Impact of nucleation on global CCN School of Earth and Environment, University of Leeds, Leeds, UK
Received: 27 Apr 2009 – Published in Atmos. Chem. Phys. Discuss.: 09 Jun 2009 Abstract. Cloud condensation nuclei (CCN) are derived from particles emitted directly
into the atmosphere (primary emissions) or from the growth of nanometer-sized
particles nucleated in the atmosphere. It is important to separate these two
sources because they respond in different ways to gas and particle emission
control strategies and environmental changes. Here, we use a global aerosol
microphysics model to quantify the contribution of primary and nucleated
particles to global CCN. The model considers primary emissions of sea spray,
sulfate and carbonaceous particles, and nucleation processes appropriate for
the free troposphere and boundary layer. We estimate that 45% of global
low-level cloud CCN at 0.2% supersaturation are secondary aerosol derived
from nucleation (ranging between 31–49% taking into account uncertainties
in primary emissions and nucleation rates), with the remainder from primary
emissions. The model suggests that 35% of CCN (0.2%) in global low-level
clouds were created in the free and upper troposphere. In the marine boundary
layer 55% of CCN (0.2%) are from nucleation, with 45% entrained from the
free troposphere and 10% nucleated directly in the boundary layer. Combinations
of model runs show that primary and nucleated CCN are non-linearly 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 (0.2%) 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.
Revised: 31 Oct 2009 – Accepted: 03 Nov 2009 – Published: 12 Nov 2009
Citation: Merikanto, J., Spracklen, D. V., Mann, G. W., Pickering, S. J., and Carslaw, K. S.: Impact of nucleation on global CCN, Atmos. Chem. Phys., 9, 8601-8616, doi:10.5194/acp-9-8601-2009, 2009.