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

Abstract

Abstract. 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/m3 in winter, but there is a clear seasonal cycle of the WSOC fraction, with minima occurring during winter. This reflects 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 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 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 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 involving processes that could be responsible for the large concentrations of WSOC in the particulate phase at our sites during winter.

Relationships between spectroscopic properties of high‐altitude organic aerosols and Sun photometry from ground‐based remote sensing

“…The higher PM loading observed for Saharan‐dominated periods than for marine‐dominated periods is consistent with previous findings by Mladenov et al [2009a] and supports other studies [ Morales‐Baquero et al , 2006; Pulido‐Villena et al , 2006; Mladenov et al , 2008] that attribute high dry deposition loadings during summers to Saharan dust events. The mean WSOC fraction of the TOC loading, however, was lower (14%) than other studies that report it between 38% and 77% for rural and high‐altitude sites [ Jaffrezo et al , 2005, and references therein]. Although there are methodological differences between these studies (all particle sizes in passive collection versus only particles <10 μm (PM 10 ) in active collection), this difference may suggest that the organic material deposited in the Sierra Nevada is less soluble than organic aerosols at other remote sites.…”

Carbonaceous species and humic like substances (HULIS) in Arctic snowpack during OASIS field campaign in Barrow

“…WinOC and DOC together represent the major fraction (over 95%) of carbon in the surface snowpack and remobilized snow. WinOC represent a very significant fraction of TCC in fine grained snows (Figure 5), with some values above 55% (median value 38%; half the values between 28% and 42%), higher than found in Greenland [e.g., Hagler et al , 2007] or more generally in aerosols [e.g., Jaffrezo et al , 2005]. This is not so surprising, as Barrow is a coastal site covered with tundra snowpack, directly in contact with plant and soil material (see Figure 3).…”

Chemical characteristics of water‐soluble organic carbon in the Asian outflow

“…The ratio is also within the range of values (0.10–0.50) reported by Mader et al [2004] for airborne filter samples obtained over downwind regions of China in spring. More comprehensive summaries of WSOC/OC ratios from previous studies available in the literature are given by Mader et al [2004] and Jaffrezo et al [2005].…”