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The global aerosol-climate model ECHAM5-HAM was modified to improve therepresentation of new particle formation in the boundary layer.Activation-type nucleation mechanism was introduced to produce observednucleation rates in the lower troposphere. A simple and computationallyefficient model for biogenic secondary organic aerosol (BSOA) formation wasimplemented. Here we study the sensitivity of the aerosol and cloud dropletnumber concentrations (CDNC) to these additions. Activation-type nucleationsignificantly increases aerosol number concentrations in the boundary layer.Increased particle number concentrations have a significant effect also oncloud droplet number concentrations and therefore on cloud properties. Weperformed calculations with activation nucleation coefficient values of2×10−7s−1, 2×10−6s−1 and 2×10−5s−1 to evaluate the sensitivity to this parameter. For BSOAwe have used yields of 0.025, 0.07 and 0.15 to estimate the amount ofmonoterpene oxidation products available for condensation. The hybrid BSOAformation scheme induces large regional changes to size distribution oforganic carbon, and therefore affects particle optical properties and clouddroplet number concentrations locally. Although activation-type nucleationimproves modeled aerosol number concentrations in the boundary layer, theuse of a global activation coefficient generally leads to overestimation ofaerosol number. Overestimation can also arise from underestimation ofprimary emissions

Sensitivity of aerosol concentrations and cloud properties to nucleation and secondary organic distribution in ECHAM5-HAM global circulation model