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Concentrations of Water Soluble Organic Carbon (WSOC) and WSOC fraction toOrganic Carbon (OC) were measured at two urban sites in valleys of theFrench Alps during a period of two and a half years. Concentrations were ashigh as 10–15μg C/m<sup>3</sup> in winter, but there is a clear seasonalcycle of the WSOC fraction, with minima occurring during winter. Thisreflects a marked dependency on temperature, with the average WSOC fractionbeing stable at 54.8±7.7% and 75.9±6.3% for temperaturesin the ranges −10 to +3°C and 12 to 24°C, respectively. Severalpoints are noteworthy in this evolution. First, there are limiting factorsthat prevent lower mass fractions in the low temperature range and highermass fractions in the high temperature range. Second, the mass fraction atthe lower temperature is rather high, in apparent contradiction with OCbeing mainly insoluble close to the emission sources. Third, the range of20% for the change of the WSOC fraction between these extreme conditionsis indeed rather narrow when compared to evaluations of the secondary (andsupposedly water soluble) OC fraction proposed in the literature, with mostof the published values being in the range 40 to 70%. A comparison of theevolution of WSOC concentrations with that of dicarboxylic acids (DCA)clearly indicates the influence of two regimes in the formation of WSOC: oneat higher temperatures classically linked with the increase of DCAconcentrations and associated with oxidation processes, and another at lowertemperatures involving a much lower increase of DCA concentrations. Weproposed several hypotheses involving processes that could be responsiblefor the large concentrations of WSOC in the particulate phase at our sitesduring winter time

Seasonal variations of the water soluble organic carbon mass fraction of aerosol in two valleys of the French Alps