Seasonal variations of the Water Soluble Organic Carbon mass fraction of aerosol in two valleys of the French Alps

J.-L. Jaffrezo, G. Aymoz, C. Delaval, J. Cozic
2005 Atmospheric Chemistry and Physics Discussions  
Concentrations of Water Soluble Organic Carbon and WSOC fraction to OC were measured at two urban sites in valleys of the French Alps during a period of two and a half years. Concentrations were as high as 10-15 µg/m 3 in winter, but there is a clear seasonal cycle of the WSOC fraction, with minima occurring during winter. This re-5 flects a marked dependency on temperature, with the average WSOC fraction being stable at 54.8±7.7% and 75.9±6.3% for temperatures in the ranges −10 to +3 • C and
more » ... −10 to +3 • C and 12 to 24 • C, respectively. Several points are noteworthy in this evolution. First, there are limiting factors that prevent lower mass fractions in the low temperature range and higher mass fractions in the high temperature range. Second, the mass fraction at 10 the lower temperature is rather high, in apparent contradiction with OC being mainly insoluble close to the emission sources. Third, the range of 20% for the change of the WSOC fraction between these extreme conditions is indeed rather narrow when compared to evaluations of the secondary ( and supposedly water soluble) OC fraction proposed in the literature, with most of the published values being in the range 40 to 15 70%. A comparison of the evolution of WSOC concentrations with that of dicarboxylic acids (DCA) clearly indicates the influence of two regimes in the formation of WSOC: one at higher temperatures classically linked with the increase of DCA concentrations and associated with oxidation processes, and another at lower temperatures involving a much lower increase of DCA concentrations. We proposed several hypotheses in-20 volving processes that could be responsible for the large concentrations of WSOC in the particulate phase at our sites during winter. aspects related to global climate (Kanakidou et al., 2005 and references therein) and 4000 ACPD 5, 3999-4036, 2005 EGU the impact of aerosols on human health (Turpin, 1999; Dreher, 2000) . Several investigations are currently available in the literature that outline studies conducted in urban or rural environments. However, the number of samples in most of these studies is rather low (see Sect. 3), and little ancillary data exists to delineate the respective impacts of sources and aging on the evolution of WSOC concentrations and the mass fraction rel-5 ative to organic carbon (OC). Furthermore, these data were obtained through a large array of methods, both in relation to the collection of samples and the analyses of OC and WSOC concentrations, making comparisons and the construction of an overall picture rather difficult. The current belief maintains that the WSOC fraction is "low" for primary OC and that it increases with aging of the aerosol, together with the general 10
doi:10.5194/acpd-5-3999-2005 fatcat:4lqtgtk26bhwxbnysqxdbh6i54