Abstract. A GLObal Model of Aerosol Processes (GLOMAP) has been developed as an extension to the TOMCAT 3-D Eulerian off-line chemical transport model. GLOMAP simulates the evolution of the global aerosol size distribution using a sectional two-moment scheme and includes the processes of aerosol nucleation, condensation, growth, coagulation, wet and dry deposition and cloud processing. We describe the results of a global simulation of sulfuric acid and sea spray aerosol. The model captures features of the aerosol size distribution that are well established from observations in the marine boundary layer and free troposphere. Modelled condensation nuclei (CN>3 nm) vary between about 250–500 cm-3 in remote marine boundary layer regions and between 2000 and 10 000 cm-3 (at standard temperature and pressure) in the upper troposphere. Cloud condensation nuclei (CCN) at 0.2% supersaturation vary between about 1000 cm-3 in polluted regions and between 10 and 500 cm-3 in the remote marine boundary layer. New particle formation through sulfuric acid-water binary nucleation occurs predominantly in the upper troposphere, but the model results show that these particles contribute greatly to aerosol concentrations in the marine boundary layer. It is estimated that sea spray emissions account for only ~10% of CCN in the tropical marine boundary layer, but between 20 and 75% in the mid-latitude Southern Ocean.

Abstract. We use the new GLOMAP model of global aerosol microphysics to investigate the sensitivity of modelled sulfate and sea salt aerosol properties to uncertainties in the driving microphysical processes and compare these uncertainties with those associated with aerosol and precursor gas emissions. Overall, we conclude that uncertainties in microphysical processes have a larger effect on global condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations than uncertainties in present-day sulfur emissions. Our simulations suggest that uncertainties in predicted sulfate and sea salt CCN abundances due to poorly constrained microphysical processes are likely to be of a similar magnitude to long-term changes in CCN due to changes in anthropogenic emissions. A microphysical treatment of the global sulfate aerosol allows the uncertainty in climate-relevant aerosol properties to be attributed to specific processes in a way that has not been possible with simpler aerosol schemes. In particular we conclude that: (1) changes in the binary H2SO4-H2O nucleation rate and condensation rate of gaseous H2SO4 cause a shift in the vertical location of the upper tropospheric CN layer by as much as 3 km, while changes in absolute concentration are relatively small; (2) uncertainties in the binary H2SO4-H2O nucleation rate have a relatively insignificant effect on boundary layer aerosol properties; (3) production of sulfate particles in power plant plumes below the scale of the model grid (which is of the order of 300 km) has the potential to change the global mean sulfate-derived CN concentration by a factor 2 or more at the surface, and changes of up to a factor 20 can occur in polluted regions; (4) predicted global mean sulfate and sea salt CCN concentrations change by 10 to 40% at the surface when several microphysical processes are changed within reasonable uncertainty ranges; (5) CCN concentrations are particularly sensitive to primary sulfate particle emissions, with global mean CCN changing by up to 40% and local concentrations changing by more than 100% when the percentage of anthropogenic SO2 emitted as particulates in plumes is changed from 0 to 5%; (6) uncertainties in CCN due to the mode of sulfate emission (i.e., the fraction of sulfur emitted as primary particles) are larger than those (~15%) caused by a ±25% change in total sulfur emissions; (7) large changes in sea spray flux have insignificant effects on global sulfate aerosol except when the mass accommodation coefficient of sulfuric acid on the salt particles is set unrealistically low.