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The contribution of boundary layer (BL) nucleation events to totalparticle concentrations on the global scale has been studied byincluding a new particle formation mechanism in a global aerosolmicrophysics model. The mechanism is based on an analysis of extensiveobservations of particle formation in the BL at a continental surfacesite. It assumes that molecular clusters form at a rate proportionalto the gaseous sulfuric acid concentration to the power of 1. Theformation rate of 3 nm diameter observable particles is controlled bythe cluster formation rate and the existing particle surface area,which acts to scavenge condensable gases and clusters duringgrowth. Modelled sulfuric acid vapour concentrations, particleformation rates, growth rates, coagulation loss rates, peak particleconcentrations, and the daily timing of events in the global modelagree well with observations made during a 22-day period of March 2003at the SMEAR II station in Hyytiälä, Finland. The nucleationbursts produce total particle concentrations (>3 nm diameter) oftenexceeding 10<sup>4</sup> cm<sup>−3</sup>, which are sustained for a period ofseveral hours around local midday. The predicted global distributionof particle formation events broadly agrees with what is expected fromavailable observations. Over relatively clean remote continentallocations formation events can sustain mean total particleconcentrations up to a factor of 8 greater than those resulting fromanthropogenic sources of primary organic and black carbonparticles. However, in polluted continental regions anthropogenicprimary particles dominate particle number and formation events leadto smaller enhancements of up to a factor of 2. Our results thereforesuggest that particle concentrations in remote continental regions aredominated by nucleated particles while concentrations in pollutedcontinental regions are dominated by primary particles. The effect ofBL particle formation over tropical regions and the Amazonis negligible. These first global particle formation simulationsreveal some interesting sensitivities.  We show, for example, thatsignificant reductions in primary particle emissions may lead to anincrease in total particle concentration because of the couplingbetween particle surface area and the rate of new particleformation. This result suggests that changes in emissions may have acomplicated effect on global and regional aerosol properties. Overall,our results show that new particle formation is a significantcomponent of the aerosol particle number budget

The contribution of boundary layer nucleation events to total particle concentrations on regional and global scales