The sample preparation line for radiocarbon measurements on atmospheric aerosol at LABEC

In atmospheric aerosols, carbonaceous particles consist of soot and of a wide variety of organic compounds. In Europe, carbon appears to be a major aerosol constituent (ten Brink et al., 2004). Carbonaceous aerosols are thought to play a major role in both the health and the climatic effects of aerosols; however, due to their complexity, the level of understanding and the knowledge of the sources for this aerosol fraction are still very low. Radiocarbon (14C) guarantees the opportunity of an unambiguous carbonaceous aerosol source apportionment because of its unique capability of assessing the contribution due to fossil fuel combustion. Radiocarbon decays with a half-life of about 5700 years. Living organisms are in equilibrium with the atmosphere and, as a good approximation, have the same radiocarbon concentration; after death, they behave as a close system and therefore they are more and more radiocarbon depleted due to its decay. Thanks to this mechanism, fossil fuels are indeed radiocarbon-free, while biogenic and biomass burning sources are characterised by about the same radiocarbon concentration of the present atmosphere. Radiocarbon measurements on the carbonaceous aerosol sub-fractions (EC and OC) allow an improvement in carbonaceous aerosol source apportionment, leading to a full and unambiguous distinction and quantification of the contributions to the aerosol burden in atmosphere, coming from natural or anthropogenic sources (Szidat et al., 2006). For many years, the INFN-LABEC laboratory of Florence (Italy) has been involved in AMS (Accelerator Mass Spectrometry) measurements for radiocarbon dating. However, in order to perform radiocarbon measurements on TC, EC and OC in aerosol samples, a new sample preparation line was required, to fulfil all the pre-treatment procedures necessary to separate the carbonaceous fractions. The new sample preparation line was developed to analyse samples characterised by smaller quantity than those typically prepared for to the radiocarbon dating (~600 μgC). The new sample preparation line can be briefly described as follows: the carrier gases (He and O2) after being suitably purified proceed in a combustion oven, where the filter loaded with atmospheric particulate matter is placed. The thermal program and the sample preparation were optimised to allow the EC/OC separation. After the combustion oven, a CuO catalyser ensures that all the combustion products evolve to CO2. After the removal of halogens, gaseous sulphur oxides and NO2, CO2 is cryogenically trapped with a newly designed trap. CO2 is then transferred from the trap to the graphitisation line, which forms the last part of the new sample preparation line. Later on, the produced graphite is pressed in capsules to be inserted in the accelerator ion source for analysis. Results will be reported from the first TC (total carbon) measurements that were carried out on samples of ~450 _g of carbon. These tests were aimed to fully characterise the line itself, to control the suitability of the produced samples for AMS measurements and, finally, to check the accuracy (Figure 1) of the AMS results obtained for these samples