Variable CCN formation potential of regional sulfur emissions

Abstract. Aerosols are short lived so their geographical distribution and impact on climate depends on where they are emitted. Previous model studies have shown that the mass of sulfate aerosol produced per unit sulfur emission (the sulfate burden potential) and the associated direct radiative forcing vary regionally because of differences in meteorology and photochemistry. Using a global model of aerosol microphysics, we show that the total number of aerosol particles produced per unit sulfur emission (the aerosol number potential) has a different regional variation to that of sulfate mass. The aerosol number potential of N. American and Asian emissions is calculated to be a factor of 3 to 4 times greater than that of European emissions, even though Europe has a higher sulfate burden potential. Pollution from N. America and Asia tends to reach higher altitudes than European pollution so forms more new particles through nucleation. Regional differences in particle production and growth mean that sulfur emissions from N. America and E. Asia produce 50 nm diameter cloud condensation nuclei up to 70% more efficiently than Europe. For 80 nm diameter CCN, N. America and Europe produce CCN 2.5 times more efficiently than E. Asia. The impact of regional sulfur emissions on particle concentrations is also much more widely spread than the impact on sulfate mass, due to efficient particle production in the free troposphere during long range transport. These results imply that regional sulfur emissions will have different climate forcing potentials through changes in cloud drop number.