234Th has been widely applied as a tracer of particulate organic carbon (POC) fluxes in the upper ocean. Fundamental to this approach is the determination of 234Th fluxes from water column measurements of the 234Th-238U disequilibria, and the conversion of 234Th flux to POC export, using the measured POC/234Th ratio on particles. As such, POC/234Th ratios are one of the most critical factors in quantifying the carbon export flux in ocean interior when using this approach. However, the POC/234Th ratios show significant temporal and spatial variations, but cannot be predicted at this time. Therefore, it is important to elucidate factors controlling the variations of the POC/234Th ratios. To achieve this purpose, we should understand the chemical interactions between POC and 234Th. In the open ocean, POC/234Th ratios have been determined together with other oceanographic parameters. We examined here the relationship between POC/234Th and primary production. The POC/234Th ratios were linearly related to logarithmic values of primary production. Taken into account the complexation between surface ligand on particulate organic matter (POM) and 234Th, a complexation model suggests that the size of particles adsorbing 234Th is related to primary production; in the equatorial Pacific, the size of particles adsorbing 234Th apparently decreases with increasing primary production, whereas opposite phenomenon occurs in the North Atlantic. Since the POC/234Th ratios were determined in filtered particulate matter, this finding suggests that aggregation of small particles would be dominant in the equatorial Pacific, which can be explained by a chemical aggregation model.
This paper presents the first description of a new three-dimensional aerosol chemical transport model, called the Model of Aerosol Species IN the Global AtmospheRe (MASINGAR), which has been developed to study the distributions of atmospheric aerosols and related trace species. MASINGAR is an on-line chemical transport model (CTM) coupled with the MRI/JMA98 GCM. MASINGAR includes nss-sulfate, carbonaceous, mineral dust, and sea-salt aerosols, and accounts for advective transport, subgrid-scale eddy diffusive and convective transport, surface emission, and dry/wet depositions, as well as chemical reactions. The advective transport is calculated using the semi-Lagrangian transport scheme. Parameterization of convective transport is based on the convective mass flux derived by the Arakawa-Schubert scheme. The space and time resolutions of the model are variable, with a standard spatial resolution of T42(2.8°×2.8°) and 30 vertical layers (up to 0.8hPa) with a 20-minute time step. In addition, the model has a built-in, four-dimensional data assimilation (FDDA) system with a nudging scheme incorporating an assimilated meteorological field, which enables the model to realistically simulate a specific period and a short-period forecast of aerosols. The model simulation of mineral dust aerosol in April 2002 suggests that MASINGAR simulates the synoptic scale aerosol events.