand the effect of solar-modulated GCR ionization on atmospheric particle formation, CCN abundance and cloud properties. The GCR-CN-CCN-cloud hypothesis, if confirmed, may provide a mechanism to amplify the influence of the solar variability on Earth's climate though the magnitude of such amplification is currently unknown.