Spatial distributions of global cloud condensation nuclei: Modelling and comparison with measurements

Atmospheric particles perturb the Earth's energy budget indirectly by acting as cloud condensation nuclei (CCN) and thus changing cloud properties and influencing precipitation. The aerosol indirect radiative forcing is the most important source of uncertainties in assessing the climate changes. The aerosol indirect radiative forcing is largely determined by the number abundance of particles that can act as CCN. At a given water supersaturation ratio (S), CCN number concentrations depend on the number size distribution and composition of atmospheric particles. Aerosol particles appear in the troposphere due to either in-situ nucleation (i.e, secondary particles) or direct emissions (i.e., primary particles). A size-resolved (sectional) particle microphysics model with a number of computationally efficient schemes has been incorporated into a global chemistry transport model (GEOS-Chem) to simulate particle number size distributions and CCN concentrations in the troposphere (Yu and Luo, 2009). The growth of nucleated particles through the condensation of sulfuric acid vapor and equilibrium uptake of nitrate, ammonium, and secondary organic aerosol is explicitly simulated, along with the coating of primary particles (dust, black carbon, organic carbon, and sea salt) by volatile components via condensation and coagulation with secondary particles. Our simulations indicate that secondary particles formed via ion-mediated nucleation appear to be able to account for the total particle number concentrations observed in many parts of troposphere. Secondary particles contribute up to ~ 60-90% of CCN0.4 (at S = 0.4%) in most parts of the troposphere, while primary particles (with coated volatile materials) can contribute up to 40-80% of CCN0.4 in the boundary layer of South America, West Africa, East and South Asia, and the associated continental outflow regions. In the lower troposphere (surface to ~ 2 km), CCN0.4 concentrations are typically several hundreds per cm3 over remote continental areas but they can reach several thousands cm-3 or higher over polluted regions. The remote marine CCN0.4 concentrations are typically ~ 40-100 cm-3 but can drop to a few tens per cm3 in some regions under certain seasons. The modeling results are compared with a large number of CCN measurements compiled recently by Andreae (2009), and our simulated CCN spatial distributions are generally consistent with the measurements. The major factors controlling global CCN abundance and key uncertainties in the simulations will also be discussed. References: Yu, F., and G. Luo, Simulation of particle size distribution with a global aerosol model: Contribution of nucleation to aerosol and CCN number concentrations, Atmos. Chem. Phys. Discuss., 9, 10597-10645, 2009. Andreae, M. O., Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions, Atmos. Chem. Phys., 9, 543-556, 2009.