Abstract
Abstract. The flux of cosmic rays to the atmosphere has been reported to correlate with cloud and aerosol properties. One proposed mechanism for these correlations is the "ion-aerosol clear-air" mechanism where the cosmic rays modulate atmospheric ion concentrations, ion-induced nucleation of aerosols and cloud condensation nuclei (CCN) concentrations. We use a global chemical transport model with online aerosol microphysics to explore the dependence of CCN concentrations on the cosmic-ray flux. Expanding upon previous work, we test the sensitivity of the cosmic-ray/CCN connection to several uncertain parameters in the model including primary emissions, Secondary Organic Aerosol (SOA) condensation and charge-enhanced condensational growth. The sensitivity of CCN to cosmic rays increases when simulations are run with decreased primary emissions, but show location-dependent behavior from increased amounts of secondary organic aerosol and charge-enhanced growth. For all test cases, the change in the concentration of particles larger than 80 nm between solar minimum (high cosmic ray flux) and solar maximum (low cosmic ray flux) simulations is less than 0.2 %. The change in the total number of particles larger than 10 nm was larger, but always less than 1 %. The simulated change in the column-integrated Ångström exponent was negligible for all test cases. Additionally, we test the predicted aerosol sensitivity to week-long Forbush decreases of cosmic rays and find that the maximum change in aerosol properties for these cases is similar to steady-state aerosol differences between the solar maximum and solar minimum. These results provide evidence that the effect of cosmic rays on CCN and clouds through the ion-aerosol clear-sky mechanism is limited by dampening from aerosol processes.
Highlights
The effect of the sun and other factors outside of the Earth system on the Earth’s climate remains a controversial aspect of climate science
Simulations of GEOS-Chem using TwO-Moment Aerosol Sectional (TOMAS) with Yu’s ionmediated nucleation (IMN) scheme have not yet been published; we compare with measurements of the Angstrom exponent (AE) between 340 and 440 nm and the total number of particles larger than 10 nm (CN10)
We estimated the sensitivity of our predicted changes in aerosol concentrations to several uncertain model inputs that affect the ability of nucleated particles to grow to cloud condensation nuclei (CCN) sizes
Summary
The effect of the sun and other factors outside of the Earth system on the Earth’s climate remains a controversial aspect of climate science. In Marsh and Svensmark (2000a, b) the low cloud cover fraction was found to change from 30 % during the solar minimum to 28 % during the solar maximum. The low clouds were estimated to have a net forcing of −16.7 W m−2, so the estimated forcing change between solar maximum and minimum is about 1.2 W m−2 (Marsh and Svensmark, 2000a). This forcing change is similar to the estimates of the magnitude of the cooling from anthropogenic aerosol indirect effects and warming from the anthropogenic increase in CO2 (Forster et al, 2007). Subsequent evaluations of trends between cosmic rays and clouds have shown both strong correlations (Todd and Kniveton, 2001; Harrison and Stephenson, 2006) and weaker or no correlation (Sun and Bradley, 2002; Kristjansson et al, 2004; Todd and Kniveton, 2004)
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