Atmos. Chem. Phys. Atmospheric Chemistry and Physics, vol. 7 (2007) pp. 2073-2090
A statistical synthesis of marine aerosol measure-ments from experiments in four different oceans is used to evaluate a global aerosol microphysics model (GLOMAP). We compare the model against observed size resolved parti-cle concentrations, probability distributions, and the tempo-ral persistence of different size particles. We attempt to ex-plain the observed sub-micrometre size distributions in terms of sulfate and sea spray and quantify the possible contribu-tions of anthropogenic sulfate and carbonaceous material to the number and mass distribution. The model predicts a bi-modal size distribution that agrees well with observations as a grand average over all regions, but there are large regional differences. Notably, observed Aitken mode number concen-trations are more than a factor 10 higher than in the model for the N Atlantic but a factor 7 lower than the model in the NW Pacific. We also find that modelled Aitken mode and accumulation mode geometric mean diameters are gen-erally smaller in the model by 10–30%. Comparison with ob-served free tropospheric Aitken mode distributions suggests that the model underpredicts growth of these particles dur-ing descent to the marine boundary layer (MBL). Recent ob-servations of a substantial organic component of free tropo-spheric aerosol could explain this discrepancy. We find that anthropogenic continental material makes a substantial con-tribution to N Atlantic MBL aerosol, with typically 60–90% of sulfate across the particle size range coming from anthro-pogenic sources, even if we analyse air that has spent an aver-age of >120 h away from land. However, anthropogenic pri-mary black carbon and organic carbon particles (at the emis-sion size and quantity assumed here) do not explain the large discrepancies in Aitken mode number. Several explanations for the discrepancy are suggested. The lack of lower atmo-spheric particle formation in the model may explain low N Atlantic particle concentrations. However, the observed and Correspondence to: D. V. Spracklen (email@example.com) modelled particle persistence at Cape Grim in the Southern Ocean, does not reveal a diurnal cycle consistent with a pho-tochemically driven local particle source. We also show that a physically based cloud drop activation scheme better ex-plains the observed change in accumulation mode geometric mean diameter with particle number.
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