More than 25000 individual aerosol particles in 51 particulate matter samples, all taken from a research vessel over the North Sea and the English Channel, in a time range of 4 years, were analyzed by automated electron probe X-ray microanalysis (EPXMA). Multivariate methods were used to reduce the total data set. Singleparticle analysis combined with hierarchical cluster analysis yields nine major particle types. The North Sea aerosol is predominantly composed of sea salt, sulfur-rich particles, silicates, and calcium sulfate particles. Their abundance is dependent on meteorological conditions and sample location. Differences between all samples were studied on the basis of the abundance variations by using principal component analysis. Three factors explain 91% of the total covariance between the samples. The first component represents the marine-derived aerosol fraction and is more important as wind speed increases or at more remote sampling locations. The second component differentiates anthropogenically derived CaS04-rich samples.Their relative abundance is much more pronounced as the sampled air masses spend longer residence times over the south of England. The samples of the third cluster are related to high silicate and sulfur abundances. Source apportionment of this group was obtained by a second principal component analysis. Two different clusters separate mixed marine/ continental samples from pure continental-derived silicate and sulfur-rich particulate samples.

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Carbon diffusion in austenite plays a critical role in phase transformation in steel. However, it can only be estimated in the fully austenitic range and has then to be extrapolated to the temperature range of the phase transformation. Therefore, published data are limited to temperatures above 750°C. In this study, new experiments are carried out to determine the carbon diffusion coefficient in austenite at temperatures as low as 500°C. Carburization experiments are performed in the austenitic range for a Fe-1.5 pct Mn 0.13 pct C and a Fe-31 pct Ni alloy (wt pct). Composition profile measurements, which are done using glow discharge optical emission spectrometry (GDOES), show that the surface composition is not constant with time. A methodology has been developed to assess the diffusion coefficient of carbon in austenite combining the measured carbon profiles and a numerical method to compute the diffusion profile taking into account the time evolution of the boundary condition. This method is first validated on the Fe-C-Mn steel. Carburization experiments are carried out on a Fe-31 pct Ni alloy at 900°C, 800°C, 700°C, 600°C, and 500°C. The carbon diffusion coefficient is assessed using the method described above and fitted with the following expression (T in Kelvin): D ¼ 1:23 Á 10 À6 Á e À 15;050 T K ð Þ ðm 2 /sÞ. The new expression is compared with previous experimental results measured for comparable nickel content at higher temperatures, and it shows a reasonable agreement. The model proposed by Å gren for carbon diffusion has been modified to take into account the thermodynamic contribution of nickel. This model also shows good agreement with the present experimental results, even if it was fitted to experiments performed at higher temperatures.

Applications of instrumental microanalysis techniques for the characterization of individual particles in environmental samples are reviewed. The principles of electron microprobe analysis and related techniques, the micro-version of proton-induced X-ray emission, laser microprobe mass spectrometry, secondary ion mass spectrometry and Raman microprobe analysis, are briefly reviewed and their published applications to aerosols and to aqueous suspensions are described.

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