First quantification of global CCN sources
Cloud condensation nuclei (CCN) are aerosol particles large enough to form cloud drops. It is vital to understand where these particles come from because modification of their concentration by anthropogenic emissions causes a large radiative forcing (the aerosol indirect effect on cloud reflectivity). Until recently, global models did not contain the aerosol processes required to calculate CCN concentrations explicitly. GLOMAP has that capability.
In a recent paper by Joonas Merikanto The impact of nucleation on global CCN we used GLOMAP to quantify the sources of CCN on a global scale.
CCN are derived from particles emitted directly into the atmosphere (primary emissions) or from the growth of nanometer-sized particles nucleated in the atmosphere. We estimate that 45% of global low-level cloud CCN at 0.2% supersaturation are secondary aerosol derived from nucleation. 35% of CCN at the altitude of low level clouds were created in the free and upper troposphere and 10% from nucleation in the boundary layer.
Now that we know where CCN come from we can begin to understand how human activities and changes in natural emissions will alter CCN concentrations, cloud properties and climate. The very high fraction of CCN formed in the free troposphere has implications for long range transport of CCN from pollutant emissions (see the paper by Manktelow
A surprising results was the extent to which these various CCN sources are strongly coupled. In particular, boundary layer nucleated CCN are strongly suppressed by both primary emissions and entrainment of particles nucleated in the free troposphere. Elimination of all primary emissions reduces global CCN by only 20% and elimination of upper tropospheric nucleation reduces CCN (0.2%) by only 12% because of the increased contribution from boundary layer nucleation.